Aakash Varambhia

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.

A combined approach for deposition and characterization of atomically engineered catalyst nanoparticles

The structure and composition of catalytic silver nanoparticles (Ag-NPs) fabricated through a novel gas condensation process has been characterized by Scanning Electron Microscopy (SEM) and Atom Probe Tomography (APT). SEM was used to confirm the number density and spatial distribution of Ag-NPs deposited directly onto standard silicon microposts used for APT experiments. Depositing nanoparticles (NPs) directly by this method eliminates the requirement for focussed ion beam (FIB) liftout, significantly decreasing APT specimen preparation time and enabling far more NPs to be examined. Furthermore, by encapsulating deposited particles before final FIB sharpening, the APT reconstruction methodologies have been improved over prior attempts, as demonstrated by comparison to the SEM data. Progress in these areas is vital to enable large-scale catalyst research efforts using APT, a technique, which offers significant potential to examine the detailed atomic-scale chemistry in a wide variety of catalytic NPs. Graphical Abstract

Ideal versus real: simulated annealing of experimentally derived and geometric platinum nanoparticles

Platinum nanoparticles find significant use as catalysts in industrial applications such as fuel cells. Research into their design has focussed heavily on nanoparticle size and shape as they greatly influence activity. Using high throughput, high precision electron microscopy, the structures of commercially available Pt catalysts have been determined, and we have used classical and quantum atomistic simulations to examine and compare them with geometric cuboctahedral and truncated octahedral structures. A simulated annealing procedure was used both to explore the potential energy surface at different temperatures, and also to assess the effect on catalytic activity that annealing would have on nanoparticles with different geometries and sizes. The differences in response to annealing between the real and geometric nanoparticles are discussed in terms of thermal stability, coordination number and the proportion of optimal binding sites on the surface of the nanoparticles. We find that annealing both experimental and geometric nanoparticles results in structures that appear similar in shape and predicted activity, using oxygen adsorption as a measure. Annealing is predicted to increase the catalytic activity in all cases except the truncated octahedra, where it has the opposite effect. As our simulations have been performed with a classical force field, we also assess its suitability to describe the potential energy of such nanoparticles by comparing with large scale density functional theory calculations.

Managing dose-, damage- and data-rates in multi-frame spectrum-imaging

As an instrument, the scanning transmission electron microscope is unique in being able to simultaneously explore both local structural and chemical variations in materials at the atomic scale. This is made possible as both types of data are acquired serially, originating simultaneously from sample interactions with a sharply focused electron probe. Unfortunately, such scanned data can be distorted by environmental factors, though recently fast-scanned multi-frame imaging approaches have been shown to mitigate these effects. Here, we demonstrate the same approach but optimized for spectroscopic data; we offer some perspectives on the new potential of multi-frame spectrum-imaging (MFSI) and show how dose-sharing approaches can reduce sample damage, improve crystallographic fidelity, increase data signal-to-noise, or maximize usable field of view. Further, we discuss the potential issue of excessive data-rates in MFSI, and demonstrate a file-compression approach to significantly reduce data storage and transmission burdens.

Determining EDS and EELS partial cross-sections from multiple calibration standards to accurately quantify bi-metallic nanoparticles using STEM

Spectroscopic signals such as EDS and EELS provide an effective way of characterising multi-element samples such as Pt-Co nanoparticles in STEM. The advantage of spectroscopy over imaging is the ability to decouple composition and mass-thickness effects for thin samples, into the number of various types of atoms in a sample. This is currently not possible for multi element samples using conventional ADF quantification techniques alone. With recent developments in microscope hardware and software, it is now possible to acquire the ADF, EDS and EELS signals simultaneously and at high speed. However, the methods of quantifying the signals emitted from the sample vary greatly. Most approaches use pure-element standards in the form of needles, nanoparticles and wedges to quantify the spectroscopic signal into either partial scattering cross-sections, zeta-factors or k-factors. But self-consistency between the different methods has not been verified and the units of the quantification are not standardised. We present a robust approach for measuring and combining ADF, EDS and EELS signals using needle and nanoparticle standards in units of the partial scattering cross-section. The partial scattering cross-section allows an easy interpretation of the signals emitted from the sample and enables accurate atom-counting of the sample.

Observation of metal nanoparticles at atomic resolution in Pt-based cancer chemotherapeutics

The chemotherapeutics cisplatin and oxaliplatin are important tools in the fight against cancer. Both compounds are platinum complexes. Aberration‐corrected scanning transmission electron microscopy using the annular dark‐field imaging mode now routinely provides single‐atom sensitivity with atomic number contrast. Here, this imaging mode is used to directly image the platinum within the two drugs in their dried form on an amorphous carbon support film. The oxaliplatin is found to have wetted the supporting amorphous carbon, forming disordered clusters suggesting that the platinum has remained within the complex. Conversely, the cisplatin sample reveals 1.8‐nm‐diameter metallic platinum clusters. The size and shape of the clusters do not appear to be dependent on drying rate nor formed by beam damage, which may suggest that they were present in the original drug solution.

