Laser-assisted synthesis of carbon coatings with cobalt oxide nanoparticles embedded in gradient of composition and sizes

Abstract We report on a new laser-assisted method for the synthesis of composite coatings consisting of cobalt oxide-embedded porous carbon with gradient of composition and sizes. Uniform carbon thin films containing cobalt oxide nanoparticles (CoO NPs) of various concentrations and sizes are obtained by Combinatorial Matrix-Assisted Pulsed Laser Evaporation (C-MAPLE) followed by a post-annealing treatment. Specifically, simultaneous pulsed laser irradiation of two concentric cryogenic targets was applied for gradient thin films assembling on solid substrates. Both targets contained environmentally friendly phloroglucinol/glyoxylic acid organic precursors and a template dissolved in an ethanol-chloroform mixture, whereas one of them enclosed, in addition, cobalt nitrate salt. Due to angular dispersion and crossing of the two vapor plumes during co-deposition process, thin composite polymeric nanocoatings containing Co nitrate salt were obtained. After subsequent thermal annealing treatment of samples at 600 °C, mesoporous carbon films exhibiting CoO NPs gradient distribution were then directly obtained onto the substrates. Carbon matrix formation was supported by EDX analyses and characteristic D, G and 2D bands present in the Raman spectra. Co NPs concentration varied from 3 to 32 wt% on the substrates with a minimum at furthest interaction point of the Co salt containing plume and substrate, while carbon concentration significantly decreased from 91 to 63 wt%. Oxygen amount was relatively constant at about 6 wt% for all coatings. The CoO NPs are homogenously dispersed in the carbon matrix whereas a tendency for aggregation is observed for high loadings. In addition, the CoO NPs size gradient distribution ranged from 3 to 8 nm with the minimum size correlating the minimum concentration.