Atomic Dynamics of Multi-Interfacial Migration and Transformations.

Redox-induced interconversions of metal oxidation states typically result in multiple phase boundaries that separate chemically and structurally distinct oxides and suboxides. Directly probing such multi-interfacial reactions is challenging because of the difficulty in simultaneously resolving the multiple reaction fronts at the atomic scale. Using the example of CuO reduction in H2 gas, a reaction pathway of CuO → monoclinic m-Cu4 O3 → Cu2 O is demonstrated and identifies interfacial reaction fronts at the atomic scale, where the Cu2 O/m-Cu4 O3 interface shows a diffuse-type interfacial transformation; while the lateral flow of interfacial ledges appears to control the m-Cu4 O3 /CuO transformation. Together with atomistic modeling, it is shown that such a multi-interface transformation results from the surface-reaction-induced formation of oxygen vacancies that diffuse into deeper atomic layers, thereby resulting in the formation of the lower oxides of Cu2 O and m-Cu4 O3 , and activate the interfacial transformations. These results demonstrate the lively dynamics at the reaction fronts of the multiple interfaces and have substantial implications for controlling the microstructure and interphase boundaries by coupling the interplay between the surface reaction dynamics and the resulting mass transport and phase evolution in the subsurface and bulk.

Dislocation-induced stop-and-go kinetics of interfacial transformations.

Most engineering materials are based on multiphase microstructures produced either through the control of phase equilibria or by the fabrication of different materials as in thin-film processing. In both processes, the microstructure relaxes towards equilibrium by mismatch dislocations (or geometric misfit dislocations) across the heterophase interfaces1-5. Despite their ubiquitous presence, directly probing the dynamic action of mismatch dislocations has been unachievable owing to their buried nature. Here, using the interfacial transformation of copper oxide to copper as an example, we demonstrate the role of mismatch dislocations in modulating oxide-to-metal interfacial transformations in an intermittent manner, by which the lateral flow of interfacial ledges is pinned at the core of mismatch dislocations until the dislocation climbs to the new oxide/metal interface location. Together with atomistic calculations, we identify that the pinning effect is associated with the non-local transport of metal atoms to fill vacancies at the dislocation core. These results provide mechanistic insight into solid-solid interfacial transformations and have substantial implications for utilizing structural defects at buried interfaces to modulate mass transport and transformation kinetics.

Mucoid Coating Provides a Growth Advantage to Pseudomonas aeruginosa at Oil-Water Interfaces.

Chronic lung infection with bacterial biofilms is one of the leading causes of death in cystic fibrosis (CF) patients. Among many species infecting the lung airways, Pseudomonas aeruginosa is the major pathogen colonizing and persisting throughout the patient's life. The microorganism undergoes pathoadaptation, while switching from a nonmucoid to a mucoid phenotype, improving the mechanical properties of the resulting biofilms. Previous investigation of the dynamic rheological properties of nonmucoid (PANT) and mucoid (PASL) clinical P. aeruginosa isolates exposed to interfacial stresses demonstrated that the mucoid strains formed films with stronger resistance to bending and nonlinear relaxation to compression and tension. We hypothesize that the mucoid switch provides a growth advantage to P. aeruginosa through the development of interfacial films with viscoelastic properties enabling cell survival. Here, we investigate the physiological response of the mucoid and the nonmucoid P. aeruginosa to interfacial entrapment. Our results, both macroscopic and molecular, reveal that mucoid coating plays an important role in protecting the bacteria from interfacial stresses. Cell characterizations using electron and fluorescence microscopies showed higher proportion of dead nonmucoid cells compared to mucoid cells on interfacial exposure. For example, scanning transmission electron microscopy (STEM) imaging showed that 96.6% of nonmucoid cells vs only 22.2% of mucoid cells were lysed owing to interfacial stress. Furthermore, the transcriptional profiling of P. aeruginosa cells indicated the upregulation of pel, psl, and alginate genes encoding for exopolysaccharide biomaterials is associated with mucoid cells' ability to cope with the interfacial environments. Further characterization of real-time gene regulation at interfaces will elucidate the effects of interfacial environment on the regulation of bacterial virulence.

Unusual layer-by-layer growth of epitaxial oxide islands during Cu oxidation.

