Hot-Carrier Transfer across a Nanoparticle-Molecule Junction: The Importance of Orbital Hybridization and Level Alignment

Abstract

While direct hot-carrier transfer can increase photocatalytic activity, it is difficult to discern experimentally and competes with several other mechanisms. To shed light on these aspects, here, we model from first-principles hot-carrier generation across the interface between plasmonic nanoparticles and a CO molecule. The hot-electron transfer probability depends nonmonotonically on the nanoparticle-molecule distance and can be effective at long distances, even before a strong chemical bond can form; hot-hole transfer on the other hand is limited to shorter distances. These observations can be explained by the energetic alignment between molecular and nanoparticle states as well as the excitation frequency. The hybridization of the molecular orbitals is the key predictor for hot-carrier transfer in these systems, emphasizing the necessity of ground state hybridization for accurate predictions. Finally, we show a nontrivial dependence of the hot-carrier distribution on the excitation energy, which could be exploited when optimizing photocatalytic systems.

Keywords: Adsorption; Hot-carrier; Nanoparticles; Plasmonic catalysis; TDDFT.

Figure 1

Geometry dependence of HC generation in Ag₂₀₁ NP + CO. (a) Optical spectrum of the bare NP. The frequency 3.8 eV of the driving laser is marked by an arrow above the spectrum. (b) Model of the NP with the axes along which the NP–molecule distance is varied. (c, d) Fractions of generated electrons (c) and holes (d) on the molecule (eq S20) averaged in the span 25–30 fs and binding energies (eq S1) as a function of distance and site.

Figure 2

Level alignment between molecular and NP PDOS for (111) on-top (a–c) and corner (d–f) sites. (a, d) Molecular PDOS as a function of distance. The NP PDOS is distance-independent due to the large size of the NP. As the molecule approaches the NP, the LUMO shifts to lower energies, eventually splitting into several branches. The PDOS for the NP and molecule at far separation are indicated above the plot, where the NP PDOS has been shifted by the pulse frequency. Shaded regions correspond to (a selection of) large values in the shifted NP PDOS. (b, e) Electron distribution as a function of distance. (c, f) The fraction of electrons generated in the molecule.

Figure 3

Geometry dependence of electron generation in Ag₂₀₁, Au₂₀₁, and Cu₂₀₁ NPs + CO. (a) Optical spectra of the bare NPs. (b–e) Fractions of generated electrons on the molecule (eq S20) after plasmon decay as a function of distance and site with pulse frequency 3.8 eV (Ag), 2.5 eV (Au), and 2.7 eV (Cu).

Figure 4

Pulse-frequency dependence of the electron generation in CO. Fraction of generated electrons (a–c) and holes (d–f) on the molecule as a function of distance and pulse frequency for the (111) on-top site in Ag (a, d), Au (b, e), and Cu (c, f). For reference, the crosses in the figure mark the distance corresponding to the adsorption minimum, and the pulse frequency used in Figure 3.