Yuyuan Lin

Adhesion and Atomic Structures of Gold on Ceria Nanostructures:The Role of Surface Structure and Oxidation State of Ceria Supports

We report an aberration-corrected electron microscopy analysis of the adhesion and atomic structures of gold nanoparticle catalysts supported on ceria nanocubes and nanorods. Under oxidative conditions, the as-prepared gold nanoparticles on the ceria nanocubes have extended atom layers at the metal-support interface. In contrast, regular gold nanoparticles and rafts are present on the ceria nanorod supports. Under the reducing conditions of water-gas shift reaction, the extended gold atom layers and rafts vanish. In addition, the gold particles on the nanocubes change in morphology and increase in size while those on the nanorods are almost unchanged. The size, morphology, and atomic interface structures of gold strongly depend on the surface structures of ceria supports ((100) surface versus (111) surface) and the reaction environment (reductive versus oxidative). These findings provide insights into the deactivation mechanisms and the shape-dependent catalysis of oxide supported metal catalysts.

Surface determination through atomically resolved secondary-electron imaging

Unique determination of the atomic structure of technologically relevant surfaces is often limited by both a need for homogeneous crystals and ambiguity of registration between the surface and bulk. Atomically resolved secondary-electron imaging is extremely sensitive to this registration and is compatible with faceted nanomaterials, but has not been previously utilized for surface structure determination. Here we report a detailed experimental atomic-resolution secondary-electron microscopy analysis of the c(6 × 2) reconstruction on strontium titanate (001) coupled with careful simulation of secondary-electron images, density functional theory calculations and surface monolayer-sensitive aberration-corrected plan-view high-resolution transmission electron microscopy. Our work reveals several unexpected findings, including an amended registry of the surface on the bulk and strontium atoms with unusual seven-fold coordination within a typically high surface coverage of square pyramidal TiO5 units. Dielectric screening is found to play a critical role in attenuating secondary-electron generation processes from valence orbitals.

High thermal stability of La2O3 and CeO2-stabilized tetragonal ZrO2

Catalyst support materials of tetragonal ZrO2, stabilized by either La2O3 (La2O3-ZrO2) or CeO2 (CeO2-ZrO2), were synthesized under hydrothermal conditions at 200 °C with NH4OH or tetramethylammonium hydroxide as the mineralizer. From in situ synchrotron powder X-ray diffraction and small-angle X-ray scattering measurements, the calcined La2O3-ZrO2 and CeO2-ZrO2 supports were nonporous nanocrystallites that exhibited rectangular shapes with a thermal stability of up to 1000 °C in air. These supports had an average size of ∼ 10 nm and a surface area of 59-97 m(2)/g. The catalysts Pt/La2O3-ZrO2 and Pt/CeO2-ZrO2 were prepared by using atomic layer deposition with varying Pt loadings from 6.3 to 12.4 wt %. Monodispersed Pt nanoparticles of ∼ 3 nm were obtained for these catalysts. The incorporation of La2O3 and CeO2 into the t-ZrO2 structure did not affect the nature of the active sites for the Pt/ZrO2 catalysts for the water-gas shift reaction.

Electron-induced Ti-rich surface segregation on SrTiO3 nanoparticles

Atomic surface structures of nanoparticles are of interest in catalysis and other fields. Aberration-corrected HREM facilitates direct imaging of the surfaces of nanoparticles. A remaining concern of surface imaging arises from beam damage. It is important to identify the intrinsic surface structures and the ones created by electron beam irradiation in TEM. In this study, we performed aberration-corrected HREM and EELS to demonstrate that TiO and bcc type Ti islands form due to intense electron irradiation. The formation of Ti-rich islands is in agreement with previous high temperature annealing experiments on the surfaces of SrTiO3 single crystals.

When does atomic resolution plan view imaging of surfaces work

Surface structures that are different from the corresponding bulk, reconstructions, are exceedingly difficult to characterize with most experimental methods. Scanning tunneling microscopy, the workhorse for imaging complex surface structures of metals and semiconductors, is not as effective for oxides and other insulating materials. This paper details the use of transmission electron microscopy plan view imaging in conjunction with image processing for solving complex surface structures. We address the issue of extracting the surface structure from a weak signal with a large bulk contribution. This method requires the sample to be thin enough for kinematical assumptions to be valid. The analysis was performed on two sets of data, c(6×2) on the (100) surface and (3×3) on the (111) surface of SrTiO3, and was unsuccessful in the latter due to the thickness of the sample and a lack of inversion symmetry. The limits and the functionality of this method are discussed.

Applications of Electron Microscopy in Heterogeneous Catalysis

Transmission electron microscopy (TEM) is a convenient technique for catalyst characterization. A variety of structural and chemical information of catalysts can be obtained by operating TEM in different modes. The recent developments in TEM, particularly for the aberration corrections, enable studying catalyst at the atomic scale. We briefly cover the TEM theory, developments, and other important topics at the beginning. Several case studies of using TEM as a tool to study the catalysts with well-defined structures are provided later in the chapter. The examples in case studies include atomic surface structures of oxide supports, shapes of supported metals in both vacuum and gaseous conditions, bimetallic catalysts, and single atom catalysts. In each case, the underlying mechanisms of catalytic behaviors are briefly discussed based on the results derived from TEM and other studies.

Atomic Surface Structures of Oxide Nanoparticles with Well-defined Shapes

Recent studies have shown that catalytic activities can be tuned by controlling the shape of nanoparticles such as SrTiO3, CeO2, and Co3O4 [1]. Therefore, determination of surface structure is very important to understand structure-property relationships for these oxide nanoparticles. The Argonne Chromatic-corrected TEM (ACAT) has an image corrector that corrects both spherical (Cs) and chromatic aberration (Cc). Cc correction allows the correction of Cs towards zero to improve resolution without compromising contrast. Using this unique feature, we correct both Cs and Cc to small values to achieve direct structure interpretable HREM images including oxygen atomic columns. In this study, atomic surface structures of SrTiO3, CeO2, Co3O4 nanocubes are observed by using aberration-corrected HREM.

Direct Observation of Atomic Surface Structures of CeO2 Nanoparticles

СеO2 has a wide range of applications such as catalysts and as electrodes in solid oxide fuel cells due to its unique physical, chemical and electrochemical properties [1,2]. Many of these applications are related to surface structures of CeO2. For example CeO2 nanoparticles are used as a catalytic support since surface oxygen vacancies are expected to promote catalytic metal particle dispersion [1]. However, due to the lack of clear observation of oxygen columns using TEM or scanning TEM, atomic surface structures of CeO2 nanoparticles are under debate. Using high resolution electron microscopy with Argonne Chromatic Aberration-corrected TEM, we are able to observe both Ce and O columns, allowing us to directly observe atomic structures of the (100), (110) and (111) surfaces of CeO2 nanoparticles.