Machine Learning Molecular Dynamics Shows Anomalous Entropic Effect on Catalysis through Surface Pre-melting of Nanoclusters

Due to the superior catalytic activity and efficient utilization of noble metals, nanocatalysts are extensively used in the modern industrial production of chemicals. The surface structures of these materials are significantly influenced by reactive adsorbates, leading to dynamic behavior under experimental conditions. The dynamic nature poses significant challenges in studying the structure-activity relations of catalysts. Herein, we unveil an anomalous entropic effect on catalysis via surface pre-melting of nanoclusters through machine learning accelerated molecular dynamics and free energy calculation. We find that due to the pre-melting of shell atoms, there exists a non-linear variation in the catalytic activity of the nanoclusters with temperature. Consequently, two notable changes in catalyst activity occur at the respective temperatures of melting for the shell and core atoms. We further study the nanoclusters with surface point defects, i.e. vacancy and ad-atom, and observe significant decrease in the surface melting temperatures of the nanoclusters, enabling the reaction to take place under more favorable and milder conditions. These findings not only provide novel insights into dynamic catalysis of nanoclusters but also offer new understanding of the role of point defects in catalytic processes.

Surface Chemistry and Specific Surface Area Rule the Efficiency of Gold Nanoparticle Sensitizers in Proton Therapy

Gold nanoparticles (AuNPs) are currently the most studied radiosensitizers in proton therapy (PT) applicable for the treatment of solid tumors, where they amplify production of reactive oxygen species (ROS). However, it is underexplored how this amplification is correlated with the AuNPs' surface chemistry. To clarify this issue, we fabricated ligand-free AuNPs of different mean diameters by laser ablation in liquids (LAL) and laser fragmentation in liquids (LFL) and irradiated them with clinically relevant proton fields by using water phantoms. ROS generation was monitored by the fluorescent dye 7-OH-coumarin. Our findings reveal an enhancement of ROS production driven by I) increased total particle surface area, II) utilization of ligand-free AuNPs avoiding sodium citrate as a radical quencher ligands, and III) a higher density of structural defects generated by LFL synthesis, indicated by surface charge density. Based on these findings it may be concluded that the surface chemistry is a major and underexplored contributor to ROS generation and sensitizing effects of AuNPs in PT. We further highlight the applicability of AuNPs in vitro in human medulloblastoma cells.

Evoking ordered vacancies in metallic nanostructures toward a vacated Barlow packing for high-performance hydrogen evolution

Metallic nanostructures are commonly densely packed into a few packing variants with slightly different atomic packing factors. The structural aspects and physicochemical properties related with the vacancies in such nanostructures are rarely explored because of lack of an effective way to control the introduction of vacancy sites. Highly voided metallic nanostructures with ordered vacancies are however energetically high lying and very difficult to synthesize. Here, we report a chemical method for synthesis of hierarchical Rh nanostructures (Rh NSs) composed of ultrathin nanosheets, composed of hexagonal close-packed structure embedded with nanodomains that adopt a vacated Barlow packing with ordered vacancies. The obtained Rh NSs exhibit remarkably enhanced electrocatalytic activity and stability toward the hydrogen evolution reaction (HER) in alkaline media. Theoretical calculations reveal that the exceptional electrocatalytic performance of Rh NSs originates from their unique vacancy structures, which facilitate the adsorption and dissociation of H2O in the HER.

Potential-Dependent Morphology of Copper Catalysts During CO2 Electroreduction Revealed by In Situ Atomic Force Microscopy

Electrochemical AFM is a powerful tool for the real-space characterization of catalysts under realistic electrochemical CO2 reduction (CO2 RR) conditions. The evolution of structural features ranging from the micrometer to the atomic scale could be resolved during CO2 RR. Using Cu(100) as model surface, distinct nanoscale surface morphologies and their potential-dependent transformations from granular to smoothly curved mound-pit surfaces or structures with rectangular terraces are revealed during CO2 RR in 0.1 m KHCO3 . The density of undercoordinated copper sites during CO2 RR is shown to increase with decreasing potential. In situ atomic-scale imaging reveals specific adsorption occurring at distinct cathodic potentials impacting the observed catalyst structure. These results show the complex interrelation of the morphology, structure, defect density, applied potential, and electrolyte in copper CO2 RR catalysts.

