Direct identification of the charge state in a single platinum nanoparticle on titanium oxide

Correlating Photo-Induced Changes in Surface Charge and Electronic Conductivity in Oxide Nanoparticles with In Situ Electron Microscopy

Photon induced changes in charge distributions and conductivities of oxide nanoparticles (rhodium doped strontium titanate) have been determined using in situ electron holography. The holography-based approach relies on the application of two distinct stimuli to the material of interest: electrons and photons. The high energy electron beam stimulates the formation of a layer of positive surface charge due to secondary electron emission. Light illumination reduces this charge due to enhanced electronic conductivity arising from photoelectron excitation. For moderate photon and electron illumination rates, there is a simple linear relationship between the steady state surface charge and the sample conductivity. For rhodium doped strontium titanate, we observe a factor of 3 increase in the conductivity for the illumination conditions employed here. The approach is general and can be employed to measure photoinduced changes in other semiconducting systems.

The Role of Lattice Distortion in Catalysis: Functionality and Distinctions from Strain

Achieving high-performance catalysts is imperative for clean energy and environmental applications. In this context, an expanding body of research underscores the critical significance of structural modifications, with lattice distortion emerging as an intrinsic reconfiguration of atomic arrangements that profoundly influences catalytic processes. By contrast, strain typically arises from interfacial mismatches or external forces. Building on these distinctions, this review systematically compares these concepts, examining their definitions, origins, criteria, characterization methods, and impacts on catalytic activity. Special emphasis is placed on the mechanistic roles of lattice distortion in catalysis, particularly its ability to enhance function through intrinsic structure modification, carrier migration dynamics modulation, surface chemistry modulation, and enhanced catalyst stability. Furthermore, the impact of lattice distortion on enhancing catalytic reactivity is elucidated by influencing molecular adsorption and activation, optimizing reaction pathways, tailoring active sites, and coupling with spin polarization effects to promote efficient catalytic performance. Finally, the remaining challenges and future outlook in the synergistic regulation of local distortion and strain, multi-scale dynamic in-situ characterization, and sustainable strategies for practical applications are discussed, offering valuable insights for advancing efficient and scalable chemical and energy transformation technologies.

A d-Electron Deficient Pd Trimer for Exceptional Pyridine Hydrogenation Activity and Selectivity

The selective hydrogenation of pyridines containing reducible groups such as 2-phenylpyridine (PPY) typically has low yields due to strong nitrogen coordination with the metal as well as nonselective and over-hydrogenation. We report the synthesis of a novel Pd trimer catalyst through confined growth on an ordered mesoporous carrier, characterized by a 0.42 d-electron deficiency to address this challenge. This catalyst achieved a nearly complete conversion of 2-phenylpyridine and selectivity to 2-phenylpiperidine (PPD), maintaining its performance across eight batch cycles and continuous flow in the liquid phase for 800 h with negligible loss of activity or selectivity. We discuss the roles of active sites, including Pd d charge and ensemble structure, in relation to activation entropy, a Hammett study, and the adsorption configuration of the reactant. The exceptional 2-phenylpyridine hydrogenation activity and selectivity are attributed to the adsorption constraint of the pyridyl ring and the stabilization of the negatively charged transition state in the rate-determining step produced by the d-electron deficient Pd trimer.

Resolution improvement of differential phase-contrast microscopy via tilt-series acquisition for environmental cell application

A simple method that improves the resolution of phase measurement in differential phase-contrast scanning transmission electron microscopy for closed-type environmental cell applications was developed and tested using a model sample simulating environmental cell observations. Because the top and bottom membranes of an environmental cell are typically far apart, the images from these membranes are shifted widely by tilt-series acquisition, and averaging the images after alignment can effectively eliminate undesired signals from the membranes while improving the signal from the object of interest. It was demonstrated that a phase precision of 2π/100 rad is well achievable using the proposed method for the sample in an environmental cell.

