Composite TiSiOC nanoparticles with Ti/Si ratio varying in a very large range were prepared by laser pyrolysis of a gas–spray mixture of silane and titanium tetra-isopropoxide. The as-formed nanoparticle batches exhibit intense blue colours, varying from dark to light blue while the Ti/Si ratio increases. This blue colour is attributed to the formation of sub-stoichiometric TiOcompounds induced by the presence of reducing agents such as silicon-based radicals and carbon atoms in the reaction medium. The blue colour of the powders is stable for several months at room temperature in normal atmospheric conditions. Elemental analysis, specific surface area and pycnometry measurements, as well as Photon Correlation Spectroscopy allow determining the chemical composition and size of the as-synthesized nanoparticles as a function of the Ti/Si ratio. X-ray diffraction, transmission electron microscopy and IR spectroscopy have been used to analyse their chemical organisation, nanostructure and morphology. Mean grain size is found around 20 nm. The nanoparticles exhibit a core-shell structure TiO/SiO2, with a core made of titania, surrounded by an amorphous shell, mainly of silica. Crystallites of anatase are present in the core with size increasing with the Ti/Si ratio. Annealing under air at 800°C induces the removal of carbon and the crystallisation of the powders with light beige to white colours.