Interlaced Pd-Ag nanowires rich in grain boundary defects for boosting oxygen reduction electrocatalysis

Given the high cost and poisoning issues of Pt, developing Pd-based catalysts as substitutes is highly essential. Although substantial progress has been made, the synthesis of Pd-based electrocatalysts with both high activity and stability in the oxygen reduction reaction (ORR) remains a challenge. In this work, we prepared Pd-Ag nanowires with up to micro-sized length and a diameter of ∼17 nm via a facile modified polyol method. The obtained Pd-Ag nanowires (NWs) exhibit interlaced features and are rich in grain boundary defects. Due to the continuous grain boundaries in the one-dimensional (1D) structure and the optimized composition, the synthesized Pd1Ag1 NWs show half-wave potential of 0.897 V and mass activity of 0.103 A mg-1 in alkaline media toward ORR, higher than those of both state-of-the-art Pt/C and other Pd-Ag counterparts. Significantly, after stability tests over 5000 cycles, Pd1Ag1 NWs shows a 2 mV positive shift, much better than that of Pt/C, exhibiting striking stability for ORR. This work may provide an avenue to construct advanced catalysts by surface defect engineering.

Integration of mesopores and crystal defects in metal-organic frameworks via templated electrosynthesis

Incorporation of mesopores and active sites into metal-organic framework (MOF) materials to uncover new efficient catalysts is a highly desirable but challenging task. We report the first example of a mesoporous MOF obtained by templated electrosynthesis using an ionic liquid as both electrolyte and template. The mesoporous Cu(II)-MOF MFM-100 has been synthesised in 100 seconds at room temperature, and this material incorporates crystal defects with uncoupled Cu(II) centres as evidenced by confocal fluorescence microscopy and electron paramagnetic resonance spectroscopy. MFM-100 prepared in this way shows exceptional catalytic activity for the aerobic oxidation of alcohols to produce aldehydes in near quantitative yield and selectivity under mild conditions, as well as having excellent stability and reusability over repeated cycles. The catalyst-substrate binding interactions have been probed by inelastic neutron scattering. This study offers a simple strategy to create mesopores and active sites simultaneously via electrochemical formation of crystal defects to promote efficient catalysis using MOFs.

Incorporating mesopores and active sites into metal-organic framework materials has proven advantageous for their catalytic application, but remains challenging to achieve. Here the authors obtain mesoporous, defect-rich metal-organic frameworks through templated electrosynthesis using ionic liquids as both electrolyte and template.

Bragg coherent diffractive imaging of single-grain defect dynamics in polycrystalline films

Polycrystalline material properties depend on the distribution and interactions of their crystalline grains. In particular, grain boundaries and defects are crucial in determining their response to external stimuli. A long-standing challenge is thus to observe individual grains, defects, and strain dynamics inside functional materials. Here we report a technique capable of revealing grain heterogeneity, including strain fields and individual dislocations, that can be used under operando conditions in reactive environments: grain Bragg coherent diffractive imaging (gBCDI). Using a polycrystalline gold thin film subjected to heating, we show how gBCDI resolves grain boundary and dislocation dynamics in individual grains in three-dimensional detail with 10-nanometer spatial and subangstrom displacement field resolution. These results pave the way for understanding polycrystalline material response under external stimuli and, ideally, engineering particular functions.

