Complex Alloy and Heterostructure Nanoparticles Derived from Perovskite Oxide Precursors for Catalytic Dry Methane Reforming

We have developed a general route for combining Ni, Fe, Co, Cu, and Pd in a nanoparticle using the exsolution mechanism from LaFe0.7Ni0.1Co0.1Cu0.05Pd0.05O3 perovskite oxide precursors. The strong adherence of the nanoparticles to the support yields attractive catalytic properties for dry methane reforming, such as coke resistance and thermal sintering suppression. The reduction of the precursors at 700 and 900 °C yielded NiCoCuPd or NiFeCoCuPd nanoparticles with a size-dependent phase transition from a complex concentrated alloy to phase-separated heterostructures. Our findings indicate that smaller concentrated alloy nanoparticles (∼10 nm) are more effective for methane activation than larger (>20 nm) phase-separated nanoparticles

Lithiation of Rutile TiO₂‑Coated Si NWs Observed by in Situ TEM

Lithiation of Rutile TiO₂‑Coated Si NWs Observed by in Situ TE

Lithiation of Rutile TiO₂‑Coated Si NWs Observed by in Situ TEM

Lithiation of Rutile TiO₂‑Coated Si NWs Observed by in Situ TE

ZnO/CuO Heterojunction Branched Nanowires for Photoelectrochemical Hydrogen Generation

We report a facile and large-scale fabrication of three-dimensional (3D) ZnO/CuO heterojunction branched nanowires (b-NWs) and their application as photocathodes for photoelectrochemical (PEC) solar hydrogen production in a neutral medium. Using simple, cost-effective thermal oxidation and hydrothermal growth methods, ZnO/CuO b-NWs are grown on copper film or mesh substrates with various ZnO and CuO NWs sizes and densities. The ZnO/CuO b-NWs are characterized in detail using high-resolution scanning and transmission electron microscopies exhibiting single-crystalline defect-free b-NWs with smooth and clean surfaces. The correlation between electrode currents and different NWs sizes and densities are studied in which b-NWs with longer and denser CuO NW cores show higher photocathodic current due to enhanced reaction surface area. The ZnO/CuO b-NW photoelectrodes exhibit broadband photoresponse from UV to near IR region, and higher photocathodic current than the ZnO-coated CuO (core/shell) NWs due to improved surface area and enhanced gas evolution. Significant improvement in the photocathodic current is observed when ZnO/CuO b-NWs are grown on copper mesh compared to copper film. The achieved results offer very useful guidelines in designing b-NWs mesh photoelectrodes for high-efficiency, low-cost, and flexible PEC cells using cheap, earth-abundant materials for clean solar hydrogen generation at large scales

Quantitative and Atomic-Scale View of CO-Induced Pt Nanoparticle Surface Reconstruction at Saturation Coverage via DFT Calculations Coupled with <i>in Situ</i> TEM and IR

Atomic-scale insights into how supported metal nanoparticles catalyze chemical reactions are critical for the optimization of chemical conversion processes. It is well-known that different geometric configurations of surface atoms on supported metal nanoparticles have different catalytic reactivity and that the adsorption of reactive species can cause reconstruction of metal surfaces. Thus, characterizing metallic surface structures under reaction conditions at atomic scale is critical for understanding reactivity. Elucidation of such insights on high surface area oxide supported metal nanoparticles has been limited by less than atomic resolution typically achieved by environmental transmission electron microscopy (TEM) when operated under realistic conditions and a lack of correlated experimental measurements providing quantitative information about the distribution of exposed surface atoms under relevant reaction conditions. We overcome these limitations by correlating density functional theory predictions of adsorbate-induced surface reconstruction visually with atom-resolved imaging by <i>in situ</i> TEM and quantitatively with sample-averaged measurements of surface atom configurations by <i>in situ</i> infrared spectroscopy all at identical saturation adsorbate coverage. This is demonstrated for platinum (Pt) nanoparticle surface reconstruction induced by CO adsorption at saturation coverage and elevated (>400 K) temperature, which is relevant for the CO oxidation reaction under cold-start conditions in the catalytic convertor. Through our correlated approach, it is observed that the truncated octahedron shape adopted by bare Pt nanoparticles undergoes a reversible, facet selective reconstruction due to saturation CO coverage, where {100} facets roughen into vicinal stepped high Miller index facets, while {111} facets remain intact

Tunable, Endotaxial Inclusion of Crystalline Pt-Based Nanoparticles Inside a High-Quality Bronze TiO₂ Matrix

A series of high-quality bronze titanium oxide films containing endotaxially embedded Pt-based nanoparticles was fabricated using pulsed laser deposition under various oxygen partial pressures (0 to 50 mTorr). We found that morphological control over the embedded Pt nanoparticles is possible by varying the oxygen partial pressure during growth. We also found that the titanium oxide matrix plays an important role in controlling composition, shape, and distribution of the endotaxially embedded Pt-based nanoparticles over this range of oxygen partial pressure by affecting (1) the formation of a segregated layer of Pt–Ti alloy nanoparticles, in addition to the pure Pt nanoparticles, under vacuum, (2) the generation of crystallographic twinning, steps, and kinks within the Pt nanoparticles, and (3) the localized precipitation of Pt nanoparticles spatially confined and morphologically adapted to the extended defects within the matrix

