Controlled synthesis of Pd-Pt alloy hollow nanostructures with enhanced catalytic activities for oxygen reduction

Electrifying Energy and Chemical Transformations with Single-Atom Alloy Nanoparticle Catalysts

Single-atom alloys (SAAs) have attracted considerable attention as promising electrocatalysts in reactions central to energy conversion and chemical transformation. In contrast to monometallic nanocrystals and metal alloys, SAAs possess unique and intriguing physicochemical properties, positioning them as ideal model systems for studying structure-property relationships. However, the field is still in its early stages. In this Perspective, we first review and summarize rational synthesis methods and advanced characterization techniques for SAA nanoparticle catalysts. We then emphasize the extensive applications of SAAs in a range of electrocatalytic reactions, including fuel cell reactions, water splitting, and carbon dioxide and nitrate reductions. Finally, we provide insights into existing challenges and prospects associated with the controlled synthesis, characterization, and design of SAA catalysts.

Cellulose/GO monolith covered with Pd-Pt bimetallic nanocrystals for continuous-flow catalytic reduction of hexavalent chromium

Cellulose monolith materials have interconnected open porous structures with very high porosity, making them attractive structures for use as support materials in heterogeneous catalysis applications. In this study, we developed a highly efficient and reusable continuous-flow reactor for Cr(VI) remediation by combining the advantageous features of cellulose monoliths with suitable reinforcement techniques. We fabricated a porous monolithic cellulose/graphene oxide (GO) composite with a continuous three-dimensional skeletal framework using the thermally induced phase separation technique. Pd nanocrystals were synthesized in situ on the surface of the composite monolith, and then converted to porous Pd-Pt bimetallic nanocrystals through a galvanic replacement reaction. This approach eliminated the need for additional reductants and stabilizers, making the process simpler and more environmentally friendly. Under carefully optimized conditions, the cellulose/GO/Pd-Pt nanocomposite monolith exhibited outstanding performance in continuous-flow reactions for Cr(VI) reduction, achieving a maximum conversion rate of 98 %. Moreover, the nanocomposite monolith-based heterogeneous catalyst exhibited remarkable long-term stability, maintaining its catalytic activity even after extended periods of storage in the dried state. These findings highlight the potential of cellulose-based composite monoliths as versatile and robust support materials for heterogeneous catalysis.

One-Step Electrochemical Synthesis of Multiyolk-Shell Nanocoils for Exceptional Photocatalytic Performance

Multiyolk-shell (mYS) nanostructures have garnered significant interest in various photocatalysis applications such as water splitting and waste treatment. Nonetheless, the complexity and rigorous conditions for the synthesis have hindered their widespread implementation. This study presents a one-step electrochemical strategy for synthesizing multiyolk-shell nanocoils (mYSNC), wherein multiple cores of noble metal nanoparticles, such as Au, are embedded within the hollow coil-shaped FePO4 shell structures, mitigating the challenges posed by conventional methods. By capitalizing on the dissimilar dissolution rates of bimetallic alloy nanocoils in an electrochemically programmed solution, nanocoils of different shapes and materials, including two variations of mYSNCs are successfully fabricated. The resulting Au-FePO4 mYSNCs exhibit exceptional photocatalytic performance for environmental remediation, demonstrating up to 99% degradation of methylene blue molecules within 50 min and 95% degradation of tetracycline within 100 min under ultraviolet-visible (UV-vis) light source. This remarkable performance can be attributed to the abundant electrochemical active sites, internal voids facilitating efficient light harvesting with coil morphology, amplified localized surface plasmon resonance (LSPR) at the plasmonic nanoparticle-semiconductor interface, and effective band engineering. The innovative approach utilizing bimetallic alloys demonstrates precise geometric control and design of intricate multicomponent hybrid composites, showcasing the potential for developing versatile hollow nanomaterials for catalytic applications.

