Abstract
We develop herein plasmonic-catalytic Au-IrO2 nanostructures with a morphology optimized for efficient light harvesting and catalytic surface area; the nanoparticles have a nanoflower morphology, with closely spaced Au branches all partially covered by an ultrathin (1 nm) IrO2 shell. This nanoparticle architecture optimizes optical features due to the interactions of closely spaced plasmonic branches forming electromagnetic hot spots, and the ultra-thin IrO2 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, IrO2, by harvesting plasmonic effects from visible light illumination in multimetallic nanoparticles. We find that the OER activity for the Au-IrO2 nanoflowers 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 IrO2 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.
Highlights
Plasmonic–catalytic nanostructures have been synthesized in which the plasmonic and catalytic components are in direct contact and where the two components are separated by a small distance (∼up to 5 nm).[12,50,51,58,59]
In nanoparticle designs in which plasmonic and catalytic components are not in direct contact, it has been demonstrated that the catalytic component may be exposed to regions of local electric fields induced by the localized surface plasmon resonant (LSPR) excitation of the plasmonic metal, enhancing catalytic activity.[10,12,56,62]
In contrast with nanostructures where the catalytic and plasmonic components are in direct contact, electronic effects and charge flow from the plasmonic to the catalytic component occurs upon LSPR excitation.[11,50,57]
Summary
Plasmonic catalysis relies on harvesting the energy generated by localized surface plasmon resonant (LSPR) excitations in plasmonic nanoparticles to drive, accelerate, and/or control molecular transformations.[1,2,3,4,5,6] Following LSPR excitation in plasmonic nanoparticles, non-radiative plasmon decay can LSPR-excited hot electrons and holes can electronically or vibrationally excite molecular adsorbates at the metal–molecule interface via direct or indirect mechanisms.[9,10] This can lead to improved reaction rates relative to the reaction in the absence of LSPR excitation, and provide new reaction pathways for the control over reaction selectivity relative to traditional thermochemically-driven processes.[11,12,13,14,15] Gold (Au) and silver (Ag) nanoparticles are amongst the strongest plasmonic structures, supporting LSPR excitation in the visible and near-infrared ranges with wavelengths that are tunable via the control of shape, size, composition and structure.[16,17] plasmonic catalysis has emerged as an attractive approach for solar to chemical energy conversion,[1,3,18,19,20,21,22] withAu, Ag, and aluminum (Al) nanoparticles having been applied as plasmonic catalysts towards a variety of molecular transformations under visible-light excitation.[20,23,24,25,26,27]Among several important chemical transformations, the water splitting reaction to produce hydrogen (H2) and oxygen (O2) has attracted massive attention for energy conversion and storage applications.[28,29,30,31] this reaction is. The SEM images show that the nanoflowers gradually increase in size as a function of time, which agrees with a mechanism based on agregation.[75] It is important to note that the nanoflower morphology was not observed when the synthesis was performed in the presence of only AuCl4− or IrCl3·xH2O precursors (Fig. S10†).
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