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.