Hot-carrier dynamics in catalysis
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Hot-carrier generation and transfer
The understanding of the dual particle-wave nature of light and matter was a watershed moment in our basic understanding of how energy flows in the universe; this has been leveraged extensively in advancing experimental and theoretical tools to elucidate, and even control, the flow of energy in chemical and materials systems where light and matter converge.1 Hot-carrier-driven catalysis represents a curious trinity of energy flow—incoming light imparts energy into electronic degrees of freedom, which in turn, impart energy into nuclear degrees of freedom. Not only is this interplay between photonic, electronic, and phononic/ nuclear energy fundamentally interesting, it shows potential to enable technologies that are capable of efficiently harvesting energy from solar photons or using low-intensity light sources to drive photochemical transformations under mild conditions, which are both important from the point of view of sustainability.2–4 Here, we focus on metal nanoparticles as sources of hot carriers. Such nanoparticle systems may derive their efficient hot-carrier generation properties from intrinsic optical resonances (such as localized surface plasmon resonance, LSPR)2–4 or emergent optical resonances (such as scattering mediated absorption, SMA).5,6
Hot-carrier transfer from metal nanoparticles to molecules to initiate photocatalytic reactions is an intrinsically multiscale problem—the constituents span multiple length scales and the energy-transfer events span multiple time scales. On the shortest time scale (∼10 fs), electromagnetic energy flows into the electronic degrees of freedom of the nanoparticle(s) and molecule(s) subject to their mutual physio–chemical interactions; these interactions may have a small or large impact on the electronic degrees of freedom depending on the identity of the metal and the molecules.4 Plasmonic excitation and decay also occur on this time scale.2–4 A slightly longer time scale (tens to hundreds of fs) sees an evolution of the hot-carrier distributions, including thermalization through electron–electron scattering, decay through electron–phonon scattering, and transfer to adsorbates.4,7–9 The transfer of charge from the metal to the molecule deposits energy into the degrees of freedom of the molecule, but also fundamentally changes the potential energy surface that governs the nuclear dynamics of the molecule. Nuclear motion of the adsorbate molecules evolves on the potential energy surface(s), corresponding to excited or ionized states of the adsorbate molecule; often the interatomic forces that govern the molecular motion upon excitation and ionization are considerably changed
Hayk Harutyunyan, Emory University, USA; [email protected] Figen Suchanek, William Paterson University of New Jersey, USA; [email protected] Robert Lemasters, Emory University, USA; [email protected] Jonathan J. Foley IV, William Paterson University of New Jersey, USA; [email protected] doi:10.1557/mrs.2019.291
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• VOLUME 45 • JANUARY 2020 •
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