Elucidating metal oxide growth mechanisms is essential for precisely designing and fabricating nanostructured oxides with broad applications in energy and electronics. However, current epitaxial oxide growth methods are based on macroscopic empirical knowledge, lacking fundamental guidance at the nanoscale. Using correlated in situ environmental transmission electron microscopy, statistically-validated quantitative analysis, and density functional theory calculations, we show epitaxial Cu2O nano-island growth on Cu is layer-by-layer along Cu2O(110) planes, regardless of substrate orientation, contradicting classical models that predict multi-layer growth parallel to substrate surfaces. Growth kinetics show cubic relationships with time, indicating individual oxide monolayers follow Frank-van der Merwe growth whereas oxide islands follow Stranski-Krastanov growth. Cu sources for island growth transition from step edges to bulk substrates during oxidation, contrasting with classical corrosion theories which assume subsurface sources predominate. Our results resolve alternative epitaxial island growth mechanisms, improving the understanding of oxidation dynamics critical for advanced manufacturing at the nanoscale.

Origin and Suppression of Beam Damage-Induced Oxygen-K Edge Artifact from γ-Al2O3 using Cryo-EELS.

Gamma-alumina (γ-Al2O3), like other low-Z oxides, is readily damaged when exposed to an electron beam. This typically results in the formation of a characteristic pre-edge peak in the oxygen-K edge of electron energy-loss spectra (EELS) acquired during or after the damage process. This artifact can mask the presence of intrinsic O-K edge fine structure that would reveal chemical properties of the material; therefore, its suppression is key. In this work, we systematically investigate the conditions that give rise to the damage-induced O-K pre-edge peak and show that it can be effectively suppressed by performing EELS experiments at cryogenic (cryo) temperatures. Prolonged exposure of γ-Al2O3 to a focused electron beam results in a hole bored through the sample; this was used as a reproducible beam damage condition. O-K edge EELS spectra were collected from a single-crystal γ-Al2O3 sample both during and after focused electron beam hole drilling, and at room and cryo temperatures, using a monochromated scanning transmission electron microscope (STEM). The characteristic 531 eV pre-edge peak visible in the room temperature EELS spectra was completely suppressed in the cryo-EELS spectra, even in the presence of a visible drilled hole. We then correlated these experimental observations with multiple-scattering EELS simulations to determine the likely atomistic origin of the damage-induced O-K pre-edge peak. The findings indicate that the pre-edge peak is caused primarily by the presence of surface O-O bonds formed during beam damage, and that operating at cryo temperature suppresses the formation of surface O-O bonds, thus preventing formation of the O-K pre-edge peak. Additionally, Al-L2,3 edge EELS spectra revealed Al loss primarily from tetrahedral sites during hole drilling.

Atomic-scale phase separation induced clustering of solute atoms.

Dealloying typically occurs via the chemical dissolution of an alloy component through a corrosion process. In contrast, here we report an atomic-scale nonchemical dealloying process that results in the clustering of solute atoms. We show that the disparity in the adatom-substrate exchange barriers separate Cu adatoms from a Cu-Au mixture, leaving behind a fluid phase enriched with Au adatoms that subsequently aggregate into supported clusters. Using dynamic, atomic-scale electron microscopy observations and theoretical modeling, we delineate the atomic-scale mechanisms associated with the nucleation, rotation and amorphization-crystallization oscillations of the Au clusters. We expect broader applicability of the results because the phase separation process is dictated by the inherent asymmetric adatom-substrate exchange barriers for separating dissimilar atoms in multicomponent materials.

Anomalous phonon relaxation in Au333(SR)79 nanoparticles with nascent plasmons.

Research on plasmons of gold nanoparticles has gained broad interest in nanoscience. However, ultrasmall sizes near the metal-to-nonmetal transition regime have not been explored until recently due to major synthetic difficulties. Herein, intriguing electron dynamics in this size regime is observed in atomically precise Au333(SR)79 nanoparticles. Femtosecond transient-absorption spectroscopy reveals an unprecedented relaxation process of 4-5 ps-a fast phonon-phonon relaxation process, together with electron-phonon coupling (∼1 ps) and normal phonon-phonon coupling (>100 ps) processes. Three types of -R capped Au333(SR)79 all exhibit two plasmon-bleaching signals independent of the -R group as well as solvent, indicating plasmon splitting and quantum effect in the ultrasmall core of Au333(SR)79 This work is expected to stimulate future work on the transition-size regime of nanometals and discovery of behavior of nascent plasmons.

