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 TiO2 are only able to generate H2 but not O2 in aqueous water. The nature of the hydrogen evolution reaction in these systems was investigated by correlating photochemical H2 production with atomic-resolution structure determined with aberration-corrected electron microscopy. It was found that the core/shell structure plays an important role for H2 generation but the system undergoes deactivation due to a loss of metallic Ni. During the H2 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 H2 evolution was generated by a photochemical reaction involving photocorrosion of Ni metal.