Theory of Electro-Ionic Perturbations at Supported Electrocatalyst Nanoparticles

Nanoscopic heterogeneities in composition and structure are quintessential for the properties of electrocatalyst materials. Here, we present a semiclassical model to study the electrochemical properties of supported electrocatalyst nanoparticles (NP). The model captures the correlated electronic and ionic equilibration across NP, support, and electrolyte. It reveals peculiar trends in surface charging of the supported NP, validated by comparison with first-principles calculations. Support-induced perturbations in electronic and ionic charge densities at the NP's active surface manifest as distinct potentials of zero local electronic and ionic charges that could differ by more than 0.5 V in the studied system.

Adjustment of Molecular Sorption Equilibrium on Catalyst Surface for Boosting Catalysis

ConspectusFor chemical reactions with complex pathways, it is extremely difficult to adjust the catalytic performance. The previous strategies on this issue mainly focused on modifying the fine structures of the catalysts, including optimization of the geometric/electronic structure of the metal nanoparticles (NPs), regulation of the chemical composition/morphology of the supports, and/or adjustment of the metal-support interactions to modulate the reaction kinetics on the catalyst surface. Although significant advances have been achieved, the catalytic performance is still unsatisfactory.It is accepted that the chemical equilibrium of a reaction can be disturbed by changing the concentration of the reactants or products, and the equilibrium will shift to another side to offset the perturbation until a new equilibrium is established. This is known as Le Chatelier's principle. Following this understanding, we show that the catalytic performance can be significantly modulated by adjusting the molecular sorption equilibrium on the catalyst surface. For example, enriching the reactants and/or intermediates on the catalyst surface pushes the reaction forward, thus increasing the catalytic conversion; removing the product away from the catalyst surface improves the catalytic conversion and product selectivity; and inhibiting the side reactions enhances the product selectivity and catalyst durability. Using these strategies has successfully enhanced the catalytic performances in many challenging reactions, such as increasing H2O2 concentration around the metal active sites to enhance methane oxidation, enriching olefin on the catalyst surface to boost hydroformylation, selective combustion of H2 to shift the reaction equilibrium and improve ethane conversion in ethane dehydrogenation, and removing water from the reaction system to enhance Fischer-Tropsch synthesis. The key to these successes is effectively shifting the molecular sorption equilibrium under the working conditions.In this Account, we briefly summarize recent advances in adjusting molecular sorption equilibrium for boosting catalysis, with a focus on the equilibrium shift for a desired pathway by the unique functions of zeolites and polymers such as silanol nests on zeolite for olefin adsorption, the "molecular fence" effect of zeolite for H2O2 enrichment, MFI zeolite nanosheets for olefin diffusion, and the hydrophobic zeolite sheath and polymer for water separation/diffusion. We report the adjustment of the molecular sorption equilibrium on the catalyst surface via enriching the reactants and intermediates, removing the products, and inhibiting the side reactions to enhance the catalytic performance. As a result, high activity, excellent selectivity, and outstanding durability of the catalysts were achieved. In addition, current challenges and perspectives of applying this strategy to more important industrial reactions are discussed. Applications of advanced characterization tools, machine learning, and artificial intelligence for monitoring the dynamic structural changes of the catalyst and predicting the structural evolutions under working conditions are anticipated to continuously play important roles in catalyst design. We believe that this strategy will open a door for the development of highly efficient catalysts with potential applications in the future.

Fermi Level Equilibration and Charge Transfer at the Exsolved Metal-Oxide Interface

Exsolution is a promising approach for fabricating oxide-supported metal nanocatalysts through redox-driven metal precipitation. A defining feature of exsolved nanocatalysts is their anchored metal-oxide interface, which exhibits exceptional structural stability in (electro)catalysis. However, the electronic interactions at this unique interface remain unclear, despite their known impact on catalytic performance. In this study, we confirm charge transfer between the host oxide and the exsolved metal by demonstrating a two-stage Fermi level (EF) evolution on SrTi0.65Fe0.35O3-δ (STF) during metallic iron (Fe0) exsolution. Combining ambient pressure X-ray photoelectron spectroscopy with theoretical analysis, we show that EF initially rises due to electron doping from oxygen vacancy formation in STF. Subsequently, upon Fe0 precipitation, EF stabilizes and becomes insensitive to further oxygen release in STF, driven by EF equilibration and charge transfer between STF and the exsolved Fe0. These findings highlight the importance of considering electronic metal-support interactions when optimizing exsolved nanocatalysts.