Structural characterization of heterogeneous RhAu nanoparticles from a microwave-assisted synthesis.

A microwave assisted method was used to synthesize RhAu nanoparticles (NPs). Characterization, based upon transmission electron microscopy (TEM), energy dispersive spectroscopy, and powder X-ray diffraction, provided the evidence of monomodal alloy NPs with a mean size distribution between 3 and 5 nm, depending upon the composition. Extended X-ray adsorption fine-structure spectroscopy (EXAFS) also showed evidence of alloying, but the coordination numbers of Rh and Au indicated significant segregation between the metals. More problematic were the low coordination numbers for Rh; values of ca. 9 indicate NPs smaller than 2 nm, significantly smaller than those observed with TEM. Additionally, no single-particle structural models were able to reproduce the experimental EXAFS data. Resolution of this discrepancy was achieved with high resolution aberration corrected scanning TEM imaging which showed the presence of ultra-small (<2 nm) pure Rh clusters and larger (∼3-5 nm) segregated particles with Au-rich cores and Rh-decorated shells. A heterogeneous model with a mixture of ultrasmall pure Rh clusters and larger segregated Rh/Au NPs was able to explain the experimental measurements of the NPs over the range of compositions measured. The combination of density functional theory, EXAFS, and TEM allowed us to quantify the heterogeneity in the RhAu NPs. It was only through this combination of theoretical and experimental techniques that resulted in a bimodal distribution of particle sizes that was able to explain all of the experimental characterization data.

Interface Engineering of Gold Nanoclusters for CO Oxidation Catalysis.

Catalysts based on atomically precise gold nanoclusters serve as an ideal model to relate the catalytic activity to the geometrical and electronic structures as well as the ligand effect. Herein, we investigate three series of ligand (thiolate)-protected gold nanoclusters, including Au38(SR)24, Au36(SR')24, and Au25(SR″)18, with a focus on their interface effects using carbon monoxide (CO) oxidation as a probe reaction. The first comparison is within each series, which reveals the same trend for the three series that, rather than the bulkiness of carbon tails as commonly thought, the steric hindrance of ligands at the interface between the thiolate, Au, and CeO2 inhibits CO adsorption onto Au sites and hence adversely affects the activity of CO oxidation. The second comparison is between the sets Au38(SR)24 and Au36(SR')24 of nearly the same size, which reveals that the Au36(SR')24 nanoclusters (with face centered cubic structure) are not sensitive to thermal pretreatment conditions, whereas the Au38(SR)24 catalysts (icosahedral structure) are and an optimum activity is observed at a pretreatment temperature of 150 °C. Overall, the atomically precise Au n(SR) m nanoclusters have revealed unprecedented details on the catalytic interface and atomic structure effects. It is hoped that such insights will benefit the ultimate goal of catalysis in future design of enzymelike catalysts for environmentally friendly green catalysis.

X-ray Photoelectron Spectroscopy Studies of Nanoparticles Dispersed in Static Liquid.

For nanoparticles active for chemical and energy transformations in static liquid environment, chemistries of surface or near-surface regions of these catalyst nanoparticles in liquid are crucial for fundamentally understanding their catalytic performances at a molecular level. Compared to catalysis at a solid-gas interface, there is very limited information on the surface of these catalyst nanoparticles under a working condition or during catalysis in liquid. Photoelectron spectroscopy is a surface-sensitive technique; however, it is challenging to study the surfaces of catalyst nanoparticles dispersed in static liquid because of the short inelastic mean free path of photoelectrons traveling in liquid. Here, we report a method for tracking the surface of nanoparticles dispersed in static liquid by employing graphene layers as an electron-transparent membrane to separate the static liquid containing a solvent, catalyst nanoparticles, and reactants from the high-vacuum environment of photoelectron spectrometers. The surfaces of Ag nanoparticles dispersed in static liquid sealed in such a graphene membrane liquid cell were successfully characterized using a photoelectron spectrometer equipped with a high vacuum energy analyzer. With this method, the surface of catalyst nanoparticles dispersed in liquid during catalysis at a relatively high temperature up to 150 °C can be tracked with photoelectron spectroscopy.

