(517e) Uncovering the Growth Mechanism of Hydrothermal Synthesis of Anatase Crystals

Anatase (TiO₂) is a metastable polymorph of titanium dioxide. Anatase crystals with larger surface area of the {001} faces exhibit higher catalytic activity for water dissociation reaction[1]. A better understanding of the crystal growth mechanisms is required to engineer functionally desirable morphology of anatase crystals.

Hydrothermal synthesis of anatase crystals was carried out to obtain large (~1 µm) crystals with the usual bipyramidal morphology dominated by {101} faces. The surface features were characterized using ex situ atomic force microscopy (AFM), and the presence of monomolecular steps suggests that anatase crystals grow by a layered growth mechanism such as spiral growth or birth-and-spread.

Crystal growth under surface integration-limited kinetics proceeds by the attachment of growth units into kink sites along steps present on the surface. These steps are parallel to the lattice directions of strong intermolecular interactions within the solid, also known as Periodic Bond Chains (PBCs) [2]. A systematic method developed earlier[3] was used to identify the PBC directions in anatase crystals. Crystal growth kinetics depends on the long-range electrostatic interaction energies between the kink site growth unit and its solid-state and solvent neighborhood. The kink incorporation rates on the edges of (101) and (001) faces of anatase crystals were calculated from the attachment and detachment kinetics of individual kink sites[4]. The relative growth rates of the crystal faces were calculated using the spiral growth model[5] to accurately predict the steady-state morphology of anatase crystals.

Further refinements to this mechanistic model will allow prediction of the effect of synthesis conditions such as supersaturation, pH, additives, etc. on the morphology of hydrothermally grown materials with important functionalities such as zinc oxide, gallium nitride, etc.

 

References

[1] H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H. M. Cheng, and G. Q. Lu, “Anatase TiO₂ single crystals with a large percentage of reactive facets," Nature, 453, 638, 2008.

[2] P. Hartman and W. G. Perdok, “On the relations between structure and morphology of crystals. I," Acta Cryst., 8, 49-52, 1955.

[3] P. Dandekar and M. F. Doherty, “A mechanistic growth model for inorganic crystals: Solid-state interactions," AIChE J., (submitted), 2014.

[4] P. Dandekar and M. F. Doherty, “A mechanistic growth model for inorganic crystals: Growth mechanism," AIChE J., (submitted), 2014.

[5] R. C. Snyder and M. F. Doherty, “Predicting crystal growth by spiral motion," Proc. R. Soc. A, 465, 1145-1171, 2009.