Colloidal Nanoparticles Hold Elementary Charge in Nonpolar Solvent

The net charge of individual nanoparticles in nonpolar solvents plays a critical role in their intrinsic properties like charge carrier lifetime, electron transport, and interparticle interactions. However, there is a long-standing belief that the oil-dispersed nanoparticles inherently possess no net charge. This work presents an approach for directly quantifying the net charge of individual nanoparticles. We employ an alternating electric field coupled with in situ observation under an optical microscope to analyze the force and motion of sub-50 nm silver nanoparticles. This technique directly reveals that around 1-10% of these colloidal nanoparticles carry a single elementary charge in nonpolar solvents, while the majority remain essentially neutral. This represents the first quantitative measurement of a single nanoparticle's charge in a nonpolar solvent.

The Interfacial Interpenetration Effect for Controlled Reaction Stability of Palladium Catalysts

Tailoring well-defined interfacial structures of heterogeneous metal catalysts has become an effective strategy for identifying the interface relationships and facilitating the reactions involving multiple intermediates. Here, a particle-particle heterostructure catalyst consisting of Pd and copper oxide nanoparticles is designed to achieve high-performance alkaline methanol oxidation electrocatalysis. The strong coupling particle-particle heterostructure catalyst induced a unique interfacial interpenetration effect to improve the interfacial charge redistribution and regulate the d-band structure for optimizing the adsorption of CO intermediates on the catalyst. The resulting catalyst shows impressive mass activity (4.0 A mgPd-1) and current density (215.8 mA cm-2) for the methanol oxidation reaction (MOR) in alkaline media, which is 80.0 and 154.1 times higher than 10% Pd/C. The catalyst also exhibited outstanding stability for the MOR without obvious mass activity decay after 30,000 cycles. Experimental results and theoretical simulation (DFT) studies show that the chemical bond of the Cu-O-Pd interface can be regulated by the Pd penetration effect, greatly improving the activity and stability of the MOR. The present work exhibits the superiority of the metal particle-metal oxide heterostructure interface toward the rational design of advanced electrocatalysts.

The Lattice Mismatch-Driven Photochemical Self-Assembly of Supported Heterostructures for Stable and Enhanced Electrocatalytic Carbon Dioxide Reduction Reaction

Metallic heterostructural nanocrystals (HNCs) hold immense potential in electrocatalytic carbon dioxide reduction reaction (CO2RR) owing to their abundant active sites and high intrinsic activity. However, a significant challenge still remains in achieving controlled nucleation and growth sites for HNCs on supports and comprehending the influence of the structure-activity relationship on electrocatalytic CO2RR performance. This work presents a photochemical self-assembly technique without the necessity for reducing agents or facet-specific capping agents. By controlling lattice mismatch and manipulating transfer paths of photo-generated carriers, we can precisely direct the growth sites and nucleation of nanocrystals, enabling the self-assembly of supported core-shell and Janus nanostructures. Compared to Pd(T)@Au core-shell HNCs with the same loading, Pd cube-Au Janus HNCs exhibit significantly enhanced selectivity and stability toward carbon monoxide (CO) production in CO2RR at less negative potentials. The Pd cube-Au Janus HNC electrocatalyst achieved a Faradaic efficiency (FE) of 92.6 ± 3.5% for CO electroreduction, accompanied by a current density of 72.3 mA·cm-2 at -0.58 V. This work provides an effective strategy for designing advanced supported tandem electrocatalysts to boost the selectivity and durability test of CO2RR.

Recent progresses in transmission electron microscopy studies of two-dimensional ferroelectrics

The rich potential of two-dimensional materials endows them with superior properties suitable for a wide range of applications, thereby attracting substantial interest across various fields. The ongoing trend towards device miniaturization aligns with the development of materials at progressively smaller scales, aiming to achieve higher integration density in electronics. In the realm of nano-scaling ferroelectric phenomena, numerous new two-dimensional ferroelectric materials have been predicted theoretically and subsequently validated through experimental confirmation. However, the capabilities of conventional tools, such as electrical measurements, are limited in providing a comprehensive investigation into the intrinsic origins of ferroelectricity and its interactions with structural factors. These factors include stacking, doping, functionalization, and defects. Consequently, the progress of potential applications, such as high-density memory devices, energy conversion systems, sensing technologies, catalysis, and more, is impeded. In this paper, we present a review of recent research that employs advanced transmission electron microscopy (TEM) techniques for the direct visualization and analysis of ferroelectric domains, domain walls, and other crucial features at the atomic level within two-dimensional materials. We discuss the essential interplay between structural characteristics and ferroelectric properties on the nanoscale, which facilitates understanding of the complex relationships governing their behavior. By doing so, we aim to pave the way for future innovative applications in this field.

