NORTHWESTERN UNIVERSITY Atomic Surface Structures of Oxide Materials: From Single Crystals to Nanoparticles DISSERTATION SUBMITTED TO THE GRADUATE SCHOOL IN PARTIAL FULFILLMENT OF THE REQUIREMENT For the degree DOCTOR OF PHILOSOPHY

Atomic Surface Structures of Oxide Materials: From Single Crystals to Nanoparticles Yuyuan Lin Atomic surface structures of oxide materials are very important in many areas. Despite the importance, these structures are not fully understood, especially for the nanoparticles. In this study, the atomic surface structures of SrTiO3 single crystals, SrTiO3 nanocuboids, and CeO2 nanoparticles are investigated by using a verity of techniques. The structural model for each system is proposed. In addition, the corresponding formation mechanisms and possible applications are discussed. With STM image simulation, DFT energetics, BVS, and the previous knowledge of SrTiO3 surface reconstructions, a structural model with a 4-fold symmetry is proposed for the (2×2) reconstruction on single crystal SrTiO3(001). The surface structure has a double-TiO2-layer feature and is very similar to the often observed c(4×2) surface reconstruction. With aberration-corrected HREM, this study shows for the first time that the (100) surface of SrTiO3 nanocuboids can be SrO, TiO2-rich reconstructions, or mixed with SrO and TiO2-rich reconstructions depending on the synthetic procedures. The findings are in agreement with DFT energetics, IR spectroscopic studies as well as surface acidity predictions. With the oxygen atoms clearly observed by using aberration-corrected HREM, the atomic structures of (100), (110) and (111) surfaces on CeO2 nanoparticles were determined for the first time. The (100) surface has a mixture of Ce, O and reduced CeO terminations on the outermost surface as well as the partially occupied lattice sites in the near-surface region (~1 nm from the 4 surface). The (110) surface has a combination of reduced flat CeO2-x surface layers and “sawtooth-like” (111) nanofacets. The CeO2 (111) surface is O-terminated. The distinct amount of surface defects on the three facets implies the different surface redox properties. In addition to the surface structures of as-prepared SrTiO3 and CeO2 nanoparticles, the atomic surface structures of the two nanoparticles under strong electron beam irradiation were investigated. The electron irradiation induces Ti-rich surface islands on the SrTiO3 nanocuboids and phase transformation of CeO2 to Ce2O3. The features of (100), (110), and (111) surfaces of Ce2O3 are very similar to that of the CeO2.