Understanding the growth of RuO2 colloidal nanoparticles over a solid support: An atomic force microscopy study

• Single deposition of RuO2 colloids on mica leads to amorphous supported clusters.
• Small sharp-pointed clusters and nanorod-like assemblies are formed after calcination.
• Disaggregation of particles occurs during the reduction treatment under hydrogen.
• Double coating calcined RuO2-mica leads to larger nanorods with stepped surfaces.
• This geometry would expose highly active sites for ammonia synthesis reaction.

Peak Force Tapping-mode AFM is used to investigate the three-dimensional morphology of RuO2 nanoparticles supported on a solid surface, with sub-nanometer spatial resolution. Samples were prepared by spin coating (i) a drop of RuO2 colloidal suspension onto a mica sheet, followed by further treatments (drying, calcination and/or reduction), and (ii) a second drop of colloidal suspension on fresh, dried or calcined RuO2-mica. RuO2 crystallites are supported with a certain distribution of sizes, and this heterogeneity is essentially maintained after high-temperature treatments. After calcination, single coated RuO2-mica samples present small RuO2 crystallites (4.5 nm height) as well as nanorod-like assemblies (14 nm height). A disaggregation of particles occurs during the reduction treatment. Double coating RuO2 over non-calcined RuO2-mica samples involves mechanical effects causing re-dispersion of supported clusters. An intermediate drying step is unable to prevent re-dispersion of particles and even leads to repulsion between dehydrate RuO2 supported particles and hydrated particles in suspension. Meanwhile, a calcination step prevents re-dispersion and promotes growth of supported crystallites. Nanorods formed by single RuO2 deposition exhibit mostly regular flat surfaces, while a second RuO2 deposition on calcined RuO2-mica samples leads to larger nanorods with stepped surfaces (terraces), a favourable geometry for ammonia synthesis reaction.

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