Nanocrystalline TiO 2 powders synthesized by in-flight oxidation of TiN in thermal plasma: Mechanisms of phase selection
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Titanium dioxide nanopowders were synthesized by in-flight oxidation of titanium nitride (TiN) in radio-frequency (rf) induction thermal plasma. The powders were characterized by x-ray diffraction, high-resolution transmission electron microscopy, field emission scanning electron microscopy, Raman spectroscopy, and optical microscopy to reveal the mechanisms of phase selection and particle morphology evolution. The reaction began with surface oxidation of TiN particles, leading to the formation of core-shell composites with oxidized shells and TiN cores, followed by gas-phase condensation of TiO2 nanoparticles. Phase selection of the resultant TiO2 powders was found to largely depend on the oxidation potential of the thermal plasma rather than on the heat transfer itself. Anatase content of the products increased steadily with increasing the O2 input, and TiO2 nanoparticles (∼50 nm) containing ∼90% of anatase were obtained through O2/Ar plasma treatment. Phase-pure rutile nanoparticles (∼50 nm, on average) were also synthesized in H2/Ar plasma injected with O2 as the powder carrier gas.
I. INTRODUCTION
Titanium dioxide (TiO2) has been a material of interest for a variety of applications including photocatalysis,1–4 photonic crystals,5,6 photovoltaic cells,7 nanoceramics,8 and gas sensors.9 Its key properties related to application efficiency include crystal structure, phase constituents, particle/crystallite size, and particle morphology. TiO2 is a polymorphic material that has three crystalline modifications, namely, stable rutile (tetragonal), metastable anatase (tetragonal), and metastable brookite (orthorhombic). Anatase and rutile are common polymorphs of synthetic TiO2, while brookite is a high-pressure phase and is occasionally observed as a byproduct along with either anatase or rutile. The anatase phase transforms into rutile over a wide range of temperatures, but the transformation mechanism and the effects of additives on the transition have not been fully understood. The industrial production of TiO2 is mainly by chloride and sulfate processes, and current preparation techniques tend toward sol-gel,10–12 flame synthesis,13–15 and thermal plasma treatment.4,16–18 The main disadvantages of sol-gel lie in the use of large quantities of organic
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0070 J. Mater. Res., Vol. 20, No. 2, Feb 2005
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solvents and the particle growth and aggregate formation resulting from crystallization of as-made amorphous or low-crystallinity powders during prolonged heat treatment. Flame synthesis is relatively inexpensive and simple, but phase control of the product is difficult to achieve because of the high temperature and the limited types of input gases. Vemury and Pratsinis14 have recently investigated the doping effects on the phase structure and particle size of TiO2 powders prepared by flame processing. Thermal plasma synthesis is known as a clean process with
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