Utilization of core-shell nanoparticles to evaluate subsurface contribution to water oxidation catalysis of [CoII(H2O)2]1.5[CoIII(CN)6] nanoparticles

Nanoparticles of a cyano-bridged polynuclear metal complex, [Co-II(H2O)(2)](1.5)[Co-III(CN)(6)], have been reported as highly efficient heterogeneous catalysts for photocatalytic water oxidation, however, reasons for the high catalytic activity have yet to be clarified. Herein, the water oxidation catalysis was investigated for a series of core-shell nanoparticles composed of cyano-bridged polynuclear metal complexes employing [Co-I(I)(H2O)(2)](1.5)[Co-III(CN)(6)] as the shell to assess subsurface contribution. The catalytic activity of the core-shell nanoparticles closely related to that of the core parts suggests that the subsurface contributes to the nanoparticle catalysis. Then, catalysis of the core-shell nanoparticles employing [Co-II(H2O)(2)](1.5)[Co-III(CN)(6)] as the catalytically inactive core but isostructural to [Co-II(H2O)(2)](1.5)[Co-III(CN)(6)] was examined to determine the thickness directly involved in the catalytic reaction. The catalytic activity depending on the thickness of the [Co-II(H2O)(2)](1.5[)Co-III(CN)(6)] shell suggests the involvement of Co-II ions at the subsurface up to 7 nm. These results suggest that the microporous structure of [Co-II(H2O)(2)](1.5)[Co-III(CN)(6)] enlarged by intrinsic defects is a reason for the high catalytic activity, where the photosensitizer and water molecules utilize the subsurface Co-II ions as active sites.