Transients in the
composition of Ni@NiO<sub><i>x</i></sub> core–shell
co-catalysts deposited on SrTiO<sub>3</sub> are
discussed on the basis of state-of-the-art continuous analysis of
photocatalytic water splitting, and post-XPS and TEM analyses. The
formation of excessive hydrogen (H<sub>2</sub>:O<sub>2</sub> ≫
2) in the initial stages of illumination demonstrates oxidation of
Ni(OH)<sub>2</sub> to NiOOH (nickel oxyhydroxide), with the latter
catalyzing water oxidation. A disproportionation reaction of Ni and
NiOOH, yielding Ni(OH)<sub>2</sub> with residual embedded Ni, occurs
when illumination is discontinued, which explains repetitive transients
in (excess) hydrogen and oxygen formation when illumination is reinitiated
