Abstract: A method of producing particles containing metal oxide is disclosed that includes heating metal-containing particles to oxidize metal in at least an outer shell of the particles; cooling oxidized particles; collecting the particles; and providing a distance of at least 300 mm between entry of the particles into the flame and collection of the particles. Particles may be oxidized to provide a metal oxide shell over an unoxidized metal core. A semiconductive layer of particles on a substrate may be formed by feeding, to a hot zone, such preoxidized particles; heating the particles to render the particles at least partially molten; and depositing the particles onto the substrate. The oxidation process may provide metal oxide particles in which different metals having different valencies are present in different proportions. The valencies and proportions may be selected to provide n- or p-type semiconductor layers.