Shankar Sivaramakrishnan

Visualizing Materials Chemistry at Atomic Resolution

Analytical electron microscopy--empowered by advances in electron optics and detectors--is poised to radically transform our understanding of the complex phenomena arising from atomic and electronic structure in materials chemistry. (To listen to a podcast about this article, please go to the Analytical Chemistry multimedia page at pubs.acs.org/page/ancham/audio/index.html.).

Direct Observation of Interfacial Au Atoms on TiO2 in Three Dimensions

Interfacial atoms, which result from interactions between the metal nanoparticles and support, have a large impact on the physical and chemical properties of nanoparticles. However, they are difficult to observe; the lack of knowledge has been a major obstacle toward unraveling their role in chemical transformations. Here we report conclusive evidence of interfacial Au atoms formed on the rutile (TiO2) (110) surfaces by activation using high-temperature (∼500 °C) annealing in air. Three-dimensional imaging was performed using depth-sectioning enabled by aberration-corrected scanning transmission electron microscopy. Results show that the interface between Au nanocrystals and TiO2 (110) surfaces consists of a single atomic layer with Au atoms embedded inside Ti-O. The number of interfacial Au atoms is estimated from ∼1-8 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; both are critical to Au catalysis.

Electron-Beam-Induced Growth of TiO 2 Nanostructures

We report the evolution of titanium dioxide nanostructures when Au nanoparticles, supported on single crystal TiO(2) substrates, were heated under ∼260 Pa of flowing O(2) in an environmental transmission electron microscope. Nanostructures with different morphologies were first observed around 500°C. Our measurements show that temperature, oxygen pressure, and the electron beam control the nanostructure growth. We propose a reaction-controlled growth mechanism where mobile Ti atoms generated by the electron- beam-induced reduction of TiO(2) are preferentially reoxidized at the Au-TiO(2) interface.

Structural evolution, epitaxy, and sublimation of silver nanoclusters on TiO2 (110)

The structural evolution, epitaxy, and sublimation temperature of silver nanoclusters (NCs) on TiO2 (110) surfaces prepared in two different ways are reported here based on a combination of in situ reflection high energy electron diffraction characterization and ex situ electron imaging. It is shown that silver NCs deposited at room temperature on oxidized TiO2 (110) surfaces are unable to form a single epitaxy prior to sublimation. When heated close to sublimation, two particle orientations dominate: (111)Ag∥(110)TiO2, [−110]Ag∥[001]TiO2 and (112)Ag∥(110)TiO2, [−110]Ag∥[001]TiO2. Single twinned silver NCs are found to be stable even at temperatures close to sublimation. On the other hand, silver NCs prepared similarly on reduced TiO2 (110) surfaces behave very differently when heated to higher temperatures. On the reduced surface, the NCs are able to evolve into a single epitaxy-(111)Ag∥(110)TiO2, [−110]Ag∥[001]TiO2. The sublimation temperature for silver NCs on the reduced surface is found to be less th...

Direct Observation of Interfacial Au atoms Using STEM Depth Sectioning

Interfacial atoms located between metal nanoparticles and supports are proposed active sites in catalysis, because of their distinct physical and chemical properties [1-3]. However, the atomistic details are difficult to resolve in the interface; the lack of knowledge has been a major obstacle toward unraveling their roles in chemical transformations. Here we report the detection of interfacial Au atoms on the rutile (TiO2) (110) surfaces thanks to the improved spatial resolution and depth of focus brought by aberration corrected scanning transmission electron microscopy (STEM).