Monitoring Local Electric Fields using Stark Shifts on Napthyl Nitrile-Functionalized Silicon Photoelectrodes

• Published in: Journal of physical chemistry. C 2020-08, Vol.124 (31), p.17000-17005
• Main Authors: Shi, Haotian, Pekarek, Ryan T, Chen, Ran, Zhang, Boxin, Wang, Yu, Aravind, Indu, Cai, Zhi, Jensen, Lasse, Neale, Nathan R, Cronin, Stephen B
• Format: Article
• Language: English
• Subjects: C: Surfaces, Interfaces, Porous Materials, and Catalysis; GERS; in-situ Raman; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; local electric field; monolayer graphene; silicon photoelectrode; Stark-shifts
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.0c03966

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.