Synthesis and photocatalytic activity of monodisperse single crystalline NiO octahedrons by the selective adsorption of Cl− ions

► Uniform NiO octahedrons are synthesized by heating NiCl2 solution for the first time. ► The size of NiO octahedrons can be changed by adjusting the heating temperature. ► The formation of NiO octahedrons results from the selective adsorption of Cl− ions. ► The NiO octahedrons show a good photocatalytic ability to degrade acid scarlet. ► The photocatalytic activity was enhanced with decreasing the size of NiO octahedrons.