Stabilizing Fe Single Atoms on Rutile-TiO2(110) Surface Via Atomic Substitution

Stable anchoring of dispersed metal atoms through either surface adsorption or lattice substitution on support surfaces is a prerequisite for highly efficient catalytic performance. Atomic-level insights into these processes are necessary to understand the metal-support interactions. Here, we identify multiple Fe single-atom configurations on the rutile-TiO2(110) surface using scanning tunneling microscopy (STM) and density functional theory (DFT). Our results show that an Fe atom can either adsorb on a surface O site (configuration I) or stably substitute a surface lattice Ti atom (configuration II). A transformation from configuration I to configuration II can be induced by STM manipulation. Furthermore, the substitutional Fe atom can capture an additional Fe atom to form a dual Fe-Fe complex (configuration III). DFT calculations reveal that these Fe species contribute different states in either the bandgap or the conduction band. These atomistic insights pave the way for interrogating the integrated performance of nonprecious, TiO2-supported Fe single-atom catalysts.

High-precision charge analysis in a catalytic nanoparticle by electron holography

The charge state of supported metal catalysts is the key to understand the elementary processes involved in catalytic reactions. However, high-precision charge analysis of the metal catalysts at the atomic level is experimentally challenging. To address this critical challenge, high-sensitivity electron holography has recently been successfully applied for precisely measuring the elementary charges on individual platinum nanoparticles supported on a titanium dioxide surface. In this review, we introduce the latest advancements in high-precision charge analysis and discuss the mechanisms of charge transfer at the metal-support interface. The development of charge measurements is entering a new era, and charge analyses under conditions closer to practical working environments, such as real-time, real-space, and reactive gas environments, are expected to be realized in the near future.

Removal of phase residues in electron holography

Electron holography provides quantitative phase information regarding the electromagnetic fields and the morphology of micro- to nano-scale samples. A phase image reconstructed numerically from an electron hologram sometimes includes phase residues, i.e. origins of unremovable phase discontinuities, which make it much more difficult to quantitatively analyze local phase values. We developed a method to remove the residues in a phase image by a combination of patching local areas of a hologram and denoising based on machine learning. The small patches for a hologram, which were generated using the spatial frequency information of the own fringe patterns, were pasted at each residue point by an algorithm based on sparse modeling. After successive phase reconstruction, the phase components with no dependency on the vicinity were filtered out by Gaussian process regression. We determined that the phase discontinuities that appeared around phase residues were removed and the phase distributions of an atomic resolution phase image of a Pt nanoparticle were sufficiently restored.

Local Charge Transfer Unveils Antideactivation of Ru at High Potentials for the Alkaline Hydrogen Oxidation Reaction

The activity of Ru-based alkaline hydrogen oxidation reaction (HOR) electrocatalysts usually decreases rapidly at potentials higher than 0.1 V (vs a reversible hydrogen electrode (RHE)), which significantly limits the lifetime of fuel cells. It is found that this phenomenon is caused by the overadsorption of the O species due to the overcharging of Ru nanoparticles at high potentials. Here, Mn1Ox(OH)y clusters-modified Ru nanoparticles (Mn1Ox(OH)y@Ru/C) were prepared to promote charge transfer from overcharged Ru nanoparticles to Mn1Ox(OH)y clusters. Mn1Ox(OH)y@Ru/C exhibits high HOR activity and stability over a wide potential range of 0-1.0 V. Moreover, a hydroxide exchange membrane fuel cell with a Mn1Ox(OH)y@Ru/C anode delivers a high peak power density of 1.731 W cm-2, much superior to that of a Pt/C anode. In situ X-ray absorption fine structure (XAFS) analysis and density functional theory (DFT) calculations reveal that Mn in Mn1Ox(OH)y clusters could receive more electrons from overcharged Ru at higher potentials and significantly decrease the overadsorption of the O species on Ru, thus permitting the HOR on Ru to proceed at high potentials. This study provides guidance for the design of alkaline HOR catalysts without activity decay at high potentials.

