Nanoscopic Porous Iridium/Iridium Dioxide Superstructures (15 nm): Synthesis and Thermal Conversion by In Situ Transmission Electron Microscopy

Abstract

Porous particle superstructures of about 15 nm diameter, consisting of ultrasmall nanoparticles of iridium and iridium dioxide, are prepared through the reduction of sodium hexachloridoiridate(+IV) with sodium citrate/sodium borohydride in water. The water‐dispersible porous particles contain about 20 wt % poly(N‐vinylpyrrolidone) (PVP), which was added for colloidal stabilization. High‐resolution transmission electron microscopy confirms the presence of both iridium and iridium dioxide primary particles (1–2 nm) in each porous superstructure. The internal porosity (≈58 vol%) is demonstrated by electron tomography. In situ transmission electron microscopy up to 1000 °C under oxygen, nitrogen, argon/hydrogen (all at 1 bar), and vacuum shows that the porous particles undergo sintering and subsequent compaction upon heating, a process that starts at around 250 °C and is completed at around 800 °C. Finally, well‐crystalline iridium dioxide is obtained under all four environments. The catalytic activity of the as‐prepared porous superstructures in electrochemical water splitting (oxygen evolution reaction; OER) is reduced considerably upon heating owing to sintering of the pores and loss of internal surface area.

Water-Based Synthesis of Ultrasmall Nanoparticles of Platinum Group Metal Oxides (1.8 nm)

“…For IrO 2 -GSH, a binding energy of 61.8 eV for Ir 4f 7/2 was determined in the XPS spectrum. This is in line with the earlier data on porous Ir/IrO 2 superstructures (61.7/64.5 eV) . Ultrasmall platinum nanoparticles contained two species of Pt 4f 7/2 .…”

“…For cysteine-coated ultrasmall gold nanoparticles, we found about 5% of Au­(I) and 95% of Au(0) by XPS and the gold fcc structure by XRD . For PVP-coated iridium/iridium dioxide superstructures we found a ratio of Ir(0) to Ir­(+IV) of 2:1 by XPS . The question why osmium alone was almost fully reduced to the metal in contrast to the other metals cannot yet be answered; it is obviously the effect of the applied reaction conditions that were more or less identical for all metals.…”

“…This is in line with the earlier data on porous Ir/ IrO 2 superstructures (61.7/64.5 eV). 43 Ultrasmall platinum nanoparticles contained two species of Pt 4f 7/2 . The first species with a binding energy of 70.4 eV had a lower value than expected for metallic platinum (71.2 eV).…”

“…14 For PVP-coated iridium/iridium dioxide superstructures we found a ratio of Ir(0) to Ir(+IV) of 2:1 by XPS. 43 The question why osmium alone was almost fully reduced to the metal in contrast to the other metals cannot yet be answered; it is obviously the effect of the applied reaction conditions that were more or less identical for all metals. The fact that we have a rapid synthesis of ultrasmall nanoparticles, which are probably not in thermodynamic equilibrium states, suggests that we see the result of both thermodynamic and kinetic factors which are different for each metal.…”

Recent Progress on Revealing 3D Structure of Electrocatalysts Using Advanced 3D Electron Tomography: A Mini Review

“…Nanostructures’s 3D morphology, in both terms of the particle size and shape, can be better studied by 3D electron tomography. A number of electrocatalysts with different morphology have been investigated, including nanoporous and nanodendrites ( Mourdikoudis et al, 2013 ; Geboes et al, 2016 ; Mourdikoudis et al, 2019 ; Pappert et al, 2019 ; Yu et al, 2019 ), nano platelets ( Kong et al, 2013 ; Toth et al, 2016 ; González-Jiménez et al, 2018 ), nanoframes and nanocages ( Becknell et al, 2017 ; Zhang et al, 2019a ; Gong, 2021a ), nanowires ( Bu et al, 2016 ; Liu et al, 2020 ), and the most widely studied nanoparticles ( Liu et al, 2012 ; Lu et al, 2014 ; Mourdikoudis et al, 2015 ; Londono-Calderon et al, 2017 ; Kang et al, 2018 ; Gong, 2021b ; Frank et al, 2021 ; Leteba et al, 2021 ).…”