Structural Evolution during Photocorrosion of Ni/NiO Core/Shell Cocatalyst on TiO₂

The Ni/NiO core/shell structure is one of the most efficient cocatalysts for solar water splitting when coupled with suitable semiconducting oxides. It has been shown that pretreated Ni/NiO core/shell structures are more active than pure Ni metal, pure NiO, or mixed dispersion of Ni metal and NiO nanoparticles. However, Ni/NiO core/shell structures on TiO 2 are only able to generate H 2 but not O 2 in aqueous water. The nature of the hydrogen evolution reaction in these systems was investigated by correlating photochemical H 2 production with atomic-resolution structure determined with aberration-corrected electron microscopy. It was found that the core/shell structure plays an important role for H 2 generation but the system undergoes deactivation due to a loss of metallic Ni. During the H 2 evolution reaction, the metal core initially formed partial voids which grew, and eventually all the Ni diffused out of the core/shell into solution leaving an inactive hollow void/NiO shell. The H 2 evolution was generated by a photochemical reaction involving photocorrosion of Ni metal.