Transition of surface phase of cobalt oxide during CO oxidation.

In situ/operando studies of a heterogeneous catalyst are particularly valuable for achieving a fundamental understanding of catalytic mechanisms at a molecular level by establishing a correlation between the observed catalytic performance and the corresponding surface chemistry during catalysis. Herein, CO oxidation on cobalt oxides was studied via ambient pressure X-ray photoelectron spectroscopy (AP-XPS). During CO oxidation on CoO in the temperature range of 140-180 °C, the active surface phase of CoO progressively transforms to Co3O4. Kinetic studies of CO oxidation on the surface phase CoO at 80-120 °C and on the formed Co3O4 at 160-220 °C show that CoO and Co3O4 exhibit different activation barriers: 49.3 kJ mol-1 for CoO and 36.9 kJ mol-1 for Co3O4. This study demonstrates the transition of the active surface phase of a transition metal oxide-based catalyst under catalytic conditions with no change in the bulk phase of the catalyst.

Atomically Precise Gold Nanoclusters Accelerate Hydrogen Evolution over MoS2 Nanosheets: The Dual Interfacial Effect.

Hydrogen generation via electrocatalytic water splitting holds great promise for future energy revolution. It is desirable to design abundant and efficient catalysts and achieve mechanistic understanding of hydrogen evolution reaction (HER). Here, this paper reports a strategy for improving HER performance of molybdenum disulfide (MoS2 ) via introducing gold nanoclusters as a cocatalyst. Compared to plain MoS2 nanosheets, the Au25 (SR)18 /MoS2 nanocomposite exhibits enhanced HER activity with a small onset potential of -0.20 V (vs reversible hydrogen electrode) and a higher current density of 59.3 mA cm-2 at the potential of -0.4 V. In addition to the interfacial interaction between nanoclusters and MoS2 , the interface between the Au25 core and the surface ligands (thiolate vs selenolate) is also discovered to distinctly affect the catalytic performance. This work highlights the promise of metal nanoclusters in boosting the HER performance via tailoring the interfacial electronic interactions between gold nanoclusters and MoS2 nanosheets, as well as the interface between metal core and surface ligands.

High-throughput, semi-automated quantitative STEM mass measurement of supported metal nanoparticles using a conventional TEM/STEM.

The adaptation of quantitative STEM techniques to enable atom-counting in supported metal nanoparticles with a modern, conventional (non-aberration-corrected) TEM/STEM (a JEOL JEM2100F) without the need for any modifications or special hardware is presented. No image simulation is required, either. This technique enables the practical analysis of the size, mass, and basic shape information of statistically robust populations of hundreds to thousands of nanoparticles. The methods for performing the necessary calibrations of the microscope and images are detailed. A user-friendly semi-automated analysis program was also written to facilitate high throughput. The program optimizes the analysis parameters, applying the procedure consistently across the entire dataset, enhancing the meaningfulness of the statistics as well as the reproducibility and transferability of the results. A series of atomically precise Au nanoparticles were used to validate the technique, which was determined to be accurate within a (nearly uniform) scaling factor of around two for the given instrument, and could be brought into better agreement with a calibration standard. The magnitude of the disparity was found to significantly and unexpectedly rely on the chosen magnification and spot size, the underlying reasons for which are unclear and likely instrument-dependent. The possible sources of error from the calibration and acquisition were examined and their impact on the accuracy and precision of quantification were estimated. The scattering cross-sections measured using this technique are relatively insensitive to moderate errors in the various detector calibrations but particularly sensitive to pixel size error.

Gold Nanoclusters Promote Electrocatalytic Water Oxidation at the Nanocluster/CoSe2 Interface.

