Density of states of Ru₃ and Pt₃ clusters supported on sputter-deposited TiO₂

In this work, 3-atom clusters, Ru₃ and Pt₃, were deposited onto radio frequency RF-sputter deposited TiO₂, treated with Ar⁺ ion sputtering. Ru₃ was deposited by both solution submersion and chemical vapor deposition of Ru₃(CO)₁₂, while Pt₃ was deposited under ultra-high vacuum using a laser vaporisation cluster source. The valence electronic density of states (DOS) of the deposited clusters were analysed after heat treatment using ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES), where UPS measures the top several layers while MIES measures only the top atomic layer. XPS was used to determine the cluster surface coverages. The DOS were found to be very similar between Ru₃ deposited by solution submersion and chemical vapor deposition. MIES results for Ru₃ had contributions from titania O 2p sites due to encapsulation by a reduced titania overlayer. For Pt₃ clusters the UPS and MIES results provided evidence that Pt was present on the topmost layer, and encapsulation did not occur. The proposed reason for the encapsulation of Ru₃ but not of Pt₃ is the higher surface energy of Ru over Pt. It is concluded that Pt clusters deposited onto TiO₂ can modify the outermost layer by adding discrete energy levels on the surface, whereas the Ru clusters being encapsulated just below the surface generate a broad distribution of energy states close to the Fermi level. The outcome of this work is that Pt₃-cluster-modified surfaces could be used as catalysts for reactions where the Pt₃ energy levels are suitable for the respective reaction. The implication of the DOS found for photocatalytic water splitting are discussed.