  We develop herein plasmonic-catalytic Au-IrO$_2$ nanostructures with a
morphology optimized for efficient light harvesting and catalytic surface area;
the nanoparticles have a dendritic morphology, with closely spaced Au branches
all partially covered by an ultrathin (1 nm) IrO$_2$ shell. This nanoparticle
architecture optimizes optical features due to the interactions of closely
spaced plasmonic branches forming electromagnetic hot spots, and the ultra-thin
IrO$_2$ layer maximizes efficient use of this expensive catalyst. This concept
was evaluated towards the enhancement of the electrocatalytic performances
towards the oxygen evolution reaction (OER) as a model transformation. The OER
can play a central role in meeting future energy demands but the performance of
conventional electrocatalysts in this reaction is limited by the sluggish OER
kinetics. We demonstrate an improvement of the OER performance for one of the
most active OER catalysts, IrO$_2$, by harvesting plasmonic effects from
visible light illumination in multimetallic nanoparticles. We find that the OER
activity for the Au-IrO$_2$ nanodendrites can be improved under LSPR
excitation, matching best properties reported in the literature. Our
simulations and electrocatalytic data demonstrate that the enhancement in OER
activities can be attributed to an electronic interaction between Au and
IrO$_2$ and to the activation of Ir-O bonds by LSPR excited hot holes, leading
to a change in the reaction mechanism (rate-determinant step) under visible
light illumination.
