Synthesis, characterization, and phase stability of ultrafine TiO 2 nanoparticles by pulsed laser ablation in liquid med
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We synthesized ultrafine TiO2 nanoparticles by pulsed laser ablation of a titanium target immersed in an aqueous solution of surfactant sodium dodecyl sulfate (SDS) or deionized water. The surfactant concentration dependence of TiO2 nanocrystal formation was systematically investigated by various characterization techniques. The maximum amount of ultrafine anatase nanocrystalline particles (with mean size of 3 nm in diameter) was obtained in an aqueous solution of 0.01 M SDS. A probable formation process was proposed based on the laser-induced reactive quenching and surfactant-mediated growth. The phase transformation and particle growth of as-prepared products were also investigated by heat treatment up to 500 °C. Single-phase anatase nanoparticles with a mean size of 8 nm were obtained by heat treatment of samples prepared in water or in a 0.01 M SDS solution. Particle size did not substantially increase through annealing, probably due to the relatively homogeneous size distribution and crystallinity of as-prepared titania nanoparticles.
I. INTRODUCTION
Oxide semiconductors, such as TiO2 nanocrystals, have extensively been studied for their high photocatalytic activities and chemical stability and for a variety of applications.1–4 The physical and chemical properties of TiO2 nanoparticles are highly correlated with their phase structure, morphology, and particle size. It has been shown that the adsorption of organics onto the surface of nanocrystalline anatase is size-dependent and that particle size is a crucial factor in photocatalytic decomposition of chloroform by nanocrystalline anatase.5,6 Most liquid-phase synthetic techniques applied to TiO2 nanoparticles were based on the hydrolysis and condensation of molecular precursors, such as titanium alkoxides and halides.7–9 Consequently, the synthesized nanoparticles were typically amorphous and required high temperatures for postcalcination to improve the crystallinity of TiO2 nanoparticles. This high-temperature process inevitably resulted in the size increase and agglomeration of TiO2 nanoparticles. Therefore, it is important to find a new synthetic method of producing crystalline TiO2 nanoparticles, not only to enhance photocatalytic activity, but also to study the size effects on electrical and optical properties using well-defined nanocrystals.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0208 J. Mater. Res., Vol. 19, No. 5, May 2004
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Hydrothermal processes were developed to prepare crystalline titanium oxide nanoparticles without postcalcination treatment. This was accomplished by carefully tuning processing variables, such as the nature of the solvent and additives, pH value, temperature, and pressure.10–12 Most recently, in situ hot-fluid annealing of amorphous titanium oxide particles confined in reverse micelles has been conducted to obtain smaller crystalline nanoparticles (4 to 5 nm in diameter).13 Apart from the widely used formatio
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