Spectroscopic Observation and Modeling of Photonic Modes in CeO2 Nanostructures.

Photonic modes in dielectric nanostructures, e.g., wide gap semiconductor like CeO2 (ceria), have the potential for various applications such as information transmission and sensing technology. To fully understand the properties of such phenomenon at the nanoscale, electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope was employed to detect and explore photonic modes in well-defined ceria nanocubes. To facilitate the interpretation of the observations, EELS simulations were performed with finite-element methods. The simulations allow the electric and magnetic field distributions associated with different modes to be determined. A simple analytical eigenfunction model was also used to estimate the energy of the photonic modes. In addition, by comparing various spectra taken at different location relative to the cube, the effect of the surrounding environment on the modes could be sensed. This work gives a high-resolution description of the photonic modes' properties in nanostructures, while demonstrating the advantage of EELS in characterizing optical phenomena locally.

In Situ TEM under Optical Excitation for Catalysis Research.

In situ characterization of materials in their operational state is a highly active field of research. Investigating the structure and response of materials under stimuli that simulate real working environments for technological applications can provide new insight and unique input to the synthesis and design of novel materials. Over recent decades, experimental setups that allow different stimuli to be applied to a sample inside an electron microscope have been devised, built, and commercialized. In this review, we focus on the in situ investigation of optically active materials using transmission electron microscopy. We illustrate two different approaches for exposing samples to light inside the microscope column, explaining the importance of different aspects of their mechanical construction and choice of light source and materials. We focus on the technical challenges of the setups and provide details of the construction, providing the reader with input on deciding which setup will be more useful for a specific experiment. The use of these setups is illustrated using examples from the literature of relevance to photocatalysis and nanoparticle synthesis.

Modeling surface spin polarization on ceria-supported Pt nanoparticles.

In this work, we employ density functional theory simulations to investigate possible spin polarization of CeO2-(111) surface and its impact on the interactions between a ceria support and Pt nanoparticles. With a Gaussian type orbital basis, our simulations suggest that the CeO2-(111) surface exhibits a robust surface spin polarization due to the internal charge transfer between atomic Ce and O layers. In turn, it can lower the surface oxygen vacancy formation energy and enhance the oxide reducibility. We show that the inclusion of spin polarization can significantly reduce the major activation barrier in the proposed reaction pathway of CO oxidation on ceria-supported Pt nanoparticles. For metal-support interactions, surface spin polarization enhances the bonding between Pt nanoparticles and ceria surface oxygen, while CO adsorption on Pt nanoparticles weakens the interfacial interaction regardless of spin polarization. However, the stable surface spin polarization can only be found in the simulations based on the Gaussian type orbital basis. Given the potential importance in the design of future high-performance catalysts, our present study suggests a pressing need to examine the surface ferromagnetism of transition metal oxides in both experiment and theory.

Exploring Blob Detection to Determine Atomic Column Positions and Intensities in Time-Resolved TEM Images with Ultra-Low Signal-to-Noise.

Spatially resolved in situ transmission electron microscopy (TEM), equipped with direct electron detection systems, is a suitable technique to record information about the atom-scale dynamics with millisecond temporal resolution from materials. However, characterizing dynamics or fluxional behavior requires processing short time exposure images which usually have severely degraded signal-to-noise ratios. The poor signal-to-noise associated with high temporal resolution makes it challenging to determine the position and intensity of atomic columns in materials undergoing structural dynamics. To address this challenge, we propose a noise-robust, processing approach based on blob detection, which has been previously established for identifying objects in images in the community of computer vision. In particular, a blob detection algorithm has been tailored to deal with noisy TEM image series from nanoparticle systems. In the presence of high noise content, our blob detection approach is demonstrated to outperform the results of other algorithms, enabling the determination of atomic column position and its intensity with a higher degree of precision.

Atomic level fluxional behavior and activity of CeO2-supported Pt catalysts for CO oxidation.

Reducible oxides are widely used catalyst supports that can increase oxidation reaction rates by transferring lattice oxygen at the metal-support interface. There are many outstanding questions regarding the atomic-scale dynamic meta-stability (i.e., fluxional behavior) of the interface during catalysis. Here, we employ aberration-corrected operando electron microscopy to visualize the structural dynamics occurring at and near Pt/CeO2 interfaces during CO oxidation. We show that the catalytic turnover frequency correlates with fluxional behavior that (a) destabilizes the supported Pt particle, (b) marks an enhanced rate of oxygen vacancy creation and annihilation, and (c) leads to increased strain and reduction in the CeO2 support surface. Overall, the results implicate the interfacial Pt-O-Ce bonds anchoring the Pt to the support as being involved also in the catalytically-driven oxygen transfer process, and they suggest that oxygen reduction takes place on the highly reduced CeO2 surface before migrating to the interfacial perimeter for reaction with CO.

Role of Convergence and Collection Angles in the Excitation of Long- and Short-Wavelength Phonons with Vibrational Electron Energy-Loss Spectroscopy.

Current generation electron monochromators employed as attachments to scanning transmission electron microscopes (STEM) offer the ability to obtain vibrational information from materials using electron energy-loss spectroscopy (EELS). We show here that in crystals, long- and short-wavelength phonon modes can be probed simultaneously with on-axis vibrational STEM EELS. The long-wavelength phonons are probed via dipole scattering, while the short-wavelength modes are probed via impact scattering of the incident electrons. The localized character of the short-wavelength modes is demonstrated by scanning the electron beam across the edge of a hexagonal boron nitride nanoparticle. It is found that employing convergence angles that encompass multiple Brillouin zone boundaries enhances the short-wavelength phonon contribution to the vibrational energy-loss spectrum much more than that achieved by employing collection angles that encompass multiple Brillouin zone boundaries. Probing short-wavelength phonons at high spatial resolution with on-axis vibrational STEM EELS will help develop a fundamental connection between vibrational excitations and bonding arrangements at atomic-scale heterogeneities in materials.

