(74d) Flame-Made TiO2(B)

TiO₂ is the second largest commodity made by flame reactors, outnumbered in tonnage only by carbon black. Unless dopants are employed to increase the rutile yield, e.g. for application as white pigments, product powders are a mixture of anatase and rutile polymorphs. One prominent titania nanopowder is high surface area (~50 m²/g) titania P25 (Evonik Industries AG) with approximately 88 wt% anatase and 12 wt% rutile [1] that has broad application as a photocatalyst.

However, TiO₂ can also attain other crystal phases than anatase and rutile, namely brookite, TiO₂â…¡, TiO₂(H), and monoclinic TiO₂(B). The latter has the lowest density and most open crystal structure of all titantia polymorphs, while its energy bands differ from and interact with the energy bands of the other crystal structures [2]. Consequently, it may lead to photocatalytic activity similar to or better than P25 [3].

Here, titania mixtures with monoclinic (TiO₂(B)) and anatase crystal phases are synthesized using flame spray pyrolysis (FSP). X-ray diffraction, transmission electron microscopy, and Raman spectroscopy confirmed the presence of TiO₂(B). Raman measurements detected up to 27% of TiO₂(B) in the crystalline fraction. The amount of TiO₂(B) increases as the time for nanoparticle growth in the flame decreases through quenching by mixing with ambient air. It can be controlled by varying the flow rates of precursor solution and dispersion oxygen into the flame. Those FSP-made titania outperformed commercial titania (P25) in the photocuring of epoxy.

1) Ohtani B.; Prieto-Mahaney, O. O.; Li, D.; Abe, R. What is Degussa (Evonik) P25? Crystalline Composition Analysis, Reconstruction from Isolated Pure Particles and Photocatalytic Activity Test. J. Photochem. Photobiol., A 2010, 216, 179-182.

2) Armstrong, A. R.; Armstrong, G.; Canales, J.; Bruce, P. G. TiO₂-B Nanowires. Angew. Chem., Int. Ed. 2004, 43, 2286-2288.

3) Yang, D.; Liu, H.; Zheng, Z.; Yuan, Y.; Zhao, J.; Waclawik, E. R.; Ke, X.; Zhu, H. An Efficient Photocatalyst Structure: TiO₂(B) Nanofibers with A Shell of Anatase Nanocrystals. J. Am. Chem. Soc. 2009, 131, 17885-17893.