Imaging of nanoparticles and nanocrystal interactions with oxide surfaces

Surfaces and interfaces have a major impact on the structure and properties of materials, especially in nanostructures, including supported metal nanoparticles (NPs) used as catalysts in chemical industries and environmental remediation. For example, oxide supported gold NPs demonstrate outstanding chemical activity in selective oxidation of CO at low temperatures, and strong support effect is believed to be involved in the enhancement of their catalytic activities. The interfaces formed by the Au NPs and their supports are heterogeneous. In homogeneous materials, another type of interfaces is formed at grain boundaries. Such interfaces play a critical role in materials’ microstructure development. Recent advances in electron microscopy have significantly improved our capability for studying interfaces. Here we extend the interfacial study to two types of samples, one sample is Au NPs supported on rutile (TiO2) surface and the other is transformation of Pt icosahedral NPs. In both cases, information obtained by innovative electron microscopy reveals atomic interfacial interactions at unprecedented details. In our study on the Au/TiO2 catalyst, we systematically examined the interfacial effects on the structure of Au NCs and the interface using aberration corrected scanning transmission electron microscopy (AC-STEM). Results show that by high temperature annealing, Au NCs supported on stepped TiO2 (110) surfaces develop an approximate and preferred epitaxial relationship with Au(111)||TiO2(110). The interfacial interaction can modify the shape of the NCs, and introduce significant strain in Au NCs near the surface steps. This strain is seen relaxed in Au layers away from the interface. By measuring the shape parameters, an interfacial energy of γint ~ 0.48 J/m2 is calculated for Au NCs on TiO2 (110) surface steps. This value is smaller than that for Au NCs on flat TiO2 (110) surfaces, which explains the stability of the interface. To examine the heterogeneous interface in three-dimensions, we further developed the depth-sectioning technique using STEM. Using this, we obtained conclusive evidences of interfacial Au atoms formed on the rutile (TiO2) (110) surfaces by activation using high temperature (~500ºC) annealing in air. Results show that the interface between Au nanocrystals and TiO2 (110) surfaces consists of a single atomic layer with Au atoms embedded inside the Ti-O plane. The number of interfacial Au atoms is estimated from ~1 to 8 in an interfacial atomic column. Direct impact of interfacial Au atoms is observed on an enhanced Au-TiO2 interaction and the reduction of surface TiO2. To determine the TiO2 surface structure and interface oxidation state, we performed atomic resolution imaging of the Au/TiO2 (110) interface using both annular bright field (ABF) STEM and chromatic aberration (Cc) corrected TEM. Direct interpretable images of oxygen columns are achieved, together with gold and titanium atomic columns. We located the bridging oxygen atoms on the surface and at the interface, and observed interfacial oxygen vacancies as well. Interfacial Au atoms near the vacant oxygen atomic column are seen shifting downwards, indicating a stronger Au-Ti bonding where oxygen is reduced. These experimental results suggest that Au NCs on TiO2 promote interfacial restructuring, reduction of the oxide surface and stabilization of the NCs by pinning metal atoms onto the oxide surfaces, all critical to catalysis. To explore the scientific opportunities provided by fast imaging in environmental TEM, two studies have been carried out, including the behavior of Au catalyst on top of Si nanowires in heating environment, and the transformation of Pt icosahedral NPs in reactive gases. Upon heating the Au NP on a Si nanowire, we captured the initial kinking of the Au NP and the following atomic diffusion of Au on Si NW surface. In reactive gases, the catalytic Pt icosahedral NPs transform into a FCC single crystal. We captured the dynamics employing both chemical triggering and collective atomic motion. In both studies, the processes were monitored using a fast electron camera at 2.5 ms time resolution. The results provide critical insights to nanoparticle catalyst behaviors with improvement in both time resolution and spatial resolution