XPS study of surface chemistry of tungsten carbides nanopowders produced through DC thermal plasma/hydrogen annealing process

•WC nanopowders were produced by a DC thermal plasma/H2 annealing process.•Air-exposed materials show ultra-thin chemically heterogeneous overlayers.•Carbidic, graphitic, carbon-to-oxygen, and WO3 surface bonding are involved.•Adsorbed oxygen at coverage above a monolayer is preferentially bonded to carbon.•Surface segregations of carbon are normal, even for sub-stoichiometric carbides. X-ray photoelectron spectroscopy (XPS) has been employed to characterize the surface composition and bonding of the tungsten carbides nanopowders produced through a DC thermal plasma/hydrogen annealing process. The XPS results were complemented with those from Raman spectroscopy, high-resolution transmission electron microscopy, and evolved gas analysis. The products of the DC plasma synthesis are the high-surface-area multicarbide mixtures composed mainly of crystalline WC1−x and W2C. The materials are contaminated with a pyrolitic carbonaceous deposit which forms ∼1nm thick graphitic overlayers on the nanoparticles’ surface. The underlying carbides are not oxidized in ambient air, and show no interfacial compounds underneath the graphitic overlayers. When annealed in hydrogen, the multicarbide mixtures undergo transformation into the single-phase WC nanopowders with an average particle size of 50–60nm. The surface of the passivated and air-exposed WC nanopowders is stabilized by an ultrathin, no more than 0.5nm in thickness, chemically heterogeneous overlayer, involving graphitic, carbon-to-oxygen, and WO3 bonding. Oxygen presents at coverages above a monolayer preferentially in the bonding configurations with carbon. The surface segregations of carbon are normally observed, even when the bulk content of carbon is below the stoichiometric level.