Probing the Birth and Ultrafast Dynamics of Hydrated Electrons at the Gold/Liquid Water Interface via an Optoelectronic Approach.

François Lapointe,Martin Wolf,Yujin Tong,R Kramer Campen

doi:10.1021/jacs.0c08289

Abstract

The hydrated electron has fundamental and practical significance in radiation and radical chemistry, catalysis, and radiobiology. While its bulk properties have been extensively studied, its behavior at solid/liquid interfaces is still unclear due to the lack of effective tools to characterize this short-lived species in between two condensed matter layers. In this study, we develop a novel optoelectronic technique for the characterization of the birth and structural evolution of solvated electrons at the metal/liquid interface with a femtosecond time resolution. Using this tool, we record for the first time the transient spectra (in a photon energy range from 0.31 to 1.85 eV) in situ with a time resolution of 50 fs revealing several novel aspects of their properties at the interface. Especially the transient species show state-dependent optical transition behaviors from being isotropic in the hot state to perpendicular to the surface in the trapped and solvated states. The technique will enable a better understanding of hot electron driven reactions at electrochemical interfaces.

Highlights

The hydrated electron is the most fundamental ion in aqueous solution,[1] a metastable structure of water molecules interacting with a free electron
Electrons in solution have been extensively studied from the standpoint of radiobiology, radical and radiation chemistry, and charge transfer systems for their importance in radiotherapy, physiology, catalysis, and atmospheric reactions.[3−6]
As we tune to higher second pulse photon energies, e.g., 1.55 eV (800 nm in wavelength, lower panel of Figure 1A), the rise is delayed by 50−100 fs, the signal decays much more slowly, and it persists for tens of picoseconds (Figure 1C)

Summary

■ INTRODUCTION

The hydrated electron is the most fundamental ion in aqueous solution,[1] a metastable structure of water molecules interacting with a free electron. The electrified metal/electrolyte interface[29,30] is highly relevant to technological systems e.g., batteries, fuel cells, Graẗ zel’s cells that rely on heterogeneous electron transfer[31,32] to store and convert energy or to carry out electrochemical reactions Improving such devices requires understanding the mechanism by which the electron moves across the solid/liquid interface. Through excitation by an ultrashort UV pulse (1 kHz, 267 nm) with a fluence of ≈0.6 mJ/cm[2], electrons are ejected from the electrode into the solution using the photoelectric process This creates a photovoltage ΔV1, which can be measured using a conventional potentiostat, that reaches a stationary value after 20−30 min as governed by mass transport and the cell geometry. The photoinduced electrode potential was measured in open circuit mode (open circuit potential, OCP) and corrected for the pulse energy of the first and second pulses and water absorption coefficient at the second pulse wavelength (see the Supporting Information)

■ RESULTS AND DISCUSSION

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES