Hydrothermal synthesis and electronic properties of FeWO4 and CoWO4 nanostructures

Iron tungstate (FeWO4) and cobalt tungstate (CoWO4) nanostructures were prepared by the hydrothermal method using sodium tungstate (Na2WO4·2H2O), ferrous ammonium sulfate [(NH4)2Fe(SO4)2·6H2O] and cobalt chloride (CoCl2·6H2O) solutions as precursors. The crystal structure and morphology of the as-prepared tungstates were characterized by X-ray diffraction analysis and transmission electron microscopy. The above characterizations render that the products obtained belong to the monoclinic crystal system and P2/a space group, with average sizes of nanoparticles of about 150 nm and 70 nm in the case of FeWO4 and CoWO4, respectively. Electronic properties of the FeWO4 and CoWO4 tungstates were studied using several X-ray spectroscopy methods, mainly X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS). The XPS valence-band and core-level spectra, the XAS W LIII edges (unoccupied W d-like states) as well as the XES bands reflecting the energy distribution of the valence W d- and O p-like states were recorded. The present XPS and XAS results reveal that the as-prepared FeWO4 and CoWO4 tungstates are in a composition close to a stoichiometric one. The XES results render that the W 5d- and O 2p-like states contribute throughout the whole valence-band region of the FeWO4 and CoWO4 tungstates, however maximum contributions of the O 2p-like states occur in the upper, whilst the W 5d-like states in the lower portions of the valence band, respectively.