Abstract
Energy migrations at metal nanomaterial surfaces are fundamentally important to heterogeneous reactions. Here we report two distinctive energy migration pathways of monolayer adsorbate molecules on differently sized metal nanoparticle surfaces investigated with ultrafast vibrational spectroscopy. On a 5 nm platinum particle, within a few picoseconds the vibrational energy of a carbon monoxide adsorbate rapidly dissipates into the particle through electron/hole pair excitations, generating heat that quickly migrates on surface. In contrast, the lack of vibration-electron coupling on approximately 1 nm particles results in vibrational energy migration among adsorbates that occurs on a twenty times slower timescale. Further investigations reveal that the rapid carbon monoxide energy relaxation is also affected by the adsorption sites and the nature of the metal but to a lesser extent. These findings reflect the dependence of electron/vibration coupling on the metallic nature, size and surface site of nanoparticles and its significance in mediating energy relaxations and migrations on nanoparticle surfaces.
Highlights
Energy migrations at metal nanomaterial surfaces are fundamentally important to heterogeneous reactions
We demonstrate that the real-time energy migration dynamics between molecules on different surface sites of metal nanoparticles can be monitored with an ultrafast multiple dimensional vibrational spectroscopy
Two distinctive energy pathways are observed for a monolayer of carbon monoxide (CO) molecules adsorbed on different surface sites of a series of platinum (Pt) nanoparticles
Summary
Energy migrations at metal nanomaterial surfaces are fundamentally important to heterogeneous reactions. We report two distinctive energy migration pathways of monolayer adsorbate molecules on differently sized metal nanoparticle surfaces investigated with ultrafast vibrational spectroscopy. Further investigations reveal that the rapid carbon monoxide energy relaxation is affected by the adsorption sites and the nature of the metal but to a lesser extent. Some experimental evidence[17,18,19,20,21,22] and theoretical studies[23] have shown the nonadiabatic effects of vibration/electron coupling on flat metal and some metal nanoparticle surfaces[10,24], questioning the general applicability of the approach This problem is even more complicated on the surfaces of catalytic metal nanoparticles, as the electronic properties can vary with particle size[25] and surface sites[26]. Changing the surface adsorption sites and the nature of the metal from Pt to Palladium (Pd) alters the fast energy relaxation but to a lesser extent compared with the size effect
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