Monitoring Local Electric Fields using Stark Shifts on Napthyl Nitrile-Functionalized Silicon Photoelectrodes
Haotian Shi, Ryan T. Pekarek, Ran Chen, Boxin Zhang, Yu Wang, Indu Aravind, Zhi Cai, Lasse Jensen, Nathan R. Neale, Stephen B. Cronin
Abstract
We report spectroscopic measurements of the local electric field using vibrational Stark shifts of napthyl nitrile-functionalized silicon under electrochemical working conditions. The C≡N bond is particularly sensitive to applied electric fields and serves as a good probe for the local electric fields at the silicon–aqueous interface. Here, surface-enhanced Raman spectra (SERS) are collected at a silicon surface using a water immersion lens as a function of the reference potential in a three-terminal potentiostat. In deionized (DI) water and KCl solutions, the nitrile (i.e., C≡N) stretch downshifts by 4.7 and 8.6 cm–1, respectively, under an applied potential of −1 V vs Ag/AgCl. Density functional theory (DFT) calculations of the napthyl nitrile complex carried out under various electric fields establish the Stark tuning rate to be 0.5622 cm–1/(MV cm–1). Based on this relation, electric fields of −8.4 and −15.2 MV/cm were obtained under negative applied potentials. These measurements report the electric field strength within the double (i.e., Helmholtz) layer, which is responsible for pulling positively charged ions (e.g., H+) toward the surface in reduction reaction processes.