M. Grace Burke

Correlating catalytic activity of Ag-Au nanoparticles with 3D compositional variations.

Significant elemental segregation is shown to exist within individual hollow silver-gold (Ag-Au) bimetallic nanoparticles obtained from the galvanic reaction between Ag particles and AuCl4(-). Three-dimensional compositional mapping using energy dispersive X-ray (EDX) tomography within the scanning transmission electron microscope (STEM) reveals that nanoparticle surface segregation inverts from Au-rich to Ag-rich as Au content increases. Maximum Au surface coverage was observed for nanoparticles with approximately 25 atom % Au, which correlates to the optimal catalytic performance in a three-component coupling reaction among cyclohexane carboxyaldehyde, piperidine, and phenylacetylene.

X-ray Energy-Dispersive Spectrometry During In Situ Liquid Cell Studies Using an Analytical Electron Microscope

The use of analytical spectroscopies during scanning/transmission electron microscope (S/TEM) investigations of micro- and nano-scale structures has become a routine technique in the arsenal of tools available to todays materials researchers. Essential to implementation and successful application of spectroscopy to characterization is the integration of numerous technologies, which include electron optics, specimen holders, and associated detectors. While this combination has been achieved in many instrument configurations, the integration of X-ray energy-dispersive spectroscopy and in situ liquid environmental cells in the S/TEM has to date been elusive. In this work we present the successful incorporation/modifications to a system that achieves this functionality for analytical electron microscopy.

Nanostructured Aptamer-Functionalized Black Phosphorus Sensing Platform for Label-Free Detection of Myoglobin, a Cardiovascular Disease Biomarker

We report the electrochemical detection of the redox active cardiac biomarker myoglobin (Mb) using aptamer-functionalized black phosphorus nanostructured electrodes by measuring direct electron transfer. The as-synthesized few-layer black phosphorus nanosheets have been functionalized with poly-l-lysine (PLL) to facilitate binding with generated anti-Mb DNA aptamers on nanostructured electrodes. This aptasensor platform has a record-low detection limit (∼0.524 pg mL(-1)) and sensitivity (36 μA pg(-1) mL cm(-2)) toward Mb with a dynamic response range from 1 pg mL(-1) to 16 μg mL(-1) for Mb in serum samples. This strategy opens up avenues to bedside technologies for multiplexed diagnosis of cardiovascular diseases in complex human samples.

STEM-EDX tomography of bimetallic nanoparticles: A methodological investigation

This paper presents an investigation of the limitations and optimisation of energy dispersive X-ray (EDX) tomography within the scanning transmission electron microscope, focussing on application of the technique to characterising the 3D elemental distribution of bimetallic AgAu nanoparticles. The detector collection efficiency when using a standard tomography holder is characterised using a tomographic data set from a single nanoparticle and compared to a standard low background double tilt holder. Optical depth profiling is used to investigate the angles and origin of detector shadowing as a function of specimen field of view. A novel time-varied acquisition scheme is described to compensate for variations in the intensity of spectrum images at each sample tilt. Finally, the ability of EDX spectrum images to satisfy the projection requirement for nanoparticle samples is discussed, with consideration of the effect of absorption and shadowing variations.

Sample Preparation Methodologies for In Situ Liquid and Gaseous Cell Analytical Transmission Electron Microscopy of Electropolished Specimens

In recent years, an increasing number of studies utilizing in situ liquid and/or gaseous cell scanning/transmission electron microscopy (S/TEM) have been reported. Because of the difficulty in the preparation of suitable specimens, these environmental S/TEM studies have been generally limited to studies of nanoscale structured materials such as nanoparticles, nanowires, or sputtered thin films. In this paper, we present two methodologies which have been developed to facilitate the preparation of electron-transparent samples from conventional bulk metals and alloys for in situ liquid/gaseous cell S/TEM experiments. These methods take advantage of combining sequential electrochemical jet polishing followed by focused ion beam extraction techniques to create large electron-transparent areas for site-specific observation. As an example, we illustrate the application of this methodology for the preparation of in situ specimens from a cold-rolled Type 304 austenitic stainless steel sample, which was subsequently examined in both 1 atm of air as well as fully immersed in a H2O environment in the S/TEM followed by hyperspectral imaging. These preparation techniques can be successfully applied as a general procedure for a wide range of metals and alloys, and are suitable for a variety of in situ analytical S/TEM studies in both aqueous and gaseous environments.

XEDS and EELS in the TEM at Atmospheric Pressure and High Temperature

Recent progress with environmental cell and microscope design has enabled in situ imaging studies within gaseous environments inside the (scanning) transmission electron microscope ((S)TEM) to become increasingly routine [1-2]. In contrast, complementary elemental information is more challenging to obtain in situ. Electron energy loss spectroscopy (EELS) has been reported inside an environmental TEM [3,4], although only at modest pressure (below ~30 mbar). Closed-cell design specimen holders, in which the specimen and the gaseous environment are sealed from the high vacuum of the TEM by two SiN windows, allow much higher pressures to be reached (up to ~1 bar). However, these environmental cells have two major drawbacks which limit their analytical capabilities: 1) X-ray energy dispersive spectroscopy (XEDS) is challenging, as the walls of the cell generally shadow the detectors, preventing the collection of characteristic X-rays; 2) EELS is challenging as the two 50 nm thick SiN windows cause multiple scattering, which limits signal-to-background ratio in the core-loss EEL spectra and thus significantly degrades performance [5].