Potential-Dependent Ionomer Rearrangement on the Pt Surface in Polymer Electrolyte Membrane Fuel Cells

The interface between the catalyst and the ionomer in the catalyst layer of polymer electrolyte membrane fuel cells (PEMFCs) has been a subject of keen interest, but its effect on durability has not been fully understood due to the complexity of the catalyst layer structure. Herein, we utilize a Pt nanoparticle (NP) array electrode fabricated using a block copolymer template as the platform for a focused investigation of the interfacial change between the Nafion thin film and the Pt NP under a constant potential. A set of analyses for the electrodes treated with various potentials reveals that the Nafion thin film becomes densely packed at the intermediate potentials (0.4 and 0.7 V), indicating an increased ionomer-catalyst interaction due to the positive charges formed at the Pt surface at these potentials. Even for a practical PEMFC single cell, we demonstrate that the potential holding at the intermediate potentials increases ionomer adsorption to the Pt surface and the oxygen transport resistance, negatively impacting its power performance. This work provides fresh insight into the mechanism behind the performance fade in PEMFCs caused by potential-dependent ionomer rearrangement.

Resolving Atomic-Scale Structure and Chemical Coordination in High-Entropy Alloy Electrocatalysts for Structure-Function Relationship Elucidation

The recent breakthrough in confining five or more atomic species in nanocatalysts, referred to as high-entropy alloy nanocatalysts (HEAs), has revealed the possibilities of multielemental interactions that can surpass the limitations of binary and ternary electrocatalysts. The wide range of potential surface configurations in HEAs, however, presents a significant challenge in resolving active structural motifs, preventing the establishment of structure-function relationships for rational catalyst design and optimization. We present a methodology for creating sub-5 nm HEAs using an aqueous-based peptide-directed route. Using a combination of pair distribution function and X-ray absorption spectroscopy, HEA structure models are constructed from reverse Monte Carlo modeling of experimental data sets and showcase a clear peptide-induced influence on atomic-structure and chemical miscibility. Coordination analysis of our structure models facilitated the construction of structure-function correlations applied to electrochemical methanol oxidation reactions, revealing the complex interplay between multiple metals that leads to improved catalytic properties. Our results showcase a viable strategy for elucidating structure-function relationships in HEAs, prospectively providing a pathway for future materials design.

Synergistic effect of bimetallic Pd-Pt nanocrystals for highly efficient methanol oxidation electrocatalysts

Metal nanocrystals (NCs) with controlled compositional and distributional structures have gained increasing attention due to their unique properties and broad applications, particularly in fuel cell systems. However, despite the significant importance of composition in metal NCs and their electrocatalytic behavior, comprehensive investigations into the relationship between atomic distribution and electrocatalytic activity remain scarce. In this study, we present the development of four types of nanocubes with similar sizes and controlled compositions (Pd-Pt alloy, Pd@Pt core-shell, Pd, and Pt) to investigate their influence on electrocatalytic performance for methanol oxidation reaction (MOR). The electrocatalytic activity and stability of these nanocubes exhibited variations based on their compositional structures, potentially affecting the interaction between the surface-active sites of the nanocrystals and reactive molecules. As a result, leveraging the synergistic effect of their alloy nanostructure, the Pd-Pt alloy nanocubes exhibited exceptional performance in MOR, surpassing the catalytic activity of other nanocubes, including Pd@Pt core-shell nanocubes, monometallic Pd and Pt nanocubes, as well as commercial Pd/C and Pt/C catalysts.

One-Pot Au@Pd Dendritic Nanoparticles as Electrocatalysts with Ethanol Oxidation Reaction

The one-pot synthesis strategy of Au@Pd dendrites nanoparticles (Au@Pd DNPs) was simply synthesized in a high-temperature aqueous solution condition where cetyltrimethylammonium chloride (CTAC) acted as a reducing and capping agent at a high temperature. The Au@Pd DNPs with highly monodisperse were shown in high yields by the Au:Pd rate. The nanostructure and optical and crystalline properties of the Au@Pd DNPs were characterized by UV–vis spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction. The Au@Pd DNPs showed an efficient electrochemical catalytic performance rate toward the ethanol oxidation reaction (EOR) due to their nanostructures and Au:Pd rate.

