Dogan Ozkaya

Core@shell bimetallic nanoparticle synthesis via anion coordination

Core@shell structured bimetallic nanoparticles are currently of immense interest due to their unique electronic, optical and catalytic properties. However, their synthesis is non-trivial. We report a new supramolecular route for the synthesis of core@shell nanoparticles, based on an anion coordination protocol--the first to function by binding the shell metal to the surface of the pre-formed primary metal core before reduction. The resultant gold/palladium and platinum/palladium core@shell nanoparticles have been characterized by aberration-corrected scanning transmission electron microscopy (as well as other techniques), giving striking atomic-resolution images of the core@shell architecture, and the unique catalytic properties of the structured nanoparticles have been demonstrated in a remarkable improvement of the selective production of industrially valuable chloroaniline from chloronitrobenzene.

A simple algorithm for measuring particle size distributions on an uneven background from TEM images.

Nanoparticles have a wide range of applications in science and technology. Their sizes are often measured using transmission electron microscopy (TEM) or X-ray diffraction. Here, we describe a simple computer algorithm for measuring particle size distributions from TEM images in the presence of an uneven background. The approach is based on adaptive thresholding, making use of local threshold values that change with spatial coordinate. The algorithm allows particles to be detected and characterized with greater accuracy than using more conventional methods, in which a global threshold is used. Its application to images of heterogeneous catalysts is presented.

Three‐dimensional shapes and spatial distributions of Pt and PtCr catalyst nanoparticles on carbon black

High‐angle annular dark‐field scanning transmission electron microscopy tomography is applied to the study of Pt and PtCr nanoparticles supported on carbon black, which are used as heterogeneous catalysts in the electrodes of proton exchange membrane fuel cells. By using electron tomography, the three‐dimensional architecture of the heterogeneous catalyst system can be determined, providing high‐spatial‐resolution information about the shapes, faceting and crystallographies of 5–20 nm single and multiply twinned catalyst particles, as well as their positions with respect to the carbon support. Approaches that can be used to provide improved information about the distribution and orientation of the particles on their support are proposed and discussed. Our results show that electron tomography provides important information that is complementary to high‐resolution lattice imaging. Both techniques are required to understand fully the nature and role of the surfaces of faceted catalyst particles.

Three-dimensional shapes and structures of lamellar-twinned fcc nanoparticles using ADF STEM

Small particles with face-centred cubic structures can have non-single-crystallographic shapes. Here, an approach based on annular dark-field scanning transmission electron microscopy (STEM) is used to obtain information about the crystal sub-units that make up supported and unsupported twinned Pt, Pt alloy and Au nanoparticles. The three-dimensional shapes of two types of lamellar-twinned particles (LTPs) of Pt are obtained using high-angle annular dark-field STEM. Possible growth mechanisms of the LTPs and origins for the contrast features in the recorded images are discussed.

Optimal ADF STEM imaging parameters for tilt-robust image quantification.

An approach towards experiment design and optimisation is proposed for achieving improved accuracy of ADF STEM quantification. In particular, improved robustness to small sample mis-tilts can be achieved by optimising detector collection and probe convergence angles. A decrease in cross section is seen for tilted samples due to the reduction in channelling, resulting in a quantification error, if this is not taken into account. At a smaller detector collection angle the increased contribution from elastic scattering, which initially increases with tilt, can be used to offset the decrease in the TDS signal.

Crystal size and shape analysis of Pt nanoparticles in two and three dimensions

The majority of industrial catalysts are high-surface-area solids, onto which an active component is dispersed in the form of nanoparticles that have sizes of between 1 and 20 nm. In an industrial environment, the crystal size distributions of such particles are conventionally measured by using either bright-field transmission electron microscope (TEM) images or X-ray diffraction. However, the analysis of particle sizes and shapes from two-dimensional bright-field TEM images is affected by variations in image contrast between adjacent particles, by the difficulty of distinguishing the particles from their matrix, and by overlap between particles when they are imaged in projection. High-angle annular dark-field (HAADF) electron tomography provides a convenient technique for overcoming many of these problems, by allowing the three-dimensional shapes and sizes of high atomic number nanoparticles that are supported on a low atomic number support to be recorded. Here, we discuss the three-dimensional analysis of particle sizes and shapes from such tomographic data, and we assess whether such measurements provide different information from that obtained using two-dimensional TEM images and X-ray diffraction measurements.

Predicting the oxygen-binding properties of platinum nanoparticle ensembles by combining high-precision electron microscopy and density functional theory

Many studies of heterogeneous catalysis, both experimental and computational, make use of idealized structures such as extended surfaces or regular polyhedral nanoparticles. This simplification neglects the morphological diversity in real commercial oxygen reduction reaction (ORR) catalysts used in fuel-cell cathodes. Here we introduce an approach that combines 3D nanoparticle structures obtained from high-throughput high-precision electron microscopy with density functional theory. Discrepancies between experimental observations and cuboctahedral/truncated-octahedral particles are revealed and discussed using a range of widely used descriptors, such as electron-density, d-band centers, and generalized coordination numbers. We use this new approach to determine the optimum particle size for which both detrimental surface roughness and particle shape effects are minimized.

Electron tomography of Pt nanocatalyst particles and their carbon support

Industrial nanocatalysts usually comprise crystalline particles of high atomic number that have sizes of between 1 and 20 nm and are supported or embedded in a lower atomic number matrix. The physical characterisation of the three-dimensional shapes and sizes of such particles can now be carried out using high-angle annular darkfield electron tomography. The spatial distribution of the particles with respect to their matrix is an issue of paramount importance for their performance as catalysts. Here, we show experimental electron tomography results from platinum particles dispersed in a carbon support. We show that both the high and the low atomic number regions of the same region of a sample can be characterised by using a combination of high and low angle annular dark field and bright field signals.

Tomographic Heating Holder for In Situ TEM: Study of Pt/C and PtPd/Al2O3 Catalysts as a Function of Temperature

A tomographic heating holder for transmission electron microscopy that can be used to study supported catalysts at temperatures of up to ~1,500°C is described. The specimen is placed in direct thermal contact with a tungsten filament that is oriented perpendicular to the axis of the holder without using a support film, allowing tomographic image acquisition at high specimen tilt angles with minimum optical shadowing. We use the holder to illustrate the evolution of the active phases of Pt nanoparticles on carbon black and PtPd nanoparticles on γ-alumina with temperature. Particle size distributions and changes in active surface area are quantified from tilt series of images acquired after subjecting the specimens to increasing temperatures. The porosity of the alumina support and the sintering mechanisms of the catalysts are shown to depend on distance from the heating filament.

Towards quantitative analysis of core-shell catalyst nano-particles by aberration corrected high angle annular dark field STEM and EDX

Core-shell structured heterogeneous catalyst nano-particles offer the promise of more efficient precious metal usage and also novel functionalities but are as yet poorly characterised due to large compositional variations over short ranges. High angle annular dark field detector in a scanning transmission electron microscope is frequently used to image at high resolution because of its Z-contrast and incoherent imaging process, but generally little attention is paid to quantification. Energy dispersive X-ray analysis provides information on thickness and chemical composition and, used in conjunction with HAADF-STEM, aids interpretation of imaged nano-particles. We present important calibrations and initial data for truly quantitative high resolution analysis.