Electron-Hole-Pair-Induced Vibrational Energy Relaxation of Rhenium Catalysts on Gold Surfaces

Vibrationally Mode-Specific Molecular Energy Transfer to Surface Electrons in Metastable Formaldehyde Scattering from Cesium-Covered Au(111)

Nonadiabatic interaction of adsorbate nuclear motion with the continuum of electronic states is known to affect the dynamics of chemical reactions at metal surfaces. A large body of work has probed the fundamental mechanisms of such interactions for atomic and diatomic molecules at surfaces. In polyatomic molecules, the possibility of mode-specific damping of vibrational motion due to the effects of electronic friction raises the question of whether such interactions could profoundly affect the outcome of chemistry at surfaces by selectively removing energy from a particular intramolecular adsorbate mode. However, to date, there have not been any fundamental experiments demonstrating nonadiabatic electron-vibration coupling in a polyatomic molecule at a surface. In this work, we scatter excited metastable formaldehyde and formaldehyde-d2 from a low work function surface and detect ejected exoelectrons that accompany molecular relaxation. The exoelectron ejection efficiency exhibits a strong dependence on the vibrational mode that is excited: out-of-plane bending excitation (ν4) leads to significantly more exoelectrons than does CO stretching excitation (ν2). The results provide clear evidence for mode-specific energy transfer from vibration to surface electrons.

Probing the Local Near-Field Intensity of Plasmonic Nanoparticles in the Mid-infrared Spectral Region

The enhanced local field of gold nanoparticles (AuNPs) in mid-infrared spectral regions is essential for improving the detection sensitivity of vibrational spectroscopy and mediating photochemical reactions. However, it is still challenging to measure its intensity at subnanometer scales. Here, using the NO2 symmetric stretching mode (νNO2) of self-assembled 4-nitrothiophenol (4-NTP) monolayers on AuNPs as a model, we demonstrated that the percentage of excited νNO2 mode, determined by femtosecond time-resolved sum-frequency generation vibrational spectroscopy, allows us to directly detect the local field intensity of the AuNP surface in subnanometer ranges. The local-field intensity is tuned by AuNP diameters. An approximate 17-fold enhancement was observed for the local field on 80 nm AuNPs compared to the Au film. Additionally, the local field can regulate the anharmonicity of the νNO2 mode by synergistic effect with molecular orientation. This work offers a promising approach to probe the local field intensity distribution around plasmonic NP surfaces at subnanometer scales.

Electron Transfer at Molecule-Metal Interfaces under Floquet Engineering: Rate Constant and Floquet Marcus Theory

Electron transfer (ET) at molecule-metal or molecule-semiconductor interfaces is a fundamental reaction that underlies all electrochemical processes and substrate-mediated surface photochemistry. In this study, we show that ET rates near a metal surface can be significantly manipulated by periodic driving (e.g., Floquet engineering). We employ the Floquet surface hopping and Floquet electronic friction algorithms developed previously to calculate the ET rates near the metal surface as a function of driving amplitudes and driving frequencies. We find that ET rates have a turnover effect when the driving frequencies increase. A Floquet Marcus theory is further formulated to analyze such a turnover effect. We then benchmark the Floquet Marcus theory against Floquet surface hopping and Floquet electronic friction methods, indicating that the Floquet Marcus theory works in the strong nonadiabatic regimes but fails in the weak nonadiabatic regimes. We hope these theoretical tools will be useful to study ET rates in the plasmonic cavity and plasmon-assisted photocatalysis.

Electrochemical CO2 reduction on Cu and Au electrodes studied using in situ sum frequency generation spectroscopy

As an important pathway for energy storage and a key reaction in the carbon cycle, the CO₂ electrochemical reduction reaction has recently gained significant interest. A variety of catalysts have been used to approach this topic experimentally and theoretically; however, the molecular level insight into the reaction mechanism is lacking due to the complexity of the surface processes and the challenges in probing the intermediate species. In this study, CO₂ reduction reactions on polycrystalline Cu and Au electrodes were investigated in 0.1 M CO₂-saturated NaHCO₃ solution. In situ sum frequency generation (SFG) spectroscopy has been adopted to access the intermediates and products on the metal electrodes. On the Au electrode, only linearly adsorbed CO could be detected, and the reduction produced no hydrocarbon species. On the Cu electrode, C-H stretching vibrations corresponding to surface-adsorbed ethoxy species were observed, but no CO vibrations can be detected with SFG. The results revealed that the CO randomly adsorbed on the Cu surface, and the multiple orientations of the adsorbed species may be the reason for the formation of C-C bonding. These results demonstrate direct molecular level evidence for different reaction pathways on the Cu and Au electrodes.