Controlled Synthesis of Carbon-Supported Pt-Based Electrocatalysts for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells are playing an increasing role in postpandemic economic recovery and climate action plans. However, their performance, cost, and durability are significantly related to Pt-based electrocatalysts, hampering their large-scale commercial application. Hence, considerable efforts have been devoted to improving the activity and durability of Pt-based electrocatalysts by controlled synthesis in recent years as an effective method for decreasing Pt use, and consequently, the cost. Therefore, this review article focuses on the synthesis processes of carbon-supported Pt-based electrocatalysts, which significantly affect the nanoparticle size, shape, and dispersion on supports and thus the activity and durability of the prepared electrocatalysts. The reviewed processes include (i) the functionalization of a commercial carbon support for enhanced catalyst–support interaction and additional catalytic effects, (ii) the methods for loading Pt-based electrocatalysts onto a carbon support that impact the manufacturing costs of electrocatalysts, (iii) the preparation of spherical and nonspherical Pt-based electrocatalysts (polyhedrons, nanocages, nanoframes, one- and two-dimensional nanostructures), and (iv) the postsynthesis treatments of supported electrocatalysts. The influences of the supports, key experimental parameters, and postsynthesis treatments on Pt-based electrocatalysts are scrutinized in detail. Future research directions are outlined, including (i) the full exploitation of the potential functionalization of commercial carbon supports, (ii) scaled-up one-pot synthesis of carbon-supported Pt-based electrocatalysts, and (iii) simplification of postsynthesis treatments. One-pot synthesis in aqueous instead of organic reaction systems and the minimal use of organic ligands are preferred to simplify the synthesis and postsynthesis treatment processes and to promote the mass production of commercial carbon-supported Pt-based electrocatalysts.

Graphical Abstract

This review focuses on the synthesis process of Pt-based electrocatalysts/C to develop aqueous one-pot synthesis at large-scale production for PEMFC stack application.

Direct strain correlations at the single-atom level in three-dimensional core-shell interface structures

Nanomaterials with core-shell architectures are prominent examples of strain-engineered materials. The lattice mismatch between the core and shell materials can cause strong interface strain, which affects the surface structures. Therefore, surface functional properties such as catalytic activities can be designed by fine-tuning the misfit strain at the interface. To precisely control the core-shell effect, it is essential to understand how the surface and interface strains are related at the atomic scale. Here, we elucidate the surface-interface strain relations by determining the full 3D atomic structure of Pd@Pt core-shell nanoparticles at the single-atom level via atomic electron tomography. Full 3D displacement fields and strain profiles of core-shell nanoparticles were obtained, which revealed a direct correlation between the surface and interface strain. The strain distributions show a strong shape-dependent anisotropy, whose nature was further corroborated by molecular statics simulations. From the observed surface strains, the surface oxygen reduction reaction activities were predicted. These findings give a deep understanding of structure-property relationships in strain-engineerable core-shell systems, which can lead to direct control over the resulting catalytic properties.

Understanding 3D interfacial strain at the atomic level has been a long-sought challenge in the field of core-shell nanomaterials. Here, the authors address this challenge by revealing the full 3D atomic structures of Pd@Pt core-shell nanoparticles

Polydimethylsiloxane Sponge-Supported Metal Nanoparticles as Reusable Catalyst for Continuous Flow Reactions

In this manuscript, polydimethylsiloxane (PDMS) sponges supporting metal nanoparticles (gold and palladium) were developed and their catalytic properties were studied through a model reaction such as the hydrogenation of p-nitrophenol. Different synthetic conditions for gold and palladium were studied to obtain the best catalyst in terms of nanoparticle loading. The as-prepared catalysts were characterized by different techniques such as scanning electron microscopy (SEM) and inductively coupled plasma optical emission spectroscopy (ICP-OES). The catalytic efficiency and recyclability of the supported catalyst were tested in static conditions. In addition, thanks to the porous structure of the material where the catalytic centers (metal nanoparticles) are located, the model reaction for continuous flow systems was tested, passing the reaction components through the catalyst, observing a high efficiency and recyclability for these systems.

