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Polymorphism of nanocrystalline TiO₂ prepared in a stagnation flame: formation of the TiO₂-II phase.

ABSTRACT: A metastable "high-pressure" phase known as ?-PbO₂-type TiO₂ or TiO₂-II is prepared via a single-step synthesis using a laminar premixed stagnation flame. Three other TiO₂ polymorphs, namely anatase, rutile and TiO₂-B phases, can also be obtained by tuning the oxygen/fuel ratio. TiO₂-II is observed as a mixture with rutile under oxygen-lean flame conditions. To the best of our knowledge, this is the first time that this phase has been identified in flame-synthesised TiO₂. The formation of TiO₂-II in an atmospheric pressure flame cannot be explained thermodynamically and is hypothesised to be kinetically driven through the oxidation and solid-state transformation of a sub-oxide TiO_2-x intermediate. In this scenario, rutile is nucleated from the metastable TiO₂-II phase instead of directly from a molten/amorphous state. Mixtures containing three-phase heterojunctions of anatase, rutile, and TiO₂-II nanoparticles as prepared here in slightly oxygen-lean flames might be important in photocatalysis due to enhanced electron-hole separation.

SUBMITTER: Manuputty MY

PROVIDER: S-EPMC6354738 | biostudies-literature | 2019 Feb

REPOSITORIES: biostudies-literature

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Polymorphism of nanocrystalline TiO2 prepared in a stagnation flame: formation of the TiO2-II phase.

Manuputty Manoel Y MY Dreyer Jochen A H JAH Sheng Yuan Y Bringley Eric J EJ Botero Maria L ML Akroyd Jethro J Kraft Markus M

Chemical science 20181114 5

A metastable "high-pressure" phase known as α-PbO2-type TiO2 or TiO2-II is prepared via a single-step synthesis using a laminar premixed stagnation flame. Three other TiO2 polymorphs, namely anatase, rutile and TiO2-B phases, can also be obtained by tuning the oxygen/fuel ratio. TiO2-II is observed as a mixture with rutile under oxygen-lean flame conditions. To the best of our knowledge, this is the first time that this pha ...[more]

PMID: 30809349

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Project description:Titanium dioxide (TiO2) in the form of thin films has attracted enormous attention for photocatalysis. It combines the fundamental properties of TiO2 as a large bandgap semiconductor with the advantage of thin films, making it competitive with TiO2 powders for recycling and maintenance in photocatalytic applications. There are many aspects affecting the photocatalytic performance of thin film structures, such as the nanocrystalline size, surface morphology, and phase composition. However, the quantification of each influencing aspect needs to be better studied and correlated. Here, we prepared a series of TiO2 thin films using a sol-gel process and spin-coated on p-type, (100)-oriented silicon substrates with a native oxide layer. The as-deposited TiO2 thin films were then annealed at different temperatures from 400 °C to 800 °C for 3 h in an ambient atmosphere. This sample synthesis provided systemic parameter variation regarding the aspects mentioned above. To characterize thin films, several techniques were used. Spectroscopic ellipsometry (SE) was employed for the investigation of the film thickness and the optical properties. The results revealed that an increasing annealing temperature reduced the film thickness with an increase in the refractive index. Atomic force microscopy (AFM) was utilized to examine the surface morphology, revealing an increased surface roughness and grain sizes. X-ray diffractometry (XRD) and UV-Raman spectroscopy were used to study the phase composition and crystallite size. The annealing process initially led to the formation of pure anatase, followed by a transformation from anatase to rutile as the annealing temperature increased. An overall enhancement in crystallinity was also observed. The photocatalytic properties of the thin films were tested using the photocatalytic decomposition of acetone gas in a home-built solid (photocatalyst)-gas (reactant) reactor. The composition of the gas mixture in the reaction chamber was monitored using in situ Fourier transform infrared spectroscopy. Finally, all of the structural and spectroscopic characteristics of the TiO2 thin films were quantified and correlated with their photocatalytic properties using a correlation matrix. This provided a good overview of which film properties affect the photocatalytic efficiency the most.

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Polymorphism of nanocrystalline TiO₂ prepared in a stagnation flame: formation of the TiO₂-II phase.

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Polymorphism of nanocrystalline TiO<sub>2</sub> prepared in a stagnation flame: formation of the TiO<sub>2</sub>-II phase.

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