Accordion Strain Accommodation Mechanism within the Epitaxially Constrained Electrode

The tremendous benefits of all-solid-state Li-ion batteries will only be reaped if the cycle-induced strain mismatch across the electrode/electrolyte interfaces can be managed at the atomic scale to ensure that structural coherency is maintained over the lifetime of the battery. We have discovered a unique strain accommodation mechanism within an epitaxially constrained high-performance bronze TiO₂ (TiO₂-B) electrode that relieves coherency stresses that arise upon Li insertion. In situ transmission electron microscopy observation reveals the formation of anatase shear bands within the TiO₂-B crystal that play the same role that interface dislocations do to relieve growth stresses. While first-principles calculations indicate that anatase is the favored crystal structure of TiO₂ in the lithiated state, its continued propagation is suppressed by the epitaxial constraints of the substrate. This discovery reveals an accordion-like mechanism relying on an otherwise undesirable structural transformation that can be exploited to manage the cyclic strain mismatch across the electrode/electrolyte interfaces that plague all solid-state batteries

Catalyst Architecture for Stable Single Atom Dispersion Enables Site-Specific Spectroscopic and Reactivity Measurements of CO Adsorbed to Pt Atoms, Oxidized Pt Clusters, and Metallic Pt Clusters on TiO₂

Oxide-supported precious metal nanoparticles are widely used industrial catalysts. Due to expense and rarity, developing synthetic protocols that reduce precious metal nanoparticle size and stabilize dispersed species is essential. Supported atomically dispersed, single precious metal atoms represent the most efficient metal utilization geometry, although debate regarding the catalytic activity of supported single precious atom species has arisen from difficulty in synthesizing homogeneous and stable single atom dispersions, and a lack of site-specific characterization approaches. We propose a catalyst architecture and characterization approach to overcome these limitations, by depositing ∼1 precious metal atom per support particle and characterizing structures by correlating scanning transmission electron microscopy imaging and CO probe molecule infrared spectroscopy. This is demonstrated for Pt supported on anatase TiO₂. In these structures, isolated Pt atoms, Pt_iso, remain stable through various conditions, and spectroscopic evidence suggests Pt_iso species exist in homogeneous local environments. Comparing Pt_iso to ∼1 nm preoxidized (Pt_ox) and prereduced (Pt_metal) Pt clusters on TiO₂, we identify unique spectroscopic signatures of CO bound to each site and find CO adsorption energy is ordered: Pt_iso ≪ Pt_metal < Pt_ox. Pt_iso species exhibited a 2-fold greater turnover frequency for CO oxidation than 1 nm Pt_metal clusters but share an identical reaction mechanism. We propose the active catalytic sites are cationic interfacial Pt atoms bonded to TiO₂ and that Pt_iso exhibits optimal reactivity because every atom is exposed for catalysis and forms an interfacial site with TiO₂. This approach should be generally useful for studying the behavior of supported precious metal atoms

New Atomic-Scale Insight into Self-Regeneration of Pt-CaTiO₃ Catalysts: Incipient Redox-Induced Structures Revealed by a Small-Angle Tilting STEM Technique

A small-angle tilting technique, applied to scanning transmission electron microscopy (STEM), was used together with multislice image simulation to reveal new atomic-scale information about the structural evolution of single-crystalline Pt-doped CaTiO₃ thin films, grown by pulsed laser deposition, that occurs in response to reduction and reoxidation treatments. Specifically, we were able to confirm that Pt atoms are randomly dispersed throughout the as-grown film, most often occupying Ti sites, show that the smallest (∼1 nm) Pt-rich clusters embedded within CaTiO₃ after reduction have a structure consistent with metallic Pt, and demonstrate that the Pt atoms from these clusters occupy mainly Ca sites, in the form of ordered Pt-atom arrays, after reoxidation

Surface-Engineered PtNi‑O Nanostructure with Record-High Performance for Electrocatalytic Hydrogen Evolution Reaction

Hydrogen holds the potential of replacing nonrenewable fossil fuel. Improving the efficiency of hydrogen evolution reaction (HER) is critical for environmental friendly hydrogen generation through electrochemical or photoelectrochemical water splitting. Here we report the surface-engineered PtNi-O nanoparticles with enriched NiO/PtNi interface on surface. Notably, PtNi-O/C showed a mass activity of 7.23 mA/μg at an overpotential of 70 mV, which is 7.9 times higher compared to that of the commercial Pt/C, representing the highest reported mass activity for HER in alkaline conditions. The HER overpotential can be lowered to 39.8 mV at 10 mA/cm² when platinum loading was only 5.1 μg_pt/cm², showing exceptional HER efficiency. Meanwhile, the prepared PtNi-O/C nanostructures demonstrated significantly improved stability as well as high current performance which are well over those of the commercial Pt/C and demonstrated capability of scaled hydrogen generation