•A simple method is developed for (Ni–ZnO)@C core– and yolk–shell nanoreactors.•Splitting of seawater for H2 can be photocatalyzed by (Ni–ZnO)@C nanoreactors.•Collision frequencies of H2O and photoactive sites can be increased (10 times). Novel photocatalysts i.e., metallic nickel and zinc oxide nanoparticles embedded in the carbon-shell ((Ni–ZnO)@C) have been used for photocatalytic splitting of seawater to generate H2. The (Ni–ZnO)@C core–shell nanoparticles having the Zn/Ni ratios of 0–3 were prepared by carbonization of Ni2+- and Zn2+-β-cyclodextrin at 673K for 2h. To increase the collision frequency of water and photoactive sites within the carbon-shell, Ni and ZnO are partially etched from the (Ni–ZnO)@C core–shell to form yolk–shell nanoparticles with a H2SO4 solution (2N). By X-ray diffraction spectroscopy, mainly Ni and ZnO crystallites are observed in the core– and yolk–shell nanoparticles. The sizes of the Ni and ZnO in the (Ni–ZnO)@C nanoreactors are between 7 and 23nm in diameters determined by TEM and small angel scattering spectroscopy. Under a 5-h UV–Vis light irradiation, 5.01μmol/hgcat of H2 are yielded from photocatalytic splitting of seawater effected by (Ni–ZnO)@C nanoreactors.