Study of mechanism of electroless copper coating of fly-ash cenosphere particles

Electroless Cu coating process involving Sn–Pd catalyst system is successfully utilized to coat Cu on the surface of fly-ash cenosphere particles to impart electrical conductivity to these non-conducting oxide ceramic particles. The low density Cu-coated cenosphere particles may be utilized for manufacturing conducting polymers for EMI shielding applications. This is the first report in the open literature to investigate the electroless Cu coating of fly-ash cenosphere particles in detail. Extensive characterization of coated particles is carried out by scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), focused ion beam spectroscopy (FIB), and X-ray diffraction (XRD) techniques to study the coating process and to show successful deposition of pure Cu. The mechanism of electroless Cu coating is mainly studied with the help of XPS, which shows the reduction of PdCl2 (activator) catalyst on the surface of cenosphere particles by SnCl2 (sensitizer) to produce pure Pd0 clusters, which subsequently act as nucleation sites for Cu deposition. The concept of XPS core-level binding energy (BE) shift due to small cluster size is utilized to predict the size of pure Pd0 clusters deposited on the fly-ash particle surface after the activation step. For the first time, the use of FIB technique is described and demonstrated to determine directly the Cu coating thickness.