An optical configuration for fastidious STEM detector calibration and the effect of the objective-lens pre-field: AN OPTICAL CONFIGURATION FOR FASTIDIOUS STEM DETECTOR CALIBRATION

In the scanning transmission electron microscope, an accurate knowledge of detector collection angles is paramount in order to quantify signals on an absolute scale. Here we present an optical configuration designed for the accurate measurement of collection angles for both image‐detectors and energy‐loss spectrometers. By deflecting a parallel electron beam, carefully calibrated using a diffraction pattern from a known material, we can directly observe the projection‐distortion in the post‐specimen lenses of probe‐corrected instruments, the 3‐fold caustic when an image‐corrector is fitted, and any misalignment of imaging detectors or spectrometer apertures. We also discuss for the first time, the effect that higher‐order aberrations in the objective‐lens pre‐field has on such an angle‐based detector mapping procedure.

From High-precision Imaging to High-performance Computing: Leveraging ADF-STEM Atom-counting and DFT for Catalyst Nano-metrology

Z-contrast imaging in the scanning transmission electron microscope (STEM) is a powerful tool to image precious metal heterogeneous catalysts at the atomic scale. When the annular dark-field (ADF) images from the STEM are quantified onto an absolute scale (Figure 1), it has been shown that it is possible to count the number of atoms in individual atomic columns of metallic nanoparticles and to estimate their three-dimensional structure [1]. In recent years further progress has been made in identifying the possible sources of error in the recording and analysis of quantitative annular dark-field (ADF STEM) images [2], in experiment-design, and in verifying the metrology by tomographic techniques. Of these developments, the move to fast multi-frame image-acquisition and -averaging has enabled the correction of experimental scanning-distortions, reductions in electron beam-damage of samples, and improvements in signal-noise ratio (SNR) [3]. Very recently, a new ADF image analysis best-practice, melding the benefits of both reference-simulation and unbiased statistical interpretation based analysis methods, has produced an atom counting method with even greater robustness [4,5]. Exploiting these recent technical developments, we obtain optimised raw data which is fed into highthroughput image processing tools revealing particle size, atom-counts etc. Unfortunately, our increased analysis throughput merely shifts the investigation bottleneck from data-processing to interpretation. To remedy this, we have developed a computationally-efficient genetic-algorithm based structure solving code (requiring a few tens of CPU hours per structure on a standard desktop PC) to retrieve likely lowenergy 3D particle structures which match the experimental observations.

Quantitative STEM of Catalyst Nanoparticles using ADF Imaging with Simultaneous EDS and EELS Spectroscopy.

Some of the key properties that affect the catalytic performance of nanoparticle ensembles are particle size, shape, surface-strain and composition. These parameters need to be measured from an ensemble of nanoparticles to obtain useful parameters to compare to catalytic activity. Using Scanning Transmission Electron Microscopy (STEM) it is possible to measure these parameters simultaneously from a nanoparticle. The Annular Dark Field (ADF) image can be used to obtain atomic positions [1,2], whereas Energy Dispersive X-ray (EDS) and Electron Energy Loss Spectroscopy (EELS) can be used to obtain high resolution compositional information. With careful instrument calibrations all three of these signals can be converted into quantitative scattering crosssections [3,4] to count the number of atoms along an atomic column of a nanoparticle.

Electrochemical CO Oxidation at Platinum on Carbon Studied through Analysis of Anomalous in Situ IR Spectra

The oxidation of adsorbed CO is a key reaction in electrocatalysis. It has been studied extensively on both extended model surfaces and on nanoparticles; however, correlation between the two is far from simple. Molecular insight into the reaction is often provided using in situ IR spectroscopy; however, practical challenges mean in situ studies on nanoparticles have yet to provide the same level of detail as those on model surfaces. Here we use a new approach to in situ IR spectroscopy to study the mechanism of CO adlayer oxidation on a commercial carbon-supported Pt catalyst. We observe bipolar IR absorption bands but develop a simple model to enable fitting. Quantitative analysis of band behavior during the oxidation prepeak using the model agrees well with previous analysis based on conventional absorption bands. A second linear CO band is observed during the main oxidation region and is assigned to the distinct contribution of CO on step as opposed to terrace sites. Analysis of the step and terrace CO bands during oxidation shows that oxidation begins on the terraces of the nanoparticles before CO on steps is removed. Further correlation of this behavior with the current shows that step CO is only lost in the first of the two main oxidation peaks.