Formation Mechanism of Carbon-Supported Hollow PtNi Nanoparticles via One-Step Preparations for Use in the Oxygen Reduction Reaction

Hollow Pt-based nanoparticles are known to possess the properties of high electrocatalytic activity and durability. Nonetheless, their practical applications as catalytic materials are limited because of the requirement for exhaustive preparation. In this study, we prepared carbon-supported hollow PtNix (x = the moles of the Ni precursor to the Pt precursor in the catalyst preparation step) catalysts using a one-step preparation method, which substantially reduced the complexity of the conventional method for preparing hollow Pt-based catalysts. In particular, this hollow structure formation mechanism was proposed based on extensive characterizations. The prepared catalysts were examined to determine if they could be used as electrocatalysts for the oxygen reduction reaction (ORR). Among the investigated catalysts, the acid-treated hollow PtNi3/C catalyst demonstrated the best ORR activity, which was 3 times higher and 2.3 times higher than those of the commercial Pt/C and acid-treated particulate PtNi3/C catalysts, respectively.

One-Pot Aqueous Synthesis of Porous Hollow PdCu Alloy Nanoparticles for Enhanced Ethanol Electrooxidation

Bimetallic PdCu porous hollow nanoparticles (PHNs) with hierarchical nanostructures and well-alloyed compositions were precisely synthesized through a one-pot aqueous synthetic route. Bimetallic PdCu PHNs exhibited multiple enhancement synergies and thus performed well in ethanol oxidation electrocatalysis with remarkable activity and stability. This work expedites rational design and synthesis of the high-hierarchy alloy electrocatalysts for fuel cell electrocatalysis.

Mesoporous Core-Shell Pd@Pt Nanospheres as Oxidase Mimics with Superhigh Catalytic Efficiency at Room Temperature

Mesoporous Pt-Pd bimetallic core-shell nanospheres (mPd@Pt NSs) with palladium-rich cores and platinum-rich shells were synthesized via a simple, two-step, wet chemical strategy mediated by nitrogen-doped carbon dots. The BET surface area of mPd@Pt NSs was found to be 210.4 m²·g^-1, which is significantly higher than the currently reported unsupported Pt-based nanomaterials. Because of the large active surface area, the as-prepared mPd@Pt NSs show superhigh oxidase activity and exhibit excellent oxidase-like catalytic efficiency with a catalytic constant (K_cat) as high as 2.1 × 10³ s^-1 at room temperature, which is of the same order of magnitude as the natural horseradish peroxidase (HRP) (K_cat = 4.3 × 10³ s^-1) at 37 °C and five-fold greater than the reported K_cat values of oxidase-like nanozyme obtained at 30 °C.

Surfactant-Free Monodispersed Pd Nanoparticles Template for Core-Shell Pd@PdPt Nanoparticles as Electrocatalyst towards Methanol Oxidation Reaction (MOR)

An eco-friendly two-step synthetic method for synthesizing Pd@PdPt/CNTs nanoparticles was introduced and studied for the methanol oxidation reaction. The Pd@PdPt alloy core-shell structure was synthesized by preparing a surfactant-free monodispersed Pd/CNTs precursor through the hydrolysis of tetrachloropalladate (II) ion ([PdCl₄]²⁻) in the presence of carbon nanotubes (CNTs) and the subsequent hydrogen reduction and followed by a galvanic replacement reaction. This method opens up an eco-friendly, practical, and straightforward route for synthesizing monometallic or bimetallic nanoparticles with a clean surfactant-free electrocatalytic surface. It is quite promising for large-scale preparation. The Pd@PdPt/CNTs electrocatalyst demonstrated a high specific mass activity for methanol oxidation (400.2 mAmg_Pt⁻¹) and excellent stability towards direct methanol oxidation compared to its monometallic counterparts.