Heterogenized Molecular Catalysts: Vibrational Sum-Frequency Spectroscopic, Electrochemical, and Theoretical Investigations

Rhenium and manganese bipyridyl tricarbonyl complexes have attracted intense interest for their promising applications in photocatalytic and electrocatalytic CO₂ reduction in both homogeneous and heterogenized systems. To date, there have been extensive studies on immobilizing Re catalysts on solid surfaces for higher catalytic efficiency, reduced catalyst loading, and convenient product separation. However, in order for the heterogenized molecular catalysts to achieve the combination of the best aspects of homogeneous and heterogeneous catalysts, it is essential to understand the fundamental physicochemical properties of such heterogeneous systems, such as surface-bound structures of Re/Mn catalysts, substrate-adsorbate interactions, and photoinduced or electric-field-induced effects on Re/Mn catalysts. For example, the surface may act to (un)block substrates, (un)trap charges, (de)stabilize particular intermediates (and thus affect scaling relations), and shift potentials in different directions, just as protein environments do. The close collaboration between the Lian, Batista, and Kubiak groups has resulted in an integrated approach to investigate how the semiconductor or metal surface affects the properties of the attached catalyst. Synthetic strategies to achieve stable and controlled attachment of Re/Mn molecular catalysts have been developed. Steady-state, time-resolved, and electrochemical vibrational sum-frequency generation (SFG) spectroscopic studies have provided insight into the effects of interfacial structures, ultrafast vibrational energy relaxation, and electric field on the Re/Mn catalysts, respectively. Various computational methods utilizing density functional theory (DFT) have been developed and applied to determine the molecular orientation by direct comparison to spectroscopy, unravel vibrational energy relaxation mechanisms, and quantify the interfacial electric field strength of the Re/Mn catalyst systems. This Account starts with a discussion of the recent progress in determining the surface-bound structures of Re catalysts on semiconductor and Au surfaces by a combined vibrational SFG and DFT study. The effects of crystal facet, length of anchoring ligands, and doping of the semiconductor on the bound structures of Re catalysts and of the substrate itself are discussed. This is followed by a summary of the progress in understanding the vibrational relaxation (VR) dynamics of Re catalysts covalently adsorbed on semiconductor and metal surfaces. The VR processes of Re catalysts on TiO₂ films and TiO₂ single crystals and a Re catalyst tethered on Au, particularly the role of electron-hole pair (EHP)-induced coupling on the VR of the Re catalyst bound on Au, are discussed. The Account also summarizes recent studies in quantifying the electric field strength experienced by the catalytically active site of the Re/Mn catalyst bound on a Au electrode based on a combined electrochemical SFG and DFT study of the Stark tuning of the CO stretching modes of these catalysts. Finally, future research directions on surface-immobilized molecular catalyst systems are discussed.

CO Stretch Vibration Lives Long on Au(111)

Measured lifetimes of the CO internal stretch mode on various metal surfaces routinely lie in the picosecond regime. These short vibrational lifetimes, which are actually reproduced by current first-principles nonadiabatic calculations, are attributed to the rapid vibrational energy loss that is caused by the facile excitation of electron-hole pairs in metals. However, this explanation was recently questioned by the huge discrepancy that exists for CO on Au(111) between the experimental vibrational lifetime that is larger than 100 ps and the previous theoretical predictions of 4.8 and 1.6 ps. Here, we show that the state-of-the-art nonadiabatic theory does reproduce the long CO lifetime measured in Au(111) provided the molecule-surface interaction is properly described. Importantly, our new results confirm that the current understanding of the adsorbates' vibrational relaxation at metal surfaces is indeed valid.

Understanding the intramolecular vibrational energy transfer and structural dynamics of anionic ligands in a photo-catalytic CO2reduction catalyst

The knowledge of intramolecular vibrational energy redistribution (IVR) and structural dynamics of rhenium photo-catalysts is essential for understanding the mechanism of the photo-catalytic process of CO₂reduction.

Ultrafast Dynamics of Charge Transfer and Photochemical Reactions in Solar Energy Conversion

For decades, ultrafast time-resolved spectroscopy has found its way into an increasing number of applications. It has become a vital technique to investigate energy conversion processes and charge transfer dynamics in optoelectronic systems such as solar cells and solar-driven photocatalytic applications. The understanding of charge transfer and photochemical reactions can help optimize and improve the performance of relevant devices with solar energy conversion processes. Here, the fundamental principles of photochemical and photophysical processes in photoinduced reactions, in which the fundamental charge carrier dynamic processes include interfacial electron transfer, singlet excitons, triplet excitons, excitons fission, and recombination, are reviewed. Transient absorption (TA) spectroscopy techniques provide a good understanding of the energy/electron transfer processes. These processes, including excited state generation and interfacial energy/electron transfer, are dominate constituents of solar energy conversion applications, for example, dye-sensitized solar cells and photocatalysis. An outlook for intrinsic electron/energy transfer dynamics via TA spectroscopic characterization is provided, establishing a foundation for the rational design of solar energy conversion devices.