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Abstract: Cobalt oxide (Co3O4) nanoparticles (NPs) assembled coating have been prepared by reactive pulsed laser deposition of Co, in O2 atmosphere, on Si or glass substrate ranging from room temperature to 250°C. The NPs, having narrow size distribution with average values of around 25–50nm, were characterized by Raman, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The Co3O4 NPs synthesized at 150°C comprise a mixed amorphous–nanocrystalline phase (that is unique property for catalysis) while complete crystallization of Co3O4 occurs at 250°C with formation of spinel structure. Photocatalytic properties of Co3O4 NPs assembled coating for degradation of methylene blue solution under visible light irradiation are reported. The effect of H2O2 concentration and effect of pH variation on dye degradation rate has been reported. The reusability of the Co3O4 NPs assembled thin coating catalyst was further evaluated in several recycling runs. The activity of heterogeneous Co3O4 NPs assembled thin coating catalyst was compared with homogenous catalyst producing Co+2 ions in methylene blue solution. [Copyright &y& Elsevier]