Tailoring noble metal nanoparticle designs to enable sensitive lateral flow immunoassay

Lateral flow immunoassay (LFIA) with gold nanoparticles (AuNPs) as signal reporters is a popular point-of-care diagnostic technique. However, given the weak absorbance of traditional 20-40 nm spherical AuNPs, their sensitivity is low, which greatly limits the wide application of AuNP-based LFIA. With the rapid advances in materials science and nanotechnology, the synthesis of noble metal nanoparticles (NMNPs) has enhanced physicochemical properties such as optical, plasmonic, catalytic, and multifunctional activity by simply engineering their physical parameters, including the size, shape, composition, and external structure. Using these engineered NMNPs as an alternative to traditional AuNPs, the sensitivity of LFIA has been significantly improved, thereby greatly expanding the working range and application scenarios of LFIA, particularly in trace analysis. Therefore, in this review, we will focus on the design of engineered NMNPs and their demonstration in improving LFIA. We highlight the strategies available for tailoring NMNP designs, the effect of NMNP engineering on their performance, and the working principle of each engineering design for enhancing LFIA. Finally, current challenges and future improvements in this field are briefly discussed.

Achievements in Pt nanoalloy oxygen reduction reaction catalysts: strain engineering, stability and atom utilization efficiency

The Pt nanoalloy surfaces often show unique electronic and physicochemical properties that are distinct from those of their parent metals, which provide significant room for manipulating their oxygen reduction reaction (ORR) behaviour. In this Feature Article, we present the progress of our recent research and that of other groups in Pt nanoalloy catalysts for ORR from three aspects, namely, strain engineering, stability and atom utilization efficiency. Some new insights into Pt surface strain engineering will be firstly introduced, with a focus on discussing the effect of compressive and tensile strain on the chemisorption properties. Secondly, the design concepts and synthetic methodologies to intensify the inherent stability of Pt nanoalloys will be summarized. Then, the exciting research push in developing nanostructured alloys with high atom utilization efficiency of Pt will be presented. Finally, a brief illumination of challenges and future developing perspectives of Pt nanoalloy catalysts will be provided.

Highly Enhanced Electrocatalytic Performances with Dendritic Bimetallic Palladium-Based Nanocrystals

The exploration of efficient nanocatalysts with high activity and stability towards water electrolysis and fuel cell applications is extremely important for the advancement of electrochemical reactions. However, it remains challenging. Controlling the morphology of bimetallic Pd–Pt nanostructures can be a great way to improve their electrocatalytic properties compared with previously developed catalysts. Herein, we synthesize bimetallic Pd–Pt nanodendrites, which consist of a dense matrix of unsaturated coordination atoms and high porosity. The concentration of cetyltrimethylammonium chloride was significant for the morphology and size of the Pd–Pt nanodendrites. Pd–Pt nanodendrites prepared by cetyltrimethylammonium chloride (200 mM) showed higher activities towards both the hydrogen evolution reaction and methanol oxidation reaction compared to their different Pd–Pt nanodendrite counterparts, commercial Pd, and Pt catalysts, which was attributed to numerous unsaturated surface atoms in well-developed single branches.

One-Pot Synthesis of Ternary Alloy Hollow Nanostructures with Controlled Morphologies for Electrocatalysis

The rational design and synthesis of multimetallic hollow nanostructures (HNSs) have been attracting great attention due to their structural and compositional advantages for application in electrocatalysis. Herein, the one-pot synthesis of Pd-Pt-Ag ternary alloy HNSs with controllable morphologies through a self-templating approach without any pre-synthesized templates is reported. Simultaneous reduction of multiple metal precursors by ascorbic acid in the presence of cetyltrimethylammonium chloride (CTAC) yielded initially metastable Pd-Ag nanocrystals, which can act as a self-template, and subsequent galvanic replacement and reduction led to the formation of final Pd-Pt-Ag HNSs. The size and hollowness (the ratio of inner cavity diameter to outer diameter) of the HNSs could be tuned through control over the concentration of CTAC. This can be attributed to the manipulated reduction kinetics of multiple metal precursors with the change in the CTAC concentration. The prepared Pd-Pt-Ag HNSs exhibited improved catalytic performance for ethanol electro-oxidation due to their large active surface areas and ternary alloy composition.