Metal-Support Interaction in Pt Nanodisk-Carbon Nitride Catalyst: Insight from Theory and Experiment

Metal-support interaction plays a critical role in determining the eventual catalytic activity of metals loaded on supporting substrates. This interaction can sometimes cause a significant drop in the metallic property of the loaded metal and, hence, a drop in catalytic activity in the reactions, especially in those for which low charge carrier transfer resistance is a necessary parameter. Therefore, there should be a case-by-case experimental or theoretical (or both) in-depth investigation to understand the role of support on each metal. Here, onto a layered porous carbon nitride (g-CN), we grew single crystalline Pt nanodisks (Pt@g-CN) with a lateral average size of 21 nm, followed by various characterisations such as electron microscopy techniques, and the measurement of electrocatalytic activity in the O2 reduction reaction (ORR). We found that intercalating Pt nanodisks in the g-CN interlayers causes an increase in electrocatalytic activity. We investigated the bonding mechanism between carbon support and platinum using density functional theory and applied the d-band theory to understand the catalytic performance. Analysis of Pt's density of states and electronic population across layers sheds light on the catalytic behaviour of Pt nanoparticles, particularly in relation to their thickness and proximity to the g-CN support interface. Our simulation reveals an optimum thickness of ~11 Å, under which the catalytic performance deteriorates.

Measuring Electrical Resistivity at the Nanoscale in Phase-Change Materials

Electrical resistivity is the key parameter in the active regions of many current nanoscale devices, from memristors to resistive random-access memory and phase-change memories. The local resistivity of the materials is engineered on the nanoscale to fit the performance requirements. Phase-change memories, for example, rely on materials whose electrical resistance increases dramatically with a change from a crystalline to an amorphous phase. Electrical characterization methods have been developed to measure the response of individual devices, but they cannot map the local resistance across the active area. Here, we propose a method based on operando electron holography to determine the local resistance within working devices. Upon switching the device, we show that electrical resistance is inhomogeneous on the scale of only a few nanometers.

Strong Interfacial Chemical Bonding in Regulating Electron Transfer and Stabilizing Catalytic Sites in a Metal-Semiconductor Schottky Junction for Enhanced Photocatalysis

The weak electronic interaction at metal-photocatalyst heterointerfaces often compromises solar-to-fuel performance. Here, a trifunctional Schottky junction, involving chemically stabilized ultrafine platinum nanoparticles (Pt NPs, ≈3 nm in diameter) on graphitic carbon nitride nanosheets (CNs) is proposed. The Pt-CN electronic interaction induces a 1.5% lattice compressive strain in Pt NPs and maintains their ultrafine size, effectively preventing their aggregation during photocatalytic reactions. Density functional theory calculations further demonstrate a significant reduction in the Schottky barrier at the chemically bonded CN-Pt heterointerface, facilitating efficient interfacial electron transfer, as supported by femtosecond transient absorption spectra (fs-TAS) measurements. The combined effects of lattice strain, stabilized Pt NPs, and efficient interfacial charge transport collaboratively enhance the photocatalytic performance, leading to over an 11-fold enhancement in visible light H2 production (8.52 mmol g-1 h-1) compared to the CN nanosheets with the in situ photo-deposited Pt NPs (0.76 mmol g-1 h-1). This study highlights the effectiveness of strong metal-semiconductor electronic interactions and underscores the potential for developing high-efficiency photocatalysts.

