Cobalt/cobalt oxide nanoparticles-assembled coatings with various morphology and composition synthesized by pulsed laser deposition

Nano-particle (NP) catalysts are under intense investigation in the nanotechnology community because of their large surface-to-volume atomic ratio, size- and shape-dependent properties, and high concentration of low-coordinated active surface sites. However, there is no general strategy to synthesize NPs, possibly in one step, of various materials with tailored properties and desired morphologies. In this work, cobalt oxide NPs assembled coatings have been prepared by reactive pulsed laser deposition of Co in O2 atmosphere. Three different deposition parameters have been varied to investigate their relevance in tailoring composition and structure of the synthesized NPs: 1) the target-substrate distance, 2) the oxygen pressure in deposition chamber, and 3) post oxidation of NPs deposited in vacuum condition. It is proved that by appropriate selection of the deposition parameters, several NP types may be obtained in one single step, namely: Co3O4 or CoO NPs, Co-Co3O4 or Co-CoO core shell NPs, with size ranging from a few nm to several tens of nm. Transmission and Scanning Electron Microscopy as well as Raman Spectroscopy have been utilized to carefully investigate the compositional and structural features of the NPs. A correlation is made between final NP properties and laser-induced processes mainly related to phase explosion generating liquid droplets that cool during flight or on the substrate. Photocatalytic properties of this heterogeneous Co3O4 NPs-assembled coating catalyst for degradation of methylene blue solution under light irradiation are assessed and compared to that of homogenous catalyst producing Co2+ ions in methylene blue solution.