Synthesis-Dependent Atomic Surface Structures of Oxide Nanoparticles

Using ${\mathrm{SrTiO}}_3$ nanocuboids as a model system, we show with aberration-corrected high resolution electron microscopy at sub-Å resolution that surface relaxations or reconstructions are present on the nanocuboids, depending on the synthetic process. Oleic acid synthesis, acetic acid synthesis, and microwave-assisted acetic acid synthesis result in a SrO termination, ${\mathrm{TiO}}_2$-rich reconstruction, and mixed termination, respectively. The experimental atomic positions are in better agreement with density functional theory calculations using an exact-exchange corrected PBEsol functional than the Perdew-Burke-Ernzerhof (PBE) functional.

Figure 1

HREM study of samples from the oleic acid synthesis. (a) Experimental HREM image of ${\mathrm{SrTiO}}_3$ nanocuboid along [110], where the bright spots can be interpreted as atoms. (b) Low magnification image of (a). (c) FFT image of (b). (d) Simulated HREM image of (a). In (a) the sample thickness and defocus are increasing from left to right. Therefore, (d) is generated using 3 images simulated with continual changing defocus and thickness, increasing from left to right.

Figure 2

(a) Interlayer spacing comparison of the experimental image and simulated images for unrelaxed and DFT relaxed structures. The comparison is based on the area marked in the red box in Fig. 1a. The interlayer spacings (i) are measured from Gaussian fits to the peak positions of the SrO-Ti atomic columns from the surface into the bulk. The error bars indicate the experimental deviation with respect to the mean values. (b) Cumulative error comparison between the DFT calculation using the PBE and the PBEsol0 functional. The base line was chosen as the 10th layer in the bulk from the surface ($j=1$ is the 10th interlayer spacing from the surface).

Figure 3

HREM images from the acetic acid synthesis. (a) Experimental HREM image of a ${\mathrm{SrTiO}}_3$ nanocuboid along [110]. (b) Low magnification image of (a). (c) The FFT image of (a). (d) A simulated image using a $R\mathrm{T}13$ reconstructed surface. (e) Illustration of the RT13 atomic surface structures along the [110] viewing direction. In (a), owing to the nature of the sample, the sample thickness and defocus are increasing from right to left. Therefore, (d) was generated using 3 images simulated with different defocus and thickness, increasing from right to left.

Figure 4

HREM images from the MA-HT synthesis. (a) Experimental HREM image of a ${\mathrm{SrTiO}}_3$ nanocuboid along [110]. (b) Low magnification image of (a). (c) The FFT image of (b). (d) A simulated image using a SrO terminated surface (areas with red arrows) and a layered structure model consisting of 50% of SrO and 50% of ($3\times 3$) surface terminations (areas with blue arrows). (e) Illustration of the ($3\times 3$) and SrO atomic surface structures along [110]. In (a), owing to the nature of the sample, the sample thickness, and defocus are increasing from right to left. Therefore, (d) was generated using 4 images simulated with different defocus and thickness, increasing from right to left.