Chemical insights on the formation of colloidal iridium nanoparticles from in situ X-ray total scattering: Influence of precursors and cati-ons on the reaction pathway

Iridium nanoparticles are important catalysts for several chemical and energy conversion reactions. Studies of Ir na-noparticles have also been key for the development of kinetic models of nanomaterial formation. However, compared to other metals such as gold or platinum, there is very limited knowledge on the actual formation mechanism of iridi-um nanoparticles on the atomic and molecular level. Here, we use in situ X-ray total scattering experiments with pair distribution function analysis to study a simple, surfactant-free synthesis of colloidal iridium nanoparticles. The reac-tion is performed in methanol at 50 °C with only an iridium salt and a base as precursor. From different precursor salts - IrCl3, IrCl4, H2IrCl6, or Na2IrCl6 –colloidal nanoparticles as small as Ir55 are obtained. The nanoparticles do not show the bulk Ir face centered cubic (fcc) structure, but decahedral and icosahedral structures. Surprisingly, the for-mation mechanism is highly dependent on the precursor salt used. Using IrCl3 or IrCl4, metallic iridium nanoparticles form rapidly from IrxCly complexes, whereas using H2IrCl6 or Na2IrCl6, the iridium nanoparticle formation follows a sudden growth after an induction period and the brief appearance of a crystalline intermediate. With H2IrCl6, the formation mechanism depends on the nature of the cation in the base - LiOH, NaOH, KOH, or CsOH - and larger parti-cles are obtained with larger cations. As the particles grow, the nanoparticle structure changes from partly icosahedral to decahedral. The presented results introduce a new iridium nanoparticle synthesis model system and provide new chemical insights into nanoparticle formation and growth.