Spatially and Temporally Resolved Dynamic Response of Co-Based Composite Interface during the Oxygen Evolution Reaction

Interfacial interaction dictates the overall catalytic performance and catalytic behavior rules of the composite catalyst. However, understanding of interfacial active sites at the microscopic scale is still limited. Importantly, identifying the dynamic action mechanism of the "real" active site at the interface necessitates nanoscale, high spatial-time-resolved complementary-operando techniques. In this work, a Co3O4 homojunction with a well-defined interface effect is developed as a model system to explore the spatial-correlation dynamic response of the interface toward oxygen evolution reaction. Quasi in situ scanning transmission electron microscopy-electron energy-loss spectroscopy with high spatial resolution visually confirms the size characteristics of the interface effect in the spatial dimension, showing that the activation of active sites originates from strong interfacial electron interactions at a scale of 3 nm. Multiple time-resolved operando spectroscopy techniques explicitly capture dynamic changes in the adsorption behavior for key reaction intermediates. Combined with density functional theory calculations, we reveal that the dynamic adjustment of multiple adsorption configurations of intermediates by highly activated active sites at the interface facilitates the O-O coupling and *OOH deprotonation processes. The dual dynamic regulation mechanism accelerates the kinetics of oxygen evolution and serves as a pivotal factor in promoting the oxygen evolution activity of the composite structure. The resulting composite catalyst (Co-B@Co3O4/Co3O4 NSs) exhibits an approximately 70-fold turnover frequency and 20-fold mass activity than the monomer structure (Co3O4 NSs) and leads to significant activity (η10 ∼257 mV). The visual complementary analysis of multimodal operando/in situ techniques provides us with a powerful platform to advance our fundamental understanding of interfacial structure-activity relationships in composite structured catalysts.

Facilely Fabricated Zero-Bias Silicon-Based Plasmonic Photodetector in the Near-Infrared Region with a Schottky Barrier Properly Controlled by Nanoalloys

Plasmonic Schottky devices have attracted considerable attention for use in practical applications based on photoelectric conversion, because they enable light to be harvested below the bandgap of semiconductors. In particular, silicon-based (Si) plasmonic Schottky devices have great potential for useful photodetection in the near-infrared region. However, the internal quantum efficiency (IQE) values of previously reported devices are low because the Schottky barrier is excessively high. Here, we are the first to develop AuAg nanoalloy-n-type Si plasmonic Schottky devices by cathodic arc plasma deposition. Interestingly, it is found that a novel nanostructure, which leads to the improvement of responsivities, is formed. Moreover, these plasmonic nanostructures can be fabricated in only ∼1 min. The fabricated AuAg nanoparticle-film structure enables proper control of the Schottky barrier height and increases the area of the Schottky interface for electron transfer. As a result, the considerably enhanced IQE of our device at a telecommunication wavelength of 1310 nm (1550 nm) without external bias is 4.6 (6.5) times higher than those in previous reports, and these responsivities are a record high. This approach can be applied to realize efficient photodetection in the NIR region and extend the use of light below the bandgap of semiconductors. This paves the way for future application advancements in a variety of fields, including photodetection, imaging, photovoltaics, and photochemistry.

Significance of Epitaxial Growth of PtO2 on Rutile TiO2 for Pt/TiO2 Catalysts

TiO2-supported Pt species have been widely applied in numerous critical reactions involving photo-, thermo-, and electrochemical-catalysis for decades. Manipulation of the state of the Pt species in Pt/TiO2 catalysts is crucial for fine-tuning their catalytic performance. Here, we report an interesting discovery showing the epitaxial growth of PtO2 atomic layers on rutile TiO2, potentially allowing control of the states of active Pt species in Pt/TiO2 catalysts. The presence of PtO2 atomic layers could modulate the geometric configuration and electronic state of the Pt species under reduction conditions, resulting in a spread of the particle shape and obtaining a Pt/PtO2/TiO2 structure with more positive valence of Pt species. As a result, such a catalyst exhibits exceptional electrocatalytic activity and stability toward hydrogen evolution reaction, while also promoting the thermocatalytic CO oxidation, surpassing the performance of the Pt/TiO2 catalyst with no epitaxial structure. This novel epitaxial growth of the PtO2 structure on rutile TiO2 in Pt/TiO2 catalysts shows its potential in the rational design of highly active and economical catalysts toward diverse catalytic reactions.

