The interaction of size-selected Ru₃ clusters with TiO₂: depth-profiling of encapsulated clusters

Ru is a metal of interest in catalysis. Monodisperse Ru₃ clusters as catalytic sites are relevant for the development of catalysts because clusters use significantly lower amounts of precious materials for forming active sites due to the small size of the cluster. However, retaining the mono-dispersity of the cluster size after deposition is a challenge because surface energy could drive both agglomeration and encapsulation of the clusters. In the present work Ru₃ clusters are deposited by chemical vapor deposition (CVD) of Ru₃(CO)₁₂ and cluster source depositions of bare Ru₃ onto radio frequency sputter-deposited TiO₂ (RF-TiO₂) substrates, TiO₂(100), and SiO₂. When supported on RF-TiO₂, bare Ru₃ is encapsulated by a layer of titania substrate material during deposition with a cluster source. Ligated Ru₃(CO)₁₂ is also encapsulated by a layer of titania when deposited onto sputter-treated RF-TiO₂, but only through heat treatment which is required to remove most of the ligands. The titania overlayer thickness was determined to be 1-2 monolayers for Ru₃(CO)₁₂ clusters on RF-TiO₂, which is thin enough for catalytic or photocatalytic reactions to potentially occur even without clusters being part of the very outermost layer. The implication for catalysis of the encapsulation of Ru₃ into the RF-TiO₂ is discussed. Temperature-dependent X-ray photoelectron spectroscopy (XPS), angle-resolved XPS, and temperature-dependent low energy ion scattering (TD-LEIS) are used to probe how the cluster-surface interaction changes due to heat treatment and scanning transmission electron microscopy (STEM) was used to image the depth of the surface from side-on.