Crucible Effect on Phase Transition Temperature during Microwave Calcination of a N-Doped TiO₂ Precursor: Implications for the Preparation of TiO₂ Nanophotocatalysts

Nitrogen-doped titanium dioxide (N–TiO2) is extensively studied in the literature and widely employed as a visible-light photocatalyst. The preparation of this nanomaterial often involves a final calcination step: in this paper, we used microwave technology as an efficient and environmentally friendly tool for the calcination of a mixed anatase/brookite phase precursor containing organic compounds arising from its synthesis. A wide study was performed, comparing results obtained in quartz fiber-made crucibles, porcelain crucibles, microwave (MW) and conventional heating, and also using a pristine TiO2 as a reference material. Weight percentages of the three main TiO2 polymorphs obtained in different calcination conditions were determined by X-ray powder diffraction (XRPD) data and Rietveld refinements, and the results are discussed. We found that the calcination’s outcome is dramatically affected by the crucible used: while mixed-phase nanomaterials were obtained when using classic porcelain crucibles, only the rutile phase was obtained when working in a quartz fiber crucible at temperatures as low as 250 °C. The direct material’s temperature measurement inside the microwave oven showed that this finding is not due to a microwave effect but to a thermal effect caused by the particular crucible employed that favors better combustion of organics and prompts rutile formation. Nitrogen doping of this material was inferred by ultraviolet–visible–diffuse reflectance spectroscopy (UV–vis–DRS).