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.

Endothermic reaction at room temperature enabled by deep-ultraviolet plasmons.

Metallic nanoparticles have been used to harvest energy from a light source and transfer it to adsorbed gas molecules, which results in a reduced chemical reaction temperature. However, most reported reactions, such as ethylene epoxidation, ammonia decomposition and H-D bond formation are exothermic, and only H-D bond formation has been achieved at room temperature. These reactions require low activation energies (<2 eV), which are readily attained using visible-frequency localized surface plasmons (from ~1.75 eV to ~3.1 eV). Here, we show that endothermic reactions that require higher activation energy (>3.1 eV) can be initiated at room temperature by using localized surface plasmons in the deep-UV range. As an example, by leveraging simultaneous excitation of multiple localized surface plasmon modes of Al nanoparticles by using high-energy electrons, we initiate the reduction of CO2 to CO by carbon at room temperature. We employ an environmental transmission electron microscope to excite and characterize Al localized surface plasmon resonances, and simultaneously measure the spatial distribution of carbon gasification near the nanoparticles in a CO2 environment. This approach opens a path towards exploring other industrially relevant chemical processes that are initiated by plasmonic fields at room temperature.

Site-selective CO disproportionation mediated by localized surface plasmon resonance excited by electron beam.

Recent reports of hot-electron-induced dissociation of small molecules, such as hydrogen, demonstrate the potential application of plasmonic nanostructures for harvesting light to initiate catalytic reactions. Theories have assumed that plasmonic catalysis is mediated by the energy transfer from nanoparticles to adsorbed molecules during the dephasing of localized surface plasmon (LSP) modes optically excited on plasmonic nanoparticles. However, LSP-induced chemical processes have not been resolved at a sub-nanoparticle scale to identify the active sites responsible for the energy transfer. Here, we exploit the LSP resonance excited by electron beam on gold nanoparticles to drive CO disproportionation at room temperature in an environmental scanning transmission electron microscope. Using in situ electron energy-loss spectroscopy with a combination of density functional theory and electromagnetic boundary element method calculations, we show at the subparticle level that the active sites on gold nanoparticles are where preferred gas adsorption sites and the locations of maximum LSP electric field amplitude (resonance antinodes) superimpose. Our findings provide insight into plasmonic catalysis and will be valuable in designing plasmonic antennas for low-temperature catalytic processes.

Low-Temperature Growth of Carbon Nanotubes Catalyzed by Sodium-Based Ingredients.

Synthesis of low-dimensional carbon nanomaterials such as carbon nanotubes (CNTs) is a key driver for achieving advances in energy storage, computing, and multifunctional composites, among other applications. Here, we report high-yield thermal chemical vapor deposition (CVD) synthesis of CNTs catalyzed by reagent-grade common sodium-containing compounds, including NaCl, NaHCO3 , Na2 CO3 , and NaOH, found in table salt, baking soda, and detergents, respectively. Coupled with an oxidative dehydrogenation reaction to crack acetylene at reduced temperatures, Na-based nanoparticles have been observed to catalyze CNT growth at temperatures below 400 °C. Ex situ and in situ transmission electron microscopy (TEM) reveal unique CNT morphologies and growth characteristics, including a vaporizing Na catalyst phenomenon that we leverage to create CNTs without residual catalyst particles for applications that require metal-free CNTs. Na is shown to synthesize CNTs on numerous substrates, and as the first alkali group metal catalyst demonstrated for CNT growth, holds great promise for expanding the understanding of nanocarbon synthesis.

Observation of Sub-10 nm Transition Metal Dichalcogenide Nanocrystals in Rapidly Heated van der Waals Heterostructures.

Two-dimensional materials, such as transition metal dichalcogenides (TMDCs), have the potential to revolutionize the field of electronics and photonics due to their unique physical and structural properties. This research presents a novel method for synthesizing crystalline TMDCs crystals with <10 nm size using ultrafast migration of vacancies at elevated temperatures. Through in situ and ex situ processing and using atomic-level characterization techniques, we analyzed the shape, size, crystallinity, composition, and strain distribution of these nanocrystals. These nanocrystals exhibit electronic structure signatures that differ from the 2D bulk: i.e., uniform mono- and multilayers. Further, our in situ, vacuum-based synthesis technique allows observation and comparison of defect and phase evolution in these crystals formed under van der Waals heterostructure confinement versus unconfined conditions. Overall, this research demonstrates a solid-state route to synthesizing uniform nanocrystals of TMDCs and lays the foundation for materials science in confined 2D spaces under extreme conditions.

Steam-created grain boundaries for methane C-H activation in palladium catalysts.

Defects may display high reactivity because the specific arrangement of atoms differs from crystalline surfaces. We demonstrate that high-temperature steam pretreatment of palladium catalysts provides a 12-fold increase in the mass-specific reaction rate for carbon-hydrogen (C­H) activation in methane oxidation compared with conventional pretreatments. Through a combination of experimental and theoretical methods, we demonstrate that an increase in the grain boundary density through crystal twinning is achieved during the steam pretreatment and oxidation and is responsible for the increased reactivity. The grain boundaries are highly stable during reaction and show specific rates at least two orders of magnitude higher than other sites on the palladium on alumina (Pd/Al2O3) catalysts. Theoretical calculations show that strain introduced by the defective structure can enhance C­H bond activation. Introduction of grain boundaries through laser ablation led to further rate increases.

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.

Unveiling Defect-Mediated Charge-Carrier Recombination at the Nanometer Scale in Polycrystalline Solar Cells.

Solar cells made of polycrystalline thin-films can outperform their single-crystalline counterparts despite the presence of grain boundaries (GBs). To unveil the influence of GBs, high spatial resolution characterization techniques are needed to measure local properties in their vicinity. However, results obtained using single technique may provide limited aspects about the GB effect. Here, we employ two techniques, near-field scanning photocurrent microscopy (NSPM) and scanning transmission electron microscope based cathodoluminescence spectroscopy (STEM-CL), to characterize CdTe solar cells at the nanoscale. The signal contrast from the grain interiors (GIs) to the GBs, for high-efficiency cells where CdTe is deposited at a high substrate temperature (500 °C) and treated by CdCl2, is found reverse from one technique to another. NSPM reveals increased photocurrents at the GBs, while STEM-CL shows reduced CL intensity and energy redshifts of the spectral peak at the GBs. The results are attributed to the increased nonradiative recombination and the band bending mediated by the surface defects and the shallow-level defects at GBs, respectively. We discuss the advantages of sample geometry for room-temperature STEM-CL and present numerical simulations as well as analytical models to extract the ratio of GB recombination velocity to minority carrier diffusivity that can be used for evaluating the GB effect in other polycrystalline solar cells.

In situ oxidation and reduction of cerium dioxide nanoparticles studied by scanning transmission electron microscopy.

Cerium dioxide nanocubes and truncated octahedra were reduced and oxidized in the scanning transmission electron microscope. The reduction process was stimulated by the electron beam and oxidation was supported by background gases in the microscope environment. High-angle annular dark field imaging is sensitive to local lattice distortions that arise as oxygen vacancies are created and cerium cations reduce enabling high spatial resolution characterization of this process with temporal resolution on the order of seconds. Such measurements enable us to differentiate and infer that the observed behavior between the nanocubes and truncated octahedra may be due to the difference in crystallographic termination of surfaces. In situ measurements taken with different partial pressures of oxygen reveal the cerium oxidation state and the dose rate threshold for the onset of beam reduction are influenced by the environment. Increasing oxygen partial pressure reduces the Ce3+ content and decreases susceptibility to electron beam driven reduction.