Design of Ultrathin Pt-Based Multimetallic Nanostructures for Efficient Oxygen Reduction Electrocatalysis

Nanocatalysts with high platinum (Pt) utilization efficiency are attracting extensive attention for oxygen reduction reactions (ORR) conducted at the cathode of fuel cells. Ultrathin Pt-based multimetallic nanostructures show obvious advantages in accelerating the sluggish cathodic ORR due to their ultrahigh Pt utilization efficiency. A focus on recent important developments is provided in using wet chemistry techniques for making/tuning the multimetallic nanostructures with high Pt utilization efficiency for boosting ORR activity and durability. First, new synthetic methods for multimetallic core/shell nanoparticles with ultrathin shell sizes for achieving highly efficient ORR catalysts are reviewed. To obtain better ORR activity and stability, multimetallic nanowires or nanosheets with well-defined structure and surface are further highlighted. Furthermore, ultrathin Pt-based multimetallic nanoframes that feature 3D molecularly accessible surfaces for achieving more efficient ORR catalysis are discussed. Finally, the remaining challenges and outlooks for the future will be provided for this promising research field.

Twin defects engineered Pd cocatalyst on C3N4 nanosheets for enhanced photocatalytic performance in CO2 reduction reaction

Photocatalytic conversion of CO₂ to value-added chemicals, a potential route to addressing the depletion of fossil fuels and anthropogenic climate change, is greatly limited by the low-efficient semiconductor photocatalyst. The integration of cocatalyst with light-harvesting semiconductor is a promising approach to enhancing the photocatalytic performance in CO₂ reduction reaction. The enhancement is greatly determined by the catalytic active sites on the surface of cocatalyst. Herein, we demonstrate that the photocatalytic performance in the CO₂ reduction reaction is greatly promoted by twin defects engineered Pd cocatalyst. In this work, Pd nanoicosahedrons with twin defects were in situ grown on C₃N₄ nanosheets, which effectively improve the photocatalytic performance in reduction of CO₂ to CO and CH₄ in comparison with Pd nanotetrahedrons without twin defects. It is proposed that the twin boundary (TB) terminations on the surface of Pd cocatalysts are highly catalytic active sites for CO₂ reduction reaction. Based on the proposed mechanism, the photocatalytic activity and selectivity in CO₂ reduction were further advanced through reducing the size of Pd icosahedral cocatalyst resulted from the increased surface density of TB terminations. The defect engineering on the surface of cocatalyst represents a novel route in realizing high-performance photocatalytic applications.

Defects by design: synthesis of palladium nanoparticles with extended twin defects and corrugated surfaces

Recent catalytic work has highlighted the importance of grain boundaries in the design of highly active catalyst materials due to the high energy of atoms at strained defect sites. In addition, undercoordinated atoms have long been known to contribute to the catalytic performance of metal nanoparticles. In this work, we describe a method for deliberately increasing the coverage of defect boundaries and undercoordinated atoms at the surfaces of well-defined, symmetric palladium nanoparticles. Careful control of the competitive interactions of chloride and bromide ions with the surface of twinned palladium nanoparticles is used to drive the growth of fin-like structures to extend the area of exposed twin boundaries while also inducing corrugation at the particle surface to add further undercoordinated sites. Mechanistic studies show surface passivation by bromide and etching by chloride in the presence of a low concentration of surfactant to be the key factors that tailor the surface of these nanoparticles, while the internal defect structure is controlled by reaction kinetics. Importantly, these basic principles of competition between surface passivation and etching as well as kinetic control of twin structure are not unique to palladium, and thus this method has the potential to be extended to the enhancement of surface defect density for nanoparticles composed of other catalytically relevant metals.

Template-Free Electroless Plating of Gold Nanowires: Direct Surface Functionalization with Shape-Selective Nanostructures for Electrochemical Applications

Metal nanowires (NWs) represent a prominent nanomaterial class, the interest in which is fueled by their tunable properties as well as their excellent performance in, for example, sensing, catalysis, and plasmonics. Synthetic approaches to obtain metal NWs mostly produce colloids or rely on templates. Integrating such nanowires into devices necessitates additional fabrication steps, such as template removal, nanostructure purification, or attachment. Here, we describe the development of a facile electroless plating protocol for the direct deposition of gold nanowire films, requiring neither templates nor complex instrumentation. The method is general, producing three-dimensional nanowire structures on substrates of varying shape and composition, with different seed types. The aqueous plating bath is prepared by ligand exchange and partial reduction of tetrachloroauric acid in the presence of 4-dimethylaminopyridine and formaldehyde. Gold deposition proceeds by nucleation of new grains on existing nanostructure tips and thus selectively produces curvy, polycrystalline nanowires of high aspect ratio. The nanofabrication potential of this method is demonstrated by producing a sensor electrode, whose performance is comparable to that of known nanostructures and discussed in terms of the catalyst architecture. Due to its flexibility and simplicity, shape-selective electroless plating is a promising new tool for functionalizing surfaces with anisotropic metal nanostructures.

