Grain-growth kinetics of rutile TiO 2 nanocrystals under hydrothermal conditions
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Liping Li Department of Physics, Brigham Young University, Provo, Utah 84602
Juliana Boerio-Goates and Brian F. Woodfielda) Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602 (Received 27 April 2003; accepted 14 August 2003)
Rutile nanocrystals were directly prepared under hydrothermal conditions using TiCl4 as the starting material. The formation reactions proceeded by suppressing the crystallization of the other TiO2 polymorphs using a fixed concentration of 0.62 M [Ti4+]. With increasing reaction temperatures from 140 to 220 °C, rutile nanocrystals were found to grow from 5.4 to 26.4 nm in size, and by varying the reaction time from 2 to 120 h at 200 °C the particle size increased from 17 to 40 nm. The grain-growth kinetics of rutile TiO2 nanocrystals under hydrothermal conditions was found to follow the equation, Dn ⳱ k0 × t × e(−Ea /RT) with a grain-growth exponent n ⳱ 5 and an activation energy of Ea ⳱ 170.8 kJ mol−1. The nanocrystals thus obtained consist of an interior rutile lattice and a surface hydration layer. With decreasing particle size, the hydration effects at the surface increase, while the rutile structure shows a lattice expansion and covalency enhancement in the Ti–O bonding.
I. INTRODUCTION
TiO2 occurs in nature in three distinct crystallographic phases: rutile, anatase, and brookite.1 TiO2 of various forms has found many important technological applications in the energy industry, including lithium batteries, solar cells, and photocatalytic devices, because of its high chemical stability, low cost, nontoxicity, and excellent physical and chemical properties.2– 4 These properties are strongly dependent on the particle size,5,6 and kinetic control of the particle size in TiO2 nanocrystals appears to be very important. However, until now, there has not been a report on the grain-growth kinetics in TiO2 nanocrystals due to the difficulties in pure-phase preparation and particle size control. These limitations also have impacted further investigation into particle sizedependent physical phenomena, microstructure, and electronic structures of TiO2 nanocrystals. As an example, some controversies still exist related to the size-dependent structural stabilities of TiO2 polymorphs.7–13 TiO2 nanocrystals prepared by many wetchemical methods7–11 are usually a mixture, with anatase being the primary phase, but anatase will transform into
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J. Mater. Res., Vol. 18, No. 11, Nov 2003 Downloaded: 08 Feb 2015
the more stable rutile phase during the coarsening process at higher temperatures. Following this observation, Zhang et al.12 performed a thermodynamic analysis and concluded that the rutile phase becomes metastable relative to anatase at particle sizes smaller than approximately 14 nm. However, this conclusion is strongly questioned by Park et al.,13 since they have successfully prepared rutile nanocrystals smaller than 10 nm. In this work, we prepare
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