Multidentate Surfactant-Dependent Synthesis of Giant Iridium Superstructures

Giant superstructures of iridium, comprised of smaller spherical and slightly larger anisotropic nanoparticles, were synthesized by reducing iridium chloride in the presence of resorcinarene, a class of tetrameric macrocyclic polyphenols, with sodium borohydride in ethanol in 30 min under mild conditions. These superstructures were characterized by a range of techniques including TEM, HRTEM, EDS and other spectroscopic methods. The impact of the macrocyclic surfactant, its features and reaction conditions in dictating the formation of giant iridium superstructures was evaluated. The packing density of nanoparticles in these giant iridium superstructures could be altered qualitatively by varying the duration of the reaction, concentration, or chain length of the resorcinarene surfactant, while their overall dimensions remained in the range of 173-197 nm. Control experiments carried out in its absence and in the presence of resorcinol revealed that resorcinarene surfactant plays the role of a dispersant and further regulates the formation of near spherical aggregates. Tween 20, a nonionic surfactant, also resulted in the formation of slightly larger giant iridium superstructures with somewhat limited dispersion stability. The nature of the macrocyclic surfactant's headgroup played a major role in the formation of these giant superstructures, as replacing resorcinarene with related pyrogallolarene only led to individually dispersed spherical and anisotropic nanoparticles though both these surfactants act as effective stabilizers for iridium nanoparticles. The available evidence suggests that the formation of these giant superstructures proceeds via the growth of sparsely populated spherical aggregates, which are formed as early as 10 min. These giant iridium superstructures showed enhanced specific activity toward oxygen evolution reaction when compared to a commercially available catalyst.