Direct Vibrational Stark Shift Probe of Quasi-Fermi Level Alignment in Metal Nanoparticle Catalyst-Based Metal–Insulator–Semiconductor Junction Photoelectrodes

Photoelectrodes consisting of metal–insulator–semiconductor (MIS) junctions are a promising candidate architecture for water splitting and for the CO2 reduction 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.


Ag electrode preparation
A 0.5mm-thick, 0.5 cm × 1 cm Ag foil (thickness 0.5 mm, 99.99% trace metals basis) was used to fabricate bare Ag electrodes. To obtain the surface-enhanced Raman signal of 4-MBN on the Ag electrode, the Ag electrode was first mechanically polished and then roughened via an oxidation-reduction cycle (ORC) treatment. 1 The ORC treatment was carried out by using a threeelectrode setup, which is described in section 2. The Ag electrode was emerged in 0.1 M KCl solution, and first oxidized with an applied potential ramp from -0.15 V to 0.65 V at a scan rate of 0.01 V s -1 . Then, reduction of the Ag electrode was done by ramping the potential from -1.25 V to -1.45 V at a scan rate of 0.001 Vs -1 . The color of the Ag electrode surface changed from metallic silver to black (during the oxidation) and then to white (during the reduction). Both the Ag foil and KCl were purchased from Sigma-Aldrich.

b-Si-Ag photoelectrode preparation
Electrodes were prepared using single side polished, low doped p-type (1-10 Ω•cm) and degenerately doped n-type silicon wafers purchased from University Wafer. Wafers were cut into 1 cm × 1 cm pieces and sonicated in a mixture of acetone and water. The back side was rigorously scratched, and pieces were soaked in 5 wt% aqueous HF solution for 90 seconds. Insulated copper wires were contacted using GaIn eutectic (ThermoScientfic) and colloidal silver paste (Delta Technologies). Back contacts were covered in a thick film of Loctite EA 9460 Epoxy and allowed to cure for at least one day.
b-Si-Ag electrodes were fabricated via a metal assisted etching technique previously described. 2 Wired electrodes were initially soaked in a 5 wt% HF solution for 90 seconds. The electrodes were then transferred to a 1 mM aqueous solution of AgNO3 in 1 wt% HF and soaked for 1 minute to electrolessly deposit silver particles. The electrodes were then soaked for 15 minutes in a 1 wt% H2O2, 1 wt% HF solution to be made porous, and silver particles were then removed with a 10-minute soak in 35 wt% HNO3. Silver particles were then redeposited on the surface by repeating the initial 5 wt% HF and 1 mM AgNO3 in 1 wt% HF steps. Between each step, electrodes were rigorously rinsed with water and blown dry with inert gas. HF (47.5 wt%), AgNO3, H2O2 (30 wt%), and HNO3 (70 wt%) were purchased from Spectrum Chemical, Sigma-Aldrich, Thermo Fisher Scientific and EMD Millipore, respectively.

SAM layer preparation
Both the roughened Ag electrode and the b-Si-Ag electrode were soaked in a 10 mM acetonitrile solution of 4-MBN overnight at room temperature to form the 4-MBN self-assembled monolayer (SAM). 3 Both Ag electrode and b-Si-Ag electrode were sonicated in acetonitrile solution for 1 minute prior to the Raman measurement. 4-MBN was purchased from Biosynth. The solution for the (photo)electrochemical measurement is 0.2M KHCO3 (pH = 8.2). The working electrode (WE) is b-p-Si-Ag NPs-4-MBN or roughened Ag electrode. The reference electrode (RE) is Ag/AgCl/1M KCl (Gaoss Union). The counter electrode (CE) is Pt mesh (Gaoss Union). KHCO3 (BioUltra, ≥99.5%) was purchased from Sigma-Aldrich.

Surface-enhanced Raman spectroscopy (SERS) Measurements
The SERS measurements were conducted via a home-built Raman setup. The Raman laser source was HeNe 632.8nm (Thorlabs HNL210LB). The laser light was directed to the electrode surface, passing through an neutral density filter for tuning the fluence intensity and focused by a 10X microscope objective (10X Olympus Plan Achromat Objective, 0.25 NA, 10.6 mm WD, from Thorlabs, RMS10X) at the sample surface. The scattered light from the sample spot was collected by the 10X microscope objective. The collected Raman signal was filtered by a 633 nm singlenotch filter (StopLine, Semrock), directed to be dispersed by a spectrograph (Shamrock, Andor) and then detected by an electron-multiplied charge coupled device (EMCCD, Newton, Andor).