Chemical Insights into the Formation of Colloidal Iridium Nanoparticles from In Situ X-ray Total Scattering: Influence of Precursors and Cations on the Reaction Pathway

Abstract

Iridium nanoparticles are important catalysts for several chemical and energy conversion reactions. Studies of iridium nanoparticles have also been a key for the development of kinetic models of nanomaterial formation. However, compared to other metals such as gold or platinum, knowledge on the nature of prenucleation species and structural insights into the resultant nanoparticles are missing, especially for nanoparticles obtained from Ir x Cl y precursors investigated here. We use in situ X-ray total scattering (TS) experiments with pair distribution function (PDF) analysis to study a simple, surfactant-free synthesis of colloidal iridium nanoparticles. The reaction is performed in methanol at 50 °C with only a base and an iridium salt as precursor. From different precursor salts�IrCl 3 , IrCl 4 , H 2 IrCl 6 , or Na 2 IrCl 6 �colloidal nanoparticles as small as Ir ∼55 are obtained as the final product. The nanoparticles do not show the bulk iridium face-centered cubic (fcc) structure but show decahedral and icosahedral structures. The formation route is highly dependent on the precursor salt used. Using IrCl 3 or IrCl 4 , metallic iridium nanoparticles form rapidly from Ir x Cl y n− complexes, whereas using H 2 IrCl 6 or Na 2 IrCl 6 , the iridium nanoparticle formation follows a sudden growth after an induction period and the brief appearance of a crystalline phase. With H 2 IrCl 6 , the formation of different Ir n (n = 55, 55, 85, and 116) nanoparticles depends on the nature of the cation in the base (LiOH, NaOH, KOH, or CsOH, respectively) and larger particles are obtained with larger cations. As the particles grow, the nanoparticle structure changes from partly icosahedral to decahedral. The results show that the synthesis of iridium nanoparticles from Ir x Cl y is a valuable iridium nanoparticle model system, which can provide new compositional and structural insights into iridium nanoparticle formation and growth.

Surfactant-Free Colloidal Syntheses of Gold-Based Nanomaterials in Alkaline Water and Mono-alcohol Mixtures

“…Previous results obtained with glycerol as the reducing agent suggest that viscosity might be a reason for the stabilization of surfactant-free Au NPs . The achievement here of stable colloidal dispersions without a surfactant in low-viscosity solvents shows that viscosity is not the only and not necessarily the main stabilization mechanism, in agreement with recent results for Pt and Ir NPs obtained in mono-alcohols. , …”

“…15 The achievement here of stable colloidal dispersions without a surfactant in low-viscosity solvents shows that viscosity is not the only and not necessarily the main stabilization mechanism, in agreement with recent results for Pt and Ir NPs obtained in mono-alcohols. 29,76 Identifying in detail the stabilization mechanism(s) of surfactant-free NPs certainly remains a challenge. 23,77 In ζ potential measurements, the NPs prepared using LiOH show a value of −14.5 or −78 mV when the analysis is performed by inputting water or EtOH, respectively, as the solvent in the data treatment (analysis software, Kalliope, Anton Paar).…”

“…As described below, in situ X-ray total scattering (XRTS) was performed to follow the synthesis. For such experiments, a relatively high Au concentration, e.g., 50 mM HAuCl 4 , is required to obtain a suitable signal. , Unfortunately, upon addition of HAuCl 4 last as it is preferred at a lower HAuCl 4 concentration, the reaction proceeds too fast for any measurement to be performed. For these XRTS experiments, 150 mM base and 50 mM HAuCl 4 were used, and ROH was added last, unlike the general synthesis experiments, in which HAuCl 4 was added last.…”

“…Unfortunately, the high viscosity of polyols prevents a simple use of the as-prepared NPs. 27 It has recently been show that mono-alcohols could lead to stable surfactant-free colloidal NPs under alkaline conditions, such as Pt, 28 Ir, 28,29 Ru, 28 or Os 30,31 NPs even at high precursor concentrations around 50 mM and at temperatures close to the boiling point of methanol (∼65 °C) or ethanol (∼78 °C). However this approach does not easily lead to stable colloidal Pd NPs that are best prepared at RT in the presence of a support.…”

Surfactant-free colloidal syntheses of gold-based nanomaterials in alkaline water and mono-alcohol mixtures

“…15 The achievement here of stable colloidal dispersions without surfactant in low viscosity solvents shows that viscosity is not the only and not necessarily the main stabilization mechanism, in agreement with recent results for Pt and Ir NPs obtained in monoalcohols. 74,75 It certainly remains a challenge to identify in details the stabilization mechanism(s) of surfactant-free NPs. 23,76 In Zeta potential measurements, the NPs prepared using LiOH show values of -14.5 mV or -78 mV when the analysis is performed by inputting water or EtOH respectively as solvents in the data treatment (analysis software: Kalliope, Anton Paar).…”