Monodisperse faceted nanocrystals, with controllable shapes and sizes, have been becoming increasingly important for applications in catalysis, gas sensing, and energy conversion. Such highly shape sensitive and selective physical and chemical properties inherently stem from the atomic and electronic structures on the faceted surfaces. For elemental nanocrystals, the atomic structure on the surfaces is determined by the geometric shape itself. However, for compound materials such as alloys and complex oxides, the compositional segregation and different terminating lattice planes on the surfaces have to be taken into account. In order to understand the unique property and growth mechanism of these nanocrystals, atomic details on the faceted surfaces need to be studied on the atomic level.

Strontium titanate (SrTiO₃), strontium zirconate (SrZrO₃) and their solid solutions (SrTi_1−xZr_xO₃) are important members in the class of perovskite structures with a general formula ABO₃ (Figure 1a). These materials are of great technological and fundamental importance not only because of their interesting properties, but also because of their ability to combine and to adjust these properties by chemical substitution with a wide variety of cations. However, despite the success of the synthesis of the {1 0 0}-faceted BaTiO₃, SrTiO₃, and Ba_1−xSr_xTiO₃ nanocubes, whether the {1 0 0} facets of the nanocubes are terminated with AO (SrO) or BO₂ (TiO₂) is a question which still remains open for speculation and investigation. A comprehensive understanding of the growth mechanisms of these faceted nanocubes has not been achieved. Direct experimental evidence for the atomic structure on these nanocube surfaces has become one of the key steps in exploring the growth mechanisms.

In this work, we report on detailed studies of monodisperse {1 0 0}-faceted nanocubes of SrTi_1−xZr_xO₃ (x = 0.25 to 0.5) which were synthesized using the oil-water two-phase solvothermal method. The surface atomic structure of the monodisperse faceted nanocrystals is determined by means of aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). On the basis of the structural features on the faceted surfaces, a deeper insight into the growth mechanisms could be obtained.

References and Acknowledgements

 (1) Du, H.; Jia, C.-L.; Mayer, J. Surface Atomic Structure and Growth Mechanism of Monodisperse {1 0 0}-Faceted Strontium Titanate Zirconate Nanocubes. Chem. Mater. 2016, 28 (2), 650–656.

(2)  Du, H.; Wohlrab, S.; Weiß, M.; Kaskel, S. Preparation of BaTiO₃ Nanocrystals Using a Two-Phase Solvothermal Method. J. Mater. Chem. 2007, 17, 4605–4610.

(3) This work has been supported in parts by the Deutsche Forschungsgemeinschaft (SFB 917).

Figures:

Figure 1. Synthesis of perovskite oxide nanocubes by an oil-water two-phase solvothermal synthesis. (a) Structural model of ABO₃ perovskite structure. (b) Schematic illustration of the strategy for the oil-water two-phase solvothermal synthesis. (c, d) Bright-field TEM images, (e, f) HAADF-STEM images of the nanocubes (SrTi_0.5Zr_0.5O₃), at lower and higher magnification, respectively.

Figure 2. Surface atomic structure of the nanocubes. (a) HAADF STEM image of a SrTi_0.75Zr_0.25O₃ nanocube in the [0 0 1] zone axis, averaged over 4 frames and denoised by a nonlinear filter,35 The Sr columns appear brighter than the Ti/Zr-O columns. (b) HAADF STEM image overlaid with color-scale two-dimensional Gaussian peaks from fitting the intensity distribution of each column. (c, d) Maps of integrated peak intensity of the Sr and Ti/Zr-O columns, respectively. Arrows in (d) indicate Zr-rich columns showing exceptional brightness. (e, f) Histogram of the intensities of the Sr and Ti/Zr-O columns, respectively.

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EMC Abstracts - https://emc-proceedings.com/abstract/surface-atomic-structure-and-growth-mechanism-of-1-0-0-faceted-perovskite-oxide-nanocubes/