Lattice-Plane-Dependent Distribution of Ce3+ at Pt and CeO2 Interfaces for Pt/CeO2 Catalysts

The interaction between a metal and a support, which is known as the metal-support interaction, often plays a determining role in the catalytic properties of supported metal catalysts. Herein, we have developed model Pt/CeO2 catalysts, which enabled us to investigate the interface atomic and electronic structures between Pt and the {001}, {011}, and {111} planes of CeO2 using scanning transmission electron microscopy and electron energy-loss spectroscopy. We found that the number of Ce3+ ions around the Pt nanoparticles followed the order {001} > {011} > {111}, which was the opposite order of the generally accepted stability of low index surfaces of CeO2. Systematic first-principles calculations revealed that the presence of Pt nanoparticles facilitated the formation of oxygen vacancies and that the appearance of the Ptδ+ state was preferred when Pt nanoparticles were in contact with CeO2 {001} planes due to direct charge transfer from Pt to CeO2. These results provide important insights into the nature of the metal-support interaction for a comprehensive understanding of the properties of supported metal catalysts.

Metal-support interaction boosts the stability of Ni-based electrocatalysts for alkaline hydrogen oxidation

Ni-based hydrogen oxidation reaction (HOR) electrocatalysts are promising anode materials for the anion exchange membrane fuel cells (AEMFCs), but their application is hindered by their inherent instability for practical operations. Here, we report a TiO2 supported Ni4Mo (Ni4Mo/TiO2) catalyst that can effectively catalyze HOR in alkaline electrolyte with a mass activity of 10.1 ± 0.9 A g-1Ni and remain active even up to 1.2 V. The Ni4Mo/TiO2 anode AEMFC delivers a peak power density of 520 mW cm-2 and durability at 400 mA cm-2 for nearly 100 h. The origin for the enhanced activity and stability is attributed to the down-shifted d band center, caused by the efficient charge transfer from TiO2 to Ni. The modulated electronic structure weakens the binding strength of oxygen species, rendering a high stability. The Ni4Mo/TiO2 has achieved greatly improved stability both in half cell and single AEMFC tests, and made a step forward for feasibility of efficient and durable AEMFCs.

Enhancing performance of electron holography with mathematical and machine learning-based denoising techniques

Electron holography is a useful tool for analyzing functional properties, such as electromagnetic fields and strains of materials and devices. The performance of electron holography is limited by the 'shot noise' inherent in electron micrographs (holograms), which are composed of a finite number of electrons. A promising approach for addressing this issue is to use mathematical and machine learning-based image-processing techniques for hologram denoising. With the advancement of information science, denoising methods have become capable of extracting signals that are completely buried in noise, and they are being applied to electron microscopy, including electron holography. However, these advanced denoising methods are complex and have many parameters to be tuned; therefore, it is necessary to understand their principles in depth and use them carefully. Herein, we present an overview of the principles and usage of sparse coding, the wavelet hidden Markov model and tensor decomposition, which have been applied to electron holography. We also present evaluation results for the denoising performance of these methods obtained through their application to simulated and experimentally recorded holograms. Our analysis, review and comparison of the methods clarify the impact of denoising on electron holography research.

A resistance-driven H2 gas sensor: high-entropy alloy nanoparticles decorated 2D MoS2

The need to use hydrogen (H2) gas has increasingly become important due to the growing demand for carbon-free energy sources. However, the explosive nature of H2 gas has raised significant safety concerns, driving the development of efficient and reliable detection. Although 2D materials have emerged as promising materials for hydrogen gas sensing applications due to their relatively high sensitivity, the incorporation of other nanomaterials into 2D materials can drastically improve both the selectivity and the sensitivity of sensors. In this work, high-entropy alloy nanoparticles using non-noble metals were used to develop a sensor for H2 gas detection. This chemical sensor was realized by decorating 2D MoS2 surfaces with multicomponent body-centered cubic (BCC) equiatomic Ti-Zr-V-Nb-Hf high-entropy alloy (HEA) nanoparticles. It was selective towards H2, over NH3, H2S, CH4, and C4H10, demonstrating widespread applications of this sensor. To understand the mechanisms behind the abnormal selectivity and sensitivity, density functional theory (DFT) calculations were performed, showing that the HEA nanoparticles can act as a chemical hub for H2 adsorption and dissociation, ultimately improving the performance of 2D material-based gas sensors.