One-Step Microwave-Assisted Synthesis of PtNiCo/rGO Electrocatalysts with High Electrochemical Performance for Direct Methanol Fuel Cells

Platinum (Pt) is widely used as an activator in direct methanol fuel cells (DMFCs). However, the development of Pt catalyst is hindered due to its high cost and CO poisoning. A multi-metallic catalyst is a promising catalyst for fuel cells. We develop a simple and rapid method to synthesize PtNiCo/rGO nanocomposites (NCs). The PtNiCo/rGO NCs catalyst was obtained by microwave-assisted synthesis of graphene oxide (GO) with Pt, Ni, and Co precursors in ethylene glycol (EG) solution after heating for 20 min. The Pt-Ni-Co nanoparticles showed a narrow particle size distribution and were uniformly dispersed on the reduced graphene oxide without agglomeration. Compared with PtNiCo catalyst, PtNiCo/rGO NCs have superior electrocatalytic properties, including a large electrochemical active surface area (ECSA), the high catalytic activity of methanol, excellent anti-toxic properties, and high electrochemical stability. The ECSA can be up to 87.41 m2/g at a scan rate of 50 mV/s. They also have the lowest oxidation potential of CO. These excellent electrochemical performances are attributed to the uniform dispersion of PtNiCo nanoparticles, good conductivity, stability, and large specific surface area of the rGO carrier. The synthesized PtNiCo/rGO nanoparticles have an average size of 17.03 ± 1.93 nm. We also investigated the effect of catalyst material size on electrocatalytic performance, and the results indicate that PtNiCo/rGO NC catalysts can replace anode catalyst materials in fuel cell applications in the future.

Nickel Structures as a Template Strategy to Create Shaped Iridium Electrocatalysts for Electrochemical Water Splitting

Low-cost, highly active, and highly stable catalysts are desired for the generation of hydrogen and oxygen using water electrolyzers. To enhance the kinetics of the oxygen evolution reaction in an acidic medium, it is of paramount importance to redesign iridium electrocatalysts into novel structures with organized morphology and high surface area. Here, we report on the designing of a well-defined and highly active hollow nanoframe based on iridium. The synthesis strategy was to control the shape of nickel nanostructures on which iridium nanoparticles will grow. After the growth of iridium on the surface, the next step was to etch the nickel core to form the NiIr hollow nanoframe. The etching procedure was found to be significant in controlling the hydroxide species on the iridium surface and by that affecting the performance. The catalytic performance of the NiIr hollow nanoframe was studied for oxygen evolution reaction and shows 29 times increased iridium mass activity compared to commercially available iridium-based catalysts. Our study provides novel insights to control the fabrication of iridium-shaped catalysts using 3d transition metal as a template and via a facile etching step to steer the formation of hydroxide species on the surface. These findings shall aid the community to finally create stable iridium alloys for polymer electrolyte membrane water electrolyzers, and the strategy is also useful for many other electrochemical devices such as batteries, fuel cells, sensors, and solar organic cells.

Twin-Directed Deposition of Pt on Pd Icosahedral Nanocrystals for Catalysts with Enhanced Activity and Durability toward Oxygen Reduction

Platinum nanocrystals featuring a multiply twinned structure and uniform sizes below 5 nm are superb catalytic materials, but it is difficult to synthesize such particles owing to the high twin-boundary energy (166 mJ/m²) of Pt. Here, we report a simple route to the synthesis of such nanocrystals by selectively growing them from the vertices of Pd icosahedral seeds. The success of this synthesis critically depends on the introduction of Br^- ions to slow the reduction kinetics of the Pt(II) precursor while limiting the surface diffusion of Pt adatoms by conducting the synthesis at 30 °C. Owing to the small size and multiply twinned structure of Pt dots, the as-obtained Pd-Pt nanocrystals show remarkably enhanced activity and durability toward oxygen reduction, with a mass activity of 1.23 A mg^-1_Pt and a specific activity of 0.99 mA cm^-2_Pt, which are 8.2 and 4.5 times as high as those of the commercial Pt/C.

