Electron energy-loss study of titania particles
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Electron energy-loss study of titania particles R. J. Gonzaleza) and A. L. Ritter Department of Physics, Virginia Tech, Blacksburg, Virginia 24061 (Received 24 March 1997; accepted 7 November 1997)
Small titania particles, prepared by hydrolysis and condensation using in situ steric stabilization, have been studied by high-energy, transmission, electron energy-loss spectroscopy. Electron diffraction patterns and energy-loss spectra as a function of momentum transfer were measured for as-prepared particles (amorphous titania), particles annealed at 600 ±C (primarily anatase), and particles annealed at 1000 ±C (primarily rutile). The energy-loss spectra at low momentum disagreed with the loss function calculated from optical data (rutile) and disagreed with theory (rutile and anatase). The data was fit by an Elliot-like model for a resonant exciton interacting with a continuum of levels. The translational effective mass of the exciton derived from the fitting was quite large, indicating that it was self-trapped.
I. INTRODUCTION
A wide range of applications exist for titania particles, from paint pigment to photocatalysis, as a consequence of the particular optical properties of this material.1–3 Significant interest exist in expanding and improving these applications by tuning the optical properties through control of the particle morphology and size distribution. We have studied titania particles prepared by the hydrolysis and condensation of TiO2 from titanium tetraethoxide, Ti(OC2 H5 )4 , (TEOT), in an aqueous ethanol solution.4–6 The average size of the particles can be controlled by varying the ratio of water to TEOT with higher ratios producing smaller average particle size due to higher nucleation rates during the condensation of the gel network. Further control of the particle size is obtained by allowing the hydrolysis/condensation reaction to occur in the presence of a polymer, such as hydroxypropylcellulose (HPC), which can adsorb onto the surface of the TiO2 particles by hydrogen bonding to create a steric barrier which limits the particle growth.4–6 This paper presents results of high-energy, transmission, electron energy-loss spectroscopy (EELS) studies on titania particles synthesized by the in situ steric stabilization process. The amorphous, as-prepared samples were spun coated on thin alumina substrate films creating isolated titania islands which were approximately 80 nm in diameter. Some of these composite films were studied as prepared while others were sintered at 600 ±C and 1000 ±C. The anneal temperatures were chosen to transform the amorphous titania particles to the anatase and the rutile crystalline phases, respectively. The structure of the nanoparticles was determined by electron diffraction in the EELS spectrometer. The diffraction patterns from the as-prepared samples were a)
Present address: Incstar Corp., Stillwater, Minnesota. J. Mater. Res., Vol. 13, No. 6, Jun 1998
indistinguishable from the patterns from the alumina substrate becau
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