Photoinduced Electrogenerated Chemiluminescence Imaging of Plasmonic Photoelectrochemistry at Single Nanocatalysts

In this study, we proposed a novel imaging technique, photoinduced electrogenerated chemiluminescence microscopy (PECLM), to monitor redox reactions driven by hot carriers on single gold nanoparticles (AuNPs) on TiO₂. Under laser irradiation, plasmon-generated hot carriers were separated by an electric field, leaving hot holes on the surface of AuNPs to drive ECL reactions. PECL intensity was highly sensitive to the number of hot carriers. Through quantitative image analysis, we found that PECL density on individual AuNPs decreased significantly with an increase in particle diameter, indicating that particle size has a significant impact on photoelectrochemical conversion efficiency. For the first time, we verified the feasibility of PECLM in mapping the catalytic activity of single photocatalysts. PECLM opens a new prospect for the in situ imaging of photocatalysis in a high-throughput way, which not only facilitates the optimization of plasmonic photocatalysts but also contributes to the dynamic study of photocatalytic processes on micro/nanointerfaces.

Metal Oxide-Supported Metal Catalysts for Electrocatalytic Oxygen Reduction Reaction: Characterization Methods, Modulation Strategies, and Recent Progress

The sluggish kinetics of the oxygen reduction reaction (ORR) with complex multielectron transfer steps significantly limits the large-scale application of electrochemical energy devices, including metal-air batteries and fuel cells. Recent years witnessed the development of metal oxide-supported metal catalysts (MOSMCs), covering single atoms, clusters, and nanoparticles. As alternatives to conventional carbon-dispersed metal catalysts, MOSMCs are gaining increasing interest due to their unique electronic configuration and potentially high corrosion resistance. By engineering the metal oxide substrate, supported metal, and their interactions, MOSMCs can be facilely modulated. Significant progress has been made in advancing MOSMCs for ORR, and their further development warrants advanced characterization methods to better understand MOSMCs and precise modulation strategies to boost their functionalities. In this regard, a comprehensive review of MOSMCs for ORR is still lacking despite this fast-developing field. To eliminate this gap, advanced characterization methods are introduced for clarifying MOSMCs experimentally and theoretically, discuss critical methods of boosting their intrinsic activities and number of active sites, and systematically overview the status of MOSMCs based on different metal oxide substrates for ORR. By conveying methods, research status, critical challenges, and perspectives, this review will rationally promote the design of MOSMCs for electrochemical energy devices.

Aldehyde Hydrogenation by Pt/TiO2 Catalyst in Aqueous Phase: Synergistic Effect of Oxygen Vacancy and Solvent Water

The aldehyde hydrogenation for stabilizing and upgrading biomass is typically performed in aqueous phase with supported metal catalysts. By combining density functional theory calculations and ab initio molecular dynamics simulations, the model reaction of formaldehyde hydrogenation with a Pt/TiO₂ catalyst is investigated with explicit solvent water molecules. In aqueous phase, both the O vacancy (Ov) on support and solvent molecules could donate charges to a Pt cluster, where the Ov could dominantly reduce the Pt cluster from positive to negative. During the formaldehyde hydrogenation, the water molecules could spontaneously protonate the O in the aldehyde group by acid/base exchange, generating the OH* at the metal-support interface by long-range proton transfer. By comparing the stoichiometric and reduced TiO₂ support, it is found that the further hydrogenation of OH* is hard on the positively charged Pt cluster over stoichiometric TiO₂. However, with the presence of Ov on reduced support, the OH* hydrogenation could become not only exergonic but also kinetically more facile, which prohibits the catalyst from poisoning. This mechanism suggests that both the proton transfer from solvent water molecules and the easier OH* hydrogenation from Ov could synergistically promote aldehyde hydrogenation. That means, even for such simple hydrogenation in water, the catalytic mechanism could explicitly relate to all of the metal cluster, oxide support, and solvent waters. Considering the ubiquitous Ov defects in reducible oxide supports and the common aqueous environment, this synergistic effect may not be exclusive to Pt/TiO₂, which can be crucial for supported metal catalysts in biomass conversion.

Counting charges per metal nanoparticle

Charges on a metal nanoparticle are measured with precision by electron holography.