Nanometal Thermocatalysts: Transformations, Deactivation, and Mitigation

Nanometals have been proven to be efficient thermocatalysts in the last decades. Their enhanced catalytic activity and tunable functionalities make them intriguing candidates for a wide range of catalytic applications, such as gaseous reactions and compound synthesis/decomposition. On the other hand, the enhanced specific surface energy and reactivity of nanometals can lead to configuration transformation and thus catalytic deactivation during the synthesis and catalysis, which largely undermines the activity and service time, thereby calling for urgent research effort to understand the deactivating mechanisms and develop efficient mitigating methods. Herein, the recent progress in understanding the configuration transformation-induced catalytic deactivation within nanometals is reviewed. The major pathways of configuration transformations, and their kinetics controlled by the environmental factors are presented. The approaches toward mitigating the transformation-induced deactivation are also presented. Finally, a perspective on the future academic approaches toward in-depth understanding of the kinetics of the deactivation of nanometals is proposed.

Highly Porous Au-Pt Bimetallic Urchin-Like Nanocrystals for Efficient Electrochemical Methanol Oxidation

Highly porous Au–Pt urchin-like bimetallic nanocrystals have been prepared by a one-pot wet-chemical synthesis method. The porosity of urchin-like bimetallic nanocrystals was controlled by amounts of hydrazine used as reductant. The prepared highly porous Au-Pt urchin-like nanocrystals were superior catalysts of electrochemical methanol oxidation due to high porosity and surface active sites by their unique morphology. This approach will pave the way for the design of bimetallic porous materials with unprecedented functions.

Enhanced oxygen reduction activity of bimetallic Pd–Ag alloy-supported on mesoporous cerium oxide electrocatalysts in alkaline media

Currently, the rational design and fabrication of Pt-free electrocatalysts towards the oxygen reduction reaction for extensive applications in fuel cells is a challenging task.

Novel carbon structures as highly stable supports for electrocatalysts in acid media: regulating the oxygen functionalization behavior of carbon

The oxygen functionalization behavior of carbon can be greatly tuned by Co, promoting its electrochemical stability as a support for Pt.

Synthesis of Pd-Pt Ultrathin Assembled Nanosheets as Highly Efficient Electrocatalysts for Ethanol Oxidation

Control over composition and morphology of nanocrystals (NCs) is significant to develop advanced catalysts applicable to polymer electrolyte membrane fuel cells and further overcome the performance limitations. Here, we present a facile synthesis of Pd-Pt alloy ultrathin assembled nanosheets (UANs) by regulating the growth behavior of Pd-Pt nanostructures. Iodide ions supplied from KI play as capping agents for the {111} plane to promote 2-dimensional (2D) growth of Pd and Pt, and the optimal concentrations of cetyltrimethylammonium chloride and ascorbic acid result in the generation of Pd-Pt alloy UANs in high yield. The prepared Pd-Pt alloy UANs exhibited the remarkable enhancement of the catalytic activity and stability toward ethanol oxidation reaction compared to irregular-shaped Pd-Pt alloy NCs, commercial Pd/C, and commercial Pt/C. Our results confirm that the Pd-Pt alloy composition and ultrathin 2D morphology offer high accessible active sites and favorable electronic structure for enhancing electrocatalytic activity.

Formation Mechanism and Gram-Scale Production of PtNi Hollow Nanoparticles for Oxygen Electrocatalysis through In Situ Galvanic Displacement Reaction

