Controls over the atomic dispersity and particle shape of noble metal catalysts are the major qualities determining their usability in industrial runs, but they are usually difficult to be simultaneously realized. Inspired from the Deacon catalyst in which RuO₂ can form epitaxial layers on the surfaces of Rutile TiO₂, here we have investigated the shape evolution process of RuO₂ nanoparticles on the surface of P25 TiO₂. It is found that size effects exist in this process and RuO₂ nanoparticles with sizes ~sub-2 nm can be transformed into epitaxial layers while nanoparticles with bigger sizes are not apt to change their shapes. Based on a thermodynamic model, we infer such transformation process is jointly driven by the surface tension and interfacial lattice match between the nanoparticles and substrates, which may be suggestive for the design of noble metal catalysts integrating both active crystal planes and high atomic exposure ratios.

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