Highly efficient photocatalytic removal of methylene blue by lamellar structured nanocrystalline and amorphous CuO

•The crystalline/amorphous CuO were synthesized by chemical precipitation.•Lamellar quantum dot structure of CuO will improve photocatalytic performance.•The degradation rate under visible light is nearly the same as that under UV light.•CuO can show excellent photocatalytic activity in actual seawater with sunlight.