SCC Initiation in the Machined Austenitic Stainless Steel 316L in Simulated PWR Primary Water

Annealed and cold-worked stainless steel 316L samples with machined and polished surfaces were tested in simulated pressurized water reactor (PWR) primary water under slow strain rate tensile (SSRT) test conditions to investigate stress corrosion cracking (SCC) initiation. Roughness, residual stress and cross-sectional microstructure of the as-machined samples were characterized before SSRT tests. Plan view and cross-sectional examinations were performed after the test. Pre-test characterization indicated that a deformation layer was present on the machined surfaces. This deformation layer consisted of an ultrafine-grained layer on the top and deformation bands underneath. The thickness of the deformation layer on the annealed material was greater than that on the cold-worked material. Post-test characterization revealed that the SCC initiation behaviors of the as-machined and polished surfaces were different for both annealed and cold-worked materials. Machining increased SCC initiation susceptibility of the annealed material as many shallow cracks initiated along the machining marks in the machined surface, and it decreased the SCC initiation susceptibility of the cold-worked material as a reduced number of cracks were identified in the machined surface compared to the polished surface. The factors influencing SCC initiation are also discussed.

Practical Aspects of Electrochemical Corrosion Measurements during in situ Analytical TEM of Austenitic Stainless Steel in Aqueous Media

The capability to perform liquid in situ transmission electron microscopy (TEM) experiments provides an unprecedented opportunity to examine the real-time processes of physical and chemical/electrochemical reactions during the interaction between metal surfaces and liquid environments. This work describes the requisite steps to make the technique fully analytical, from sample preparation, through modifications of the electrodes, characterization of electrolytes, and finally to electrochemical corrosion experiments comparing in situ TEM to conventional bulk cell and microcell configurations.

Temperature Programmed Reduction of a PdCu Bimetallic Catalyst via Atmospheric Pressure in situ STEM-EDS and in situ X-Ray Adsorption Analysis

Temperature programmed reduction (TPR) is a classical method for the analysis of metal catalysts but provides little detail of what is actually occurring in the system if other methods are not employed to determine which changes are taking place. Coupling TPR with in situ X-Ray absorption analysis allows specific changes in bulk metal oxidation state via analysis of the X-Ray absorption near edge structure (TPR-XANES) or particle size and alloy structure by analysis of the extended X-Ray fine structure.(TPR-EXAFS)[1]. Recently, it has become possible to observe temperature programmed reduction via in situ scanning transmission electron microscope (STEM) energy dispersive X-ray spectroscopy (XEDS) at hydrogen pressures up to one atmosphere [2]. This correlative study uses a combination of: TPR, XANES, XEDS and S/TEM methodologies to study the evolution of particle morphologies on a PdCu/TiO2 catalyst and explain the reduction profile observed in the conventional TPR of this system. This catalyst is representative of a large group of bimetallic catalysts useful for a range of chemical conversions including catalytic reforming, hydrotreating, emissions controls, and biomass conversion.

Stability, composition and core-shell particle structure of uranium(IV)-silicate colloids

Uranium is typically the most abundant radionuclide by mass in radioactive wastes and is a significant component of effluent streams at nuclear facilities. Actinide(IV) (An(IV)) colloids formed via various pathways, including corrosion of spent nuclear fuel, have the potential to greatly enhance the mobility of poorly soluble An(IV) forms, including uranium. This is particularly important in conditions relevant to decommissioning of nuclear facilities and the geological disposal of radioactive waste. Previous studies have suggested that silicate could stabilize U(IV) colloids. Here the formation, composition, and structure of U(IV)-silicate colloids under the alkaline conditions relevant to spent nuclear fuel storage and disposal were investigated using a range of state of the art techniques. The colloids are formed across a range of pH conditions (9-10.5) and silicate concentrations (2-4 mM) and have a primary particle size 1-10 nm, also forming suspended aggregates <220 nm. X-ray absorption spectroscopy, ultrafiltration, and scanning transmission electron microscopy confirm the particles are U(IV)-silicates. Additional evidence from X-ray diffraction and pair distribution function data suggests the primary particles are composed of a UO2-rich core and a U-silicate shell. U(IV)-silicate colloids formation correlates with the formation of U(OH)3(H3SiO4)32- complexes in solution indicating they are likely particle precursors. Finally, these colloids form under a range of conditions relevant to nuclear fuel storage and geological disposal of radioactive waste and represent a potential pathway for U mobility in these systems.