Ni-NiO core-shell co-catalyst structures on Ta2O5 have been investigated for solar H-2 production. Core-shell co-catalysts with different morphologies resulted in large changes in photocatalytic activity. Increased H-2 production was found to be related to an increase in the thickness of NiO shell due to suppression of the back reaction. Atomic level transmission electron microscopy showed that the core-shell co-catalyst structures deactivated primarily due to a loss of metallic Ni from the core structure. During deactivation, the catalyst transformed either to structures consisting of NiO nanoblocks or hollow NiO shells. The phase transformations occurring during deactivation were associated with Ni diffusion processes that are driven by light illumination. (C) 2015 Elsevier B.V. All rights reserved.