Anisotropic Crystal Growth Kinetics of Anatase TiO2 Nanoparticles Synthesized in a Nonaqueous Medium
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- 作者：Grethe Vestergaard Jensen ; Martin Bremholm ; Nina Lock ; G. Roshan Deen ; Torben R. Jensen ; Bo B. Iversen ; Markus Niederberger ; Jan Skov Pedersen ; Henrik Birkedal
- 刊名：Chemistry of Materials
- 出版年：2010
- 出版时间：November 23, 2010
- 年：2010
- 卷：22
- 期：22
- 页码：6044-6055
- 全文大小：967K
- 年卷期：v.22,no.22(November 23, 2010)
- ISSN：1520-5002
文摘
The formation and growth of titania (anatase) nanoparticles in benzyl alcohol from TiCl4 was studied in situ at 85 °C via powder X-ray diffraction (PXRD), small-angle X-ray scattering (SAXS), and turbidimetry. The results provide new information on the kinetics of this process and allow for better control of particle size, shape, and aggregation. Rietveld refinement of ex situ PXRD data shows that the final crystals are anisotropic in shape and elongated along the crystallographic c-axis. In situ SAXS and PXRD show that the crystals form suddenly after a lag period. The crystals are initially isotropic in shape and the growth is isotropic; thereafter, the growth proceeds predominantly along the crystallographic c-axis to form anisotropic crystals, in agreement with the ex situ PXRD results. The relative lattice strain, which is determined as the lattice deformation relative to the lattice constants found late in the growth process, is positive along the c-axis and negative, but smaller, along the a-axis. In both directions, the strain relaxes as the particles grow. The strain anisotropy, measured as c/a, relaxes to the bulk value for particles with an equivalent linear dimension on the order of 4.5 nm. The present data provide the first strain information in anisotropic particles smaller than 5 nm. The large anisotropic strain is related to the important out-of-plane contributions to the surface energy resulting from selective ligand binding to the surfaces. In situ SAXS shows that the particles initially form small aggregates that can be modeled as either spheres or surface fractals. The aggregate radius of gyration shows a linear growth for both models. At long growth times, the turbidity suddenly increases, because of the occurrence of large-scale aggregation. The onset time follows Arrhenius behavior with an effective activation energy of 106.7 kJ/mol. The large scale aggregation is also reflected in the in situ SAXS data as the point after which the aggregate size accelerates and the aggregates can only be described as volume fractals. These types of sol−gel syntheses are typically stopped after the large-scale aggregation; however, according to the present work, discrete or only slightly aggregated nanoparticles are present at a much earlier stage.