Electrochemistry: Hot Electron-driven Catalysis via Liquid Phase Local Plasma Discharge and Plasmon-resonant Nanostructures
Abstract
Over the past 10 years, there have been a large number of studies exploring plasmon resonant and hot electron-enhanced catalysis in metal nanostructures. However, because of their extremely short lifetimes (50fsec Ð 1psec), claims of their role in catalysis have been largely speculative and mechanistic understanding remains vague. What is currently lacking are direct spectroscopic measurements of charge transfer from these hot electrons to the ions and molecules in solution. Previously, we used pump-probe spectroscopy to measure the dynamics of hot electrons in the solid-state (i.e., spectroscopic signatures of hot electrons in metals). In the proposed work, we will extend this hot electron study into the liquid domain and directly monitor hot electron charge transfer to ions/molecules in solution spectroscopically both under steady-state and time-resolved conditions. The proposed work explores two approaches to studying hot electron-driven catalysis, one involving liquid phase plasma discharge and the other plasmon-resonant nanostructures. These two approaches are synergistic, and knowledge gained in one area will inform that of the other. In the proposed work, we will tether various Òreporter moleculesÓ to plasmonic nanostructure electrodes, which will enable us to monitor charge transfer to the ions in solution and determine the potential dependence of the plasmon-driven chemistry. As the electrode potential is scanned to more reducing potentials, we should be able to shut off this plasmon-driven charge transfer reaction. Finding this threshold is related to the steady-state photovoltage hypothesis and is of fundamental value for understanding plasmon-driven chemistry. Time-resolved spectroscopy will enable us to establish the time dependence of photo-induced electric fields and charge transfer dynamics, as well as local pH. Ultimately, this will enable us to see how fast charge is transferred to ions in solution and measure the diffusion kinetics that restore the local environment to equilibrium after hot electrons are injected. The hot electrons produced in a transient plasma are much hotter (i.e., 30 eV) than those produced in metal photoexcitations (i.e., < 2 eV). As such, the plasma-based hot electron approach provides a basis for what is possible with plasmon resonant hot electrons (i.e., non-equilibrium chemistry) that may ultimately be more efficient from a power/energy standpoint. Here, the nanosecond voltage pulses enable a new regime of high fields to be applied without producing a large amount of current. Nanoparticle-enhancement enable the plasma to be discharged locally on the surface of the nanoparticles where it is most useful for catalysis. By independently controlling the local ion environment and the plasma radical species, we aim to drive difficult reaction (i.e., high barrier) systems, such as N2 À NH3, CO2 reduction to hydrocarbons, and conversion of nitrates to ammonia. We believe that introducing a transient plasma at an electrochemically-tunable electrode surface will provide new degrees of freedom in controlling the selectivity of the reaction pathways. For example, a typical limiting factor in driving electrochemistry is water splitting, which represents a low barrier reaction that short-circuits or shunts higher barrier reactions such as CO2 reduction, N2 reduction, etc. By electrochemically-tuning the electrode surface to hydrogen rich or hydroxyl-rich, we can control the radicals, intermediates, and pathways to promote/demote a given reaction of interest.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Sep 28, 2022
- Source ID
- W911NF2210284
Entities
People
- Stephen B Cronin
Organizations
- Army Contracting Command
- United States Army
- University of Southern California