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A porous coating assembled with hierarchical 3D Co 3 O 4 urchin-like particles was synthesized by Pulsed Laser Deposition (PLD) and thermal oxidation. Laser ablation of Co B powder, used as the target material, in oxygen atmosphere formed core–shell particles on the coating surface with mainly a metallic Co core and a mixture of Co, B and O accommodating the shell. The thermal oxidation of these core–shell particles in air at 600 °C induces the morphological transformation to urchin-like particles consisting of nanowires (NWs) (diameter: 30–60 nm and length 1–3 μm) grown radially from the core surface. The extrusion marks on the surface of NWs indicate that the stress induced growth process is caused by difference in the thermal expansion coefficient. XRD, Raman, EXAFS and HRTEM analysis confirmed that the NWs are polycrystalline consisting of pure Co 3 O 4 phase. A wet-chemistry hydrothermal procedure was also employed to synthesize nanostructured urchin-like particles which are hollow and the structure is held together by the radially oriented nanorods (diameter: 40–150 nm). During photocatalysis, urchin-like particles synthesized by PLD displayed significantly higher (∼5 times) degradation rates when compared to chemical urchins for degradation of methylene blue dye via a photo-Fenton reaction in presence of H 2 O 2 and visible light. This is mainly attributed to poor stability of the nanorods in the chemical urchin structure. Features such as high surface area, enhanced stability against agglomeration, polycrystalline nature of the NWs, porous surface and superior adhesion, are responsible for the enhanced photocatalytic activity of Co 3 O 4 urchin-like particles assembled in a porous coating synthesized by PLD and thermal oxidation. Reusability tests also demonstrate the robust nature of the catalyst coating. [ABSTRACT FROM AUTHOR]

Abstract: Co3O4 nanoparticles assembled coatings have been synthesized by physical (pulsed laser deposition (PLD) and electron beam deposition) and chemical (sol–gel and electroless) techniques. The morphological, structural, and optical properties of all the coatings were analyzed by SEM, XRD, Raman spectroscopy, XPS, and UV–vis spectroscopy. The catalytic activities of the several Co3O4 heterogeneous catalyst coatings have been compared by looking at their efficiency on degradation of methylene blue dye that occurs via photo Fenton reaction in presence of H2O2 or oxone oxidants. The coating synthesized by PLD exhibits the best photo-degradation rate of dye in presence of H2O2 (4h) or oxone (8min). The NPs produced by PLD process exhibit smaller average size (18nm), narrow size distribution (σ =3nm), perfect spherical shape, low degree of agglomeration, and mixed amorphous-nanocrystalline phase. These special features of NPs, along with better visible light absorption, are the main factors responsible for the enhanced photocatalytic activity of the PLD Co3O4 NPs assembled coating. It is also observed that although the concentration of oxone (0.5mM) was very small as compared to H2O2 (100mM) the photo-degradation of MB dye solution is much faster for oxone. [Copyright &y& Elsevier]

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]