Thermal Stability of Phases in TiO2/ZnO Composite Microparticles Obtained by Plasma Synthesis

The thermal stability of the phase composition of TiO2/ZnO composite particles during the post-growth heat treatment has been investigated. TiO2/ZnO composite particles were synthesized by processing a metallic mixture of titanium and zinc microparticles in a low-temperature arc discharge plasma in argon under ambient conditions. The obtained materials were characterized using electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. It was found that the synthesized structures contain up to 40% of titanium dioxide polymorphs—anatase and brookite. During heat treatment at 300°C in air, zinc atoms are displaced from the TiO2 lattice, accompanied by structural rearrangement of the oxide matrix. Annealing at temperatures of 500–700°C leads to coarsening of most of anatase crystallites and partial recrystallization of the rest into the rutile phase. Annealing at 900°C promotes the formation of zinc titanates (Zn2TiO4 and Zn2Ti3O8), which act as catalysts for the anatase-to-rutile phase transition. Upon annealing at 950°C, the rutile monophase is stabilized in the synthesized samples. A simple and environmentally friendly method is thus proposed for the large-scale synthesis of TiO2/ZnO composite particles containing a high fraction (up to 40%) of anatase and brookite, which remain stable up to 700°C and can be effectively applied in photostimulated degradation processes of organic pollutants.