Annealing thermal treatments are routinely used in the synthesis of nanoparticles to tailor their size and shape. To control particle growth at elevated temperatures, understanding the dynamics behind surface evolution is of primary importance. Time-resolved, in-situ, aberration-corrected high-resolution transmission electron microscopy (HRTEM) has been successfully used to image structural modifications of nanoparticles in response to thermal annealing, including, for example, surface faceting and sintering [1].

   This study reports the structural evolution of SrTiO₃ nanocuboids [2] in response to thermal annealing at high temperature (≥ 500 °C) using HRTEM imaging. In-situ experiments were performed using a dedicated heating holder, in a JEOL 2200MCO microscope, operating at 200 keV, under low (4 x 10⁶ e/nm²) and high (10¹⁰ e/nm²) electron dose conditions. Imaging at low electron doses reveals structural modifications to the nanoparticles that can be ascribed to heating only. At low dose, the effect of beam irradiation on the surface structure is negligible even for direct exposure times longer than 30 min. An example is illustrated in Figure 1, where a typical flat {001} facet remains unchanged after 2 min of direct beam exposure. By comparison, electron irradiation at high electron dose triggers the growth of TiO islands within a few seconds, consistent with previous observations by Lin and co-workers [3]. Figure 2 illustrates the formation of TiO islands after 3 s exposure at high electron dose (a), and the subsequent sputtering of surface atoms after 1 h 20 min of direct irradiation (b).

   Following in-situ thermal treatment at 800 C, surface faceting is observed at low dose (arrows in figure 3 (b)). The formation of the new facets is triggered by diffusion of the surface atoms, driven by the elevated temperature. Furthermore, atomic migration induces sintering of the particle (Ostwald ripening). For longer annealing times at higher temperatures, phase transformation of the facets is expected to take place, and TiO_x islands will eventually start to grow [4]. In-situ thermal annealing of the particles at higher temperatures is currently under investigation, and the results will be also reported.  

References

[1] M. Chi et al., Nat. Comm. 6 (2015) 8925.

[2] Y. Lin et al., Phy. Rev. Lett. 111 (2013) 156101.

[3] Y. Lin et al., Micron 68 (2014) 152 – 157.

[4] S. Bo Lee et al., Ultramic. 104 (2005) 30 – 38.

[5] The authors acknowledge funding from the European Union Seventh Framework Programme under Grant agreement 312483-ESTEEM2, Prof. Laurence Marks, Prof. Kenneth Poeppelmeier and Dr. Yuyuan Lin for kindly providing the specimens. 

Figures:

Figure 1. HRTEM image illustrating a flat (001) surface of a SrTiO₃ nanocuboid viewed in a [110] zone axis under low beam exposure (4 x 10⁶ e/nm²) (a). The structure remains unchanged after 120 s of direct irradiation (b).

Figure 2. HRTEM image showing the structural modification of the (001) surface in Figure 1 under high electron dose (10¹⁰ e/nm²). The electron beam triggers the formation of TiO islands (red arrows in a), and sputtering of the surface atoms (b).

Figure 3. Low dose HRTEM images illustrating the surface evolution of a SrTiO₃ nanocuboid viewed in a [111] zone axis in response to annealing at 800 C. Surface faceting (red arrows) and sintering, together with Ostwald ripening are both observed.

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EMC Abstracts - https://emc-proceedings.com/abstract/structural-evolution-of-strontium-titanate-nanocuboids-under-in-situ-electron-irradiation-and-heating/