Enhanced electrochemical evolution of oxygen by using nanoflowers made from a gold and iridium oxide composite

Abstract

We report on the synthesis of a composite made from iridium oxide and gold that has a flower-like morphology. The ratio of iridium oxide to gold can be controlled by altering the concentrations of the metal precursors or the pH of the solution containing the reductant citrate. Scanning electron microscopy, transmission electron microscopy, Xray diffraction, X-ray photoelectron spectroscopy, and laser confocal micro-Raman spectroscopy were applied to characterize the structures of the nanoflowers, and a mechanism of their formation was deduced. The nanoflowers display an electrocatalytic activity in an oxygen evolution reaction (OER) that is significantly enhanced compared to bare iridium oxide nanoparticles. The highest turnover frequency for the OER of the new nanoflowers is 10.9 s −1 , which is almost one order of magnitude better than that of the respective nanoparticles. These attractive features are attributed to the high oxidation states of iridium in the nanoflowers which is caused by the transfer of electronic charge from metal oxides to gold, and also to the flower fractal structure which is thought to provide a much more accessible surface than suspensions of the respective nanoparticle.

Design-controlled synthesis of IrO2 sub-monolayers on Au nanoflowers: marrying plasmonic and electrocatalytic properties

“…This change in color indicates the reduction of AuCl 4 − species to AuCl 2 − by IrCl 6 3− (leading to the formation of Ir 4+ species). 55 In this case, we postulate that, as AuCl 2 − has a higher standard reduction potential relative to AuCl 4 − , this species is mainly responsible for the formation of the Au nuclei during the synthesis. Then, as further Au and Ir are produced from the reduction of precursors (co-reduction), growth takes place by precursor addition at the surface of the pre-formed nuclei/seeds as well as by their aggregation in the presence of citrate, which produces the so-called nanoflowers morphology observed herein.…”

“…The synthesis of the multimetallic Au-IrO 2 nanoflowers was performed by the co-reduction of Au and Ir precursors (AuCl 4 − and IrCl 3 •xH 2 O, respectively) in the presence of sodium citrate as both reducing agent and stabilizer. 55 Fig. 1A-E shows scanning electron microscope (SEM) images ( Fig.…”

“…40 Specifically, the detected potentials required to achieve a current density of 10 mA cm −2 , which represent an important metric in the solar synthesis of fuels, decreased from 0. 72 32,40,55,[76][77][78][79][80][81] In order to provide more insights into the effect of LSPR excitation on the kinetics of the OER, an analysis of the Tafel slope was performed. As seen from Fig.…”

“…More specifically, the synthesized Au-IrO 2 plasmonic-catalytic nanoparticles displayed a tortuous flower-like morphology, in which several branches are closely spaced to each other and each branch is composed of Au partially covered by an ultrathin (1 nm) Ir-based shell. These features are very attractive to address the OER for a variety of reasons: (i) IrO 2 represents one of the most active species towards the OER; 53,54 (ii) the ultrathin and incomplete IrO 2 shell at the surface of each branch maximizes the light harvesting by Au and the subsequent flow of charge carriers from Au to the Ir-based shell; 55 (iii) the ultrathin and incomplete Ir-based shell maximises the IrO 2 surface area and therefore minimizes the loading and utilization of this expensive metal; and (iv) the high curvature and plasmonic hybridization between the closely spaced plasmonic branches allows for the generation of a high density of electromagnetic hot spots, i.e., areas of high electric field enhancements at the junctions of the branches as a result of the LSPR excitation that can be felt by the catalytic Ir-based shells. 11,50 These high electric field enhancements, for instance, can decay via absorption and lead to the formation of energetic charge carriers (hot electrons and holes) at the IrO 2 , as described in antenna-reactor complex nanoparticle designs.…”

Synthesis, characterization and electrocatalytic properties of delafossite CuGaO2

“…31 The current density of IrO 2 nanoflowers was reported to be of 0.5 mA/cm 2 in 0.1 M buffer solution using glassy carbon electrode versus Ag/AgCl. 32 The current densities of the nanocrystalline CuGaO 2 hexagons, sub-micron sized plates and micron-sized particles were also calculated using the amount of materials pasted on electrode, observed current and BET surface area of the materials. The current densities for OER were found to be 0.099, 0.062 and 0.015 mA/cm 2 of nanocrystalline hexagons, sub-micron sized plates and micron-sized particles of CuGaO 2 , respectively, at 0.60 V. The electro-catalytic performance of nanocrystalline CuGaO 2 hexagons for OER was found much higher (1.5 and 6 times) than the performances observed with sub-micron sized CuGaO 2 plates and micron-sized CuGaO 2 particles.…”