Electrocatalytic water splitting to produce hydrogen comprises the hydrogen and oxygen evolution half reactions (HER and OER), with the latter as the bottleneck process. Thus, enhancing the OER performance and understanding the mechanism are critically important. Herein, we report a strategy for OER enhancement by utilizing gold nanoclusters to form cluster/CoSe2 composites; the latter exhibit largely enhanced OER activity in alkaline solutions. The Au25/CoSe2 composite affords a current density of 10 mA cm-2 at small overpotential of ∼0.43 V (cf. CoSe2: ∼0.52 V). The ligand and gold cluster size can also tune the catalytic performance of the composites. Based upon XPS analysis and DFT simulations, we attribute the activity enhancement to electronic interactions between nanocluster and CoSe2, which favors the formation of the important intermediate (OOH) as well as the desorption of oxygen molecules over Aun/CoSe2 composites in the process of water oxidation. Such an atomic level understanding may provide some guidelines for design of OER catalysts.

Towards Direct Synthesis of Alane: A Predicted Defect-Mediated Pathway Confirmed Experimentally.

Alane (AlH3 ) is a unique energetic material that has not found a broad practical use for over 70 years because it is difficult to synthesize directly from its elements. Using density functional theory, we examine the defect-mediated formation of alane monomers on Al(111) in a two-step process: (1) dissociative adsorption of H2 and (2) alane formation, which are both endothermic on a clean surface. Only with Ti dopant to facilitate H2 dissociation and vacancies to provide Al adatoms, both processes become exothermic. In agreement, in situ scanning tunneling microscopy showed that during H2 exposure, alane monomers and clusters form primarily in the vicinity of Al vacancies and Ti atoms. Moreover, ball milling of the Al samples with Ti (providing necessary defects) showed a 10 % conversion of Al into AlH3 or closely related species at 344 bar H2 , indicating that the predicted pathway may lead to the direct synthesis of alane from elements at pressures much lower than the 10(4)  bar expected from bulk thermodynamics.

Statistical analysis of support thickness and particle size effects in HRTEM imaging of metal nanoparticles.

High-resolution transmission electron microscopy (HRTEM) examination of nanoparticles requires their placement on some manner of support - either TEM grid membranes or part of the material itself, as in many heterogeneous catalyst systems - but a systematic quantification of the practical imaging limits of this approach has been lacking. Here we address this issue through a statistical evaluation of how nanoparticle size and substrate thickness affects the ability to resolve structural features of interest in HRTEM images of metallic nanoparticles on common support membranes. The visibility of lattice fringes from crystalline Au nanoparticles on amorphous carbon and silicon supports of varying thickness was investigated with both conventional and aberration-corrected TEM. Over the 1-4nm nanoparticle size range examined, the probability of successfully resolving lattice fringes differed significantly as a function both of nanoparticle size and support thickness. Statistical analysis was used to formulate guidelines for the selection of supports and to quantify the impact a given support would have on HRTEM imaging of crystalline structure. For nanoparticles ≥1nm, aberration-correction was found to provide limited benefit for the purpose of visualizing lattice fringes; electron dose is more predictive of lattice fringe visibility than aberration correction. These results confirm that the ability to visualize lattice fringes is ultimately dependent on the signal-to-noise ratio of the HRTEM images, rather than the point-to-point resolving power of the microscope. This study provides a benchmark for HRTEM imaging of crystalline supported metal nanoparticles and is extensible to a wide variety of supports and nanostructures.

Silver cluster formation, dynamics, and chemistry in metal-organic frameworks.

Synthetic methods used to produce metal nanoparticles typically lead to a distribution of particle sizes. In addition, creation of the smallest clusters, with sizes of a few to tens of atoms, remains very challenging. Nanoporous metal-organic frameworks (MOFs) are a promising solution to these problems, since their long-range crystalline order creates completely uniform pore sizes with the potential for both steric and chemical stabilization. We report a systematic investigation of silver nanocluster formation within MOFs using three representative MOF templates. The as-synthesized clusters are spectroscopically consistent with dimensions < or =1 nm, with a significant fraction existing as Ag(3) clusters, as shown by electron paramagnetic resonance. Importantly, we show conclusively that very rapid TEM-induced MOF degradation leads to agglomeration and stable, easily imaged particles, explaining prior reports of particles larger than MOF pores. These results solve an important riddle concerning MOF-based templates and suggest that heterostructures composed of highly uniform arrays of nanoparticles within MOFs are feasible.