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

Abstract

Nitrogen-doped titanium dioxide (N−TiO 2 ) 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 TiO 2 as a reference material. Weight percentages of the three main TiO 2 polymorphs obtained in different calcination conditions were determined by Xray 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).

Phase Transformation Strategies for the Construction of Heterojunction TiO2 Photocatalysts

“…Nowadays, there are numerous approaches that can be used to control the phase composition of TiO 2 , including typical heat treatment, [66][67][68] heteroatom doping, [69][70][71] MOF induction, [72][73][74] etc. In addition, the phase transformation of TiO 2 is also affected by the reaction conditions such as pH value, reactant ratio and solvent type.…”