Reduction of 4‐Nitrothiophenol on Ag/Au Bimetallic Alloy Surfaces Studied Using Bipolar Raman Spectroelectrochemistry

Herein, we describe an experimental approach for adapting the principles of Raman spectroelectrochemistry to electrodes controlled using a bipolar circuit. This method allows the simultaneous acquisition of spectroscopic data as a function of both the electrode potential and the chemical composition of a bimetallic alloy and can be generalized to other system variables. The electrochemical reduction of 4‐nitrothiophenol (4‐NTP) was carried out on bimetallic Ag/Au alloy gradients and monitored in situ using a confocal Raman microscope with 785 nm excitation. Continuous Ag/Au alloy gradients, in which the alloy composition varied from approximately 0.5 to 1.0 mole fraction Ag, were prepared by using bipolar electrodeposition and then modified with a monolayer of 4‐NTP using self‐assembly. 4‐NTP monolayers on Au/Ag alloys were placed in a bipolar electrochemical cell and characterized as a function of applied potential and chemical composition by using surface‐enhanced Raman scattering. The E1/2 for NTP reduction was observed to be a strong function of the alloy composition, increasing by over 100 mV as the mole fraction of Ag varied from 0.5 to 1.0. In addition, spectroscopic evidence for the formation of the partially reduced intermediate, 4,4’‐dimercaptoazobenzene (DMAB) at intermediate applied potentials, was also found. Bipolar Raman spectroelectrochemistry (BRSE) is a powerful tool for acquiring in situ multidimensional Raman spectroelectrochemical data. Fork in the road: Bipolar spectroelectrochemistry is used to acquire in situ SERS data as a function of potential and alloy composition for a monolayer of 4‐nitrothiophenol (4‐NTP). The reduction proceeds by one of two pathways, depending on alloy composition: a direct electrochemical reduction (lower Ag mole fraction) or a two‐step photo‐assisted pathway (higher Ag mole fraction).