Dynamic structure of active sites in ceria-supported Pt catalysts for the water gas shift reaction.

Oxide-supported noble metal catalysts have been extensively studied for decades for the water gas shift (WGS) reaction, a catalytic transformation central to a host of large volume processes that variously utilize or produce hydrogen. There remains considerable uncertainty as to how the specific features of the active metal-support interfacial bonding-perhaps most importantly the temporal dynamic changes occurring therein-serve to enable high activity and selectivity. Here we report the dynamic characteristics of a Pt/CeO2 system at the atomic level for the WGS reaction and specifically reveal the synergistic effects of metal-support bonding at the perimeter region. We find that the perimeter Pt0 - O vacancy-Ce3+ sites are formed in the active structure, transformed at working temperatures and their appearance regulates the adsorbate behaviors. We find that the dynamic nature of this site is a key mechanistic step for the WGS reaction.

Atomic Scale Characterization of Fluxional Cation Behavior on Nanoparticle Surfaces: Probing Oxygen Vacancy Creation/Annihilation at Surface Sites.

Oxygen vacancy creation and annihilation are key processes in nonstoichiometric oxides such as CeO2. The oxygen vacancy creation and annihilation rates on an oxide's surface partly govern its ability to exchange oxygen with the ambient environment, which is critical for a number of applications including energy technologies, environmental pollutant remediation, and chemical synthesis. Experimental methods to probe and correlate local oxygen vacancy reaction rates with atomic-level structural heterogeneities would provide significant information for the rational design and control of surface functionality; however, such methods have been unavailable to date. Here, we characterize picoscale fluxional behavior in cations using time-resolved in situ aberration-corrected transmission electron microscopy to locate atomic-level variations in oxygen vacancy creation and annihilation rates on oxide nanoparticle surfaces. Low coordination number sites such as steps and edges, as well as locally strained sites, exhibited the greatest number of cation displacements, implying enhanced surface oxygen vacancy activity at these sites. The approach has potential applications to a much wider class of materials and catalysis problems involving surface and interfacial transport functionalities.

Properties of Dipole-Mode Vibrational Energy Losses Recorded From a TEM Specimen.

The authors discuss the dipole vibrational modes that predominate in the energy-loss spectra of ionic materials below 1 eV, concentrating on thin-film specimens of typical transmission electron microscopy (TEM) thickness. The thickness dependence of the intensity is shown to be a useful guide to the bulk or surface character of vibrational peaks. The lateral and depth resolution of the energy-loss signal is investigated with the aid of finite-element calculations.

Chemical kinetics for operando electron microscopy of catalysts: 3D modeling of gas and temperature distributions during catalytic reactions.

In situ environmental transmission electron microscopy (ETEM) is a powerful tool for observing structural modifications taking place in heterogeneous catalysts under reaction conditions. However, to strengthen the link between catalyst structure and functionality, an operando measurement must be performed in which reaction kinetics and catalyst structure are simultaneously determined. To determine chemical kinetics for gas-phase catalysis, it is necessary to develop a reliable chemical engineering model to describe catalysis as well as heat and mass transport processes within the ETEM cell. Here, we establish a finite element model to determine the gas and temperature profiles during catalysis in an open-cell operando ETEM experiment. The model is applied to a SiO2-supported Ru catalyst performing CO oxidation. Good agreement is achieved between simulated compositions and those measured experimentally across a temperature range of 25 - 350 °C. In general, for lower conversions, the simulations show that the temperature and gas are relatively homogeneous within the hot zone of the TEM holder where the catalyst is located. The uniformity of gas and temperature indicates that the ETEM reactor system behavior approximates that of a continuously stirred tank reactor (CSTR). The large degree of gas-phase uniformity also allows one to estimate the catalytic conversion of reactants in the cell to within 10% using electron energy-loss spectroscopy. Moreover, the findings indicate that for reactant conversions below 35%, one can reliably evaluate the steady-state reaction rate of catalyst nanoparticles that are imaged on the TEM grid.

Tracking the picoscale spatial motion of atomic columns during dynamic structural change.

In many materials systems, such as catalytic nanoparticles, the ability to characterize dynamic atomic structural changes is important for developing a more fundamental understanding of functionality. Recent developments in direct electron detection now allow image series to be acquired at frame rates on the order of 1000 frames per second in bright-field transmission electron microscopy (BF TEM), which could potentially allow dynamic changes in the atomic structure of individual nanoparticles to be characterized with millisecond temporal resolution in favorable cases. However, extracting such data from TEM image series requires the development of computational methods that can be applied to very large datasets and are robust in the presence of noise and in the non-ideal imaging conditions of some types of environmental TEM experiments. Here, we present a two-dimensional Gaussian fitting algorithm to track the position and intensities of atomic columns in temporally resolved BF TEM image series. We have tested our algorithm on experimental image series of Ce atomic columns near the surface of a ceria (CeO2) nanoparticle with electron beam doses of ~125-5000 e-Å-2 per frame. The accuracy of the algorithm for locating atomic column positions is compared to that of the more traditional centroid fitting technique, and the accuracy of intensity measurements is evaluated as a function of dose per frame. The code developed here, and the methodology used to explore the errors and limitations of the measurements, could be applied more broadly to any temporally resolved TEM image series to track dynamic atomic column motion.