Identifying Defects with Guided Algorithms in Bragg Coherent Diffractive Imaging

Crystallographic defects such as dislocations can significantly alter material properties and functionality. However, imaging these imperfections during operation remains challenging due to the short length scales involved and the reactive environments of interest. Bragg coherent diffractive imaging (BCDI) has emerged as a powerful tool capable of identifying dislocations, twin domains, and other defects in 3D detail with nanometer spatial resolution within nanocrystals and grains in reactive environments. However, BCDI relies on phase retrieval algorithms that can fail to accurately reconstruct the defect network. Here, we use numerical simulations to explore different guided phase retrieval algorithms for imaging defective crystals using BCDI. We explore different defect types, defect densities, Bragg peaks, and guided algorithm fitness metrics as a function of signal-to-noise ratio. Based on these results, we offer a general prescription for phasing of defective crystals with no a priori knowledge.

Electrodeposition of Rhodium Nanowires Arrays and Their Morphology-Dependent Hydrogen Evolution Activity

This work reports on the electrodeposition of rhodium (Rh) nanowires with a controlled surface morphology synthesized using an anodic aluminum oxide (AAO) template. Vertically aligned Rh nanowires with a smooth and coarse morphology were successfully deposited by adjusting the electrode potential and the concentration of precursor ions and by involving a complexing reagent in the electrolyte solution. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses were used to follow the morphological evolution of Rh nanowires. As a heterogeneous electrocatalyst for hydrogen evolution reactions (HER), the coarse Rh nanowire array exhibited an enhanced catalytic performance respect to smooth ones due to the larger surface area to mass ratio and the higher density of catalytically active defects, as evidenced by voltammetric measurements and TEM. Results suggest that the morphology of metallic nanomaterials could be readily engineered by electrodeposition. The controlled electrodeposition offers great potential for the development of an effective synthesis tool for heterogeneous catalysts with a superior performance for wide applications.

Submonolayered Ru Deposited on Ultrathin Pd Nanosheets used for Enhanced Catalytic Applications

Ultrathin Pd nanosheets (NSs) coated with submonolayered Ru, referred to as Pd@Ru NSs, are synthesized via a seed-mediated growth method. The underpotential deposition can be the driving force for the formation of Pd@Ru NSs. The Pd@Ru NSs exhibit superior catalytic properties in the reduction of 4-nitrophenol and the semihydrogenation of 1-octyne, compared to the pure Pd NSs and Ru NSs.

Creation of Controllable High-Density Defects in Silver Nanowires for Enhanced Catalytic Property

Structural defects have been proven to determine many of the materials' properties. Here, we demonstrate a unique approach to the creation of Ag nanowires with high-density defects through controllable nanoparticles coalescence in one-dimensional pores of mesoporous silica. The density of defects can be easily adjusted by tuning the annealing temperature during synthetic process. The high-density defects promote the adsorption and activation of more reactants on the surface of Ag nanowires during catalytic reactions. As a result, the as-prepared Ag nanowires exhibit enhanced activities in catalyzing dehydrogenative coupling reaction of silane in terms of apparent activation energy and turnover frequency (TOF). We show further that the silane conversion rate can be enhanced by maximizing the defect density and thus the number of active sites on the Ag nanowires, reaching a remarkable TOF of 8288 h(-1), which represents the highest TOF that has been achieved by far on Ag catalysts. This work not only proves the important role of structural defects in catalysis but also provides a new and general strategy for constructing high-density defects in metal catalysts.