Heterometallic antenna-reactor complexes for photocatalysis

Abstract

Metallic nanoparticles with strong optically resonant properties behave as nanoscale optical antennas, and have recently shown extraordinary promise as light-driven catalysts. Traditionally, however, heterogeneous catalysis has relied upon weakly light-absorbing metals such as Pd, Pt, Ru, or Rh to lower the activation energy for chemical reactions. Here we show that coupling a plasmonic nanoantenna directly to catalytic nanoparticles enables the light-induced generation of hot carriers within the catalyst nanoparticles, transforming the entire complex into an efficient light-controlled reactive catalyst. In Pd-decorated Al nanocrystals, photocatalytic hydrogen desorption closely follows the antenna-induced local absorption cross-section of the Pd islands, and a supralinear power dependence strongly suggests that hot-carrier-induced desorption occurs at the Pd island surface. When acetylene is present along with hydrogen, the selectivity for photocatalytic ethylene production relative to ethane is strongly enhanced, approaching 40:1. These observations indicate that antenna-reactor complexes may greatly expand possibilities for developing designer photocatalytic substrates.

Keywords: aluminum; catalysis; nanoparticle; photocatalysis; plasmon.

Fig. 1.

Absorption enhancements in heterometallic antenna−reactor systems. (A) Generalized schematic of a simple system containing a plasmonic antenna coupled through localized near-field enhancements to a catalytic reactor metal nanoparticle. (B) A simplified Pd−AlNC antenna−reactor model, consisting of a 20-nm Pd island on a 110-nm-diameter AlNC nanosphere. (C) Near-field enhancement in the Pd island for this antenna−reactor geometry. (D) Schematic of a comparative geometry, a Pd island on a 110-nm-diameter dielectric Al₂O₃ nanosphere. (E) The near-field enhancement in this geometry is substantially reduced relative to C. (F) Absorption of Pd on Al₂O₃ (black) and an antenna−reactor geometry using FDTD (red solid curve) and isolated absorption multiplied by field enhancement (red dashed curve). Near-field enhancement in the Al₂O₃ layer of the AlNC is shown in blue.

Fig. 2.

EELS plasmon maps AlNC−Pd heterometallic antenna−reactor catalysts. (A) HAADF-STEM corresponding to the particle of interest is shown in Inset. Electron energy loss spectral components corresponding to modes shown in B−E as obtained from the region indicated by the red square in Inset. A Pd-decorated Al₂O₃ protrusion is highlighted by the white arrow. (Scale bar, 50 nm.) (B) Al LSPR mode with energy loss centered at 6 eV. (C) Al LSPR mode with energy loss centered at 7 eV. (D) Al–Pd antenna mode with broad energy loss ranging from 1.5 eV to 6 eV. (E) Pd interband transition. Spectra for modes in B–E are shown in A. Normalized EELS loss probability of spatial plasmon maps correspond to color legend on left.

Fig. 3.

AlNC−Pd photocatalytic reactivity for the hydrogen−deuterium exchange reaction. (A) Wavelength dependence of H₂ desorption on AlNC–Pd (red) and pristine AlNC (green) and the calculated absorption cross-section of AlNC−Pd (black). The sample was irradiated with a power density of 5 W/cm² at all wavelengths. (B) Power dependence measurements of HD production at the dipolar plasmon resonance (492 nm, 2.52 eV) and the Al interband transition (800 nm, 1.55 eV).

Fig. 4.

AlNC−Pd photocatalytic reactivity toward selective acetylene hydrogenation. Selectivity of ethylene:ethane production by acetylene hydrogenation under illuminated (red) and thermal (black) conditions.