Galvanic displacement reaction has been considered a simple method for fabricating hollow nanoparticles. However, the formation of hollow interiors in nanoparticles is not easily achieved owing to the easy oxidization of transition metals, which results in mixed morphologies, and the presence of surfactants on the nanoparticle surface, which severely deteriorates the catalytic activity. In this study, we developed a facile gram-scale methodology for the one-pot preparation of carbon-supported PtNi hollow nanoparticles as an efficient and durable oxygen reduction electrocatalyst without using stabilizing agents or additional processes. The hollow structures were evolved from sacrificial Ni nanoparticles via an in situ galvanic displacement reaction with a Pt precursor, directly following a preannealing process. By sampling the PtNi/C hollow nanoparticles at various reaction times, the structural formation mechanism was investigated using transmission electron microscopy with energy-dispersive X-ray spectroscopy mapping/line-scan profiling. We found out that the structure and morphology of the PtNi hollow nanoparticles were controlled by the acidity of the metal precursor solution and the nanoparticle core size. The synthesized PtNi hollow nanoparticles acted as an oxygen reduction electrocatalyst, with a catalytic activity superior to that of a commercial Pt catalyst. Even after 10 000 cycles of harsh accelerated durability testing, the PtNi/C hollow electrocatalyst showed high performance and durability. We concluded that the Pt-rich layers on the PtNi hollow nanoparticles improved the catalytic activity and durability considerably.

High quality electrocatalyst by Pd-Pt alloys nanoparticles uniformly distributed on polyaniline/carbon nanotubes for effective methanol oxidation

In the present work, Pd-Pt nanoparticle/polyaniline/carbon nanotube (NP/PANI/CNT) composites, synthesized through the uniform distribution of Pd-Pt NPs, whose single size was controlled to 2-4 nm by citric acid, on a PANI/CNT compound, served as the electrode catalysts for effective methanol oxidation. The structural characteristics were identified by x-ray diffraction, x-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy, and revealed that the Pt₆₅Pd₃₅ NP/PANI/CNT composite displayed superior electrocatalytic oxidation performance and vitality in various Pd-Pt NPs. The results indicated that as an excellent anode catalyst the complex would have application prospects for effective methanol oxidation.

Observing the structure-dependent electrocatalytic activity of bimetallic Pd-Au nanorods at the single-particle level

We developed an electrochemiluminescence (ECL) microscopy technique to image the structure-dependent electrocatalytic reactivity of bimetallic Pd-Au nanorods (NRs) at the single-particle level.

Multicomponent Plasmonic Nanoparticles: From Heterostructured Nanoparticles to Colloidal Composite Nanostructures

Plasmonic nanostructures possessing unique and versatile optoelectronic properties have been vastly investigated over the past decade. However, the full potential of plasmonic nanostructure has not yet been fully exploited, particularly with single-component homogeneous structures with monotonic properties, and the addition of new components for making multicomponent nanoparticles may lead to new-yet-unexpected or improved properties. Here we define the term "multi-component nanoparticles" as hybrid structures composed of two or more condensed nanoscale domains with distinctive material compositions, shapes, or sizes. We reviewed and discussed the designing principles and synthetic strategies to efficiently combine multiple components to form hybrid nanoparticles with a new or improved plasmonic functionality. In particular, it has been quite challenging to precisely synthesize widely diverse multicomponent plasmonic structures, limiting realization of the full potential of plasmonic heterostructures. To address this challenge, several synthetic approaches have been reported to form a variety of different multicomponent plasmonic nanoparticles, mainly based on heterogeneous nucleation, atomic replacements, adsorption on supports, and biomolecule-mediated assemblies. In addition, the unique and synergistic features of multicomponent plasmonic nanoparticles, such as combination of pristine material properties, finely tuned plasmon resonance and coupling, enhanced light-matter interactions, geometry-induced polarization, and plasmon-induced energy and charge transfer across the heterointerface, were reported. In this review, we comprehensively summarize the latest advances on state-of-art synthetic strategies, unique properties, and promising applications of multicomponent plasmonic nanoparticles. These plasmonic nanoparticles including heterostructured nanoparticles and composite nanostructures are prepared by direct synthesis and physical force- or biomolecule-mediated assembly, which hold tremendous potential for plasmon-mediated energy transfer, magnetic plasmonics, metamolecules, and nanobiotechnology.

Reactive nanotemplates for synthesis of highly efficient electrocatalysts: beyond simple morphology transfer

Efficient electrocatalysts for energy conversion in general, and fuel cell operation and water electrolysis in particular, are pivotal for carbon-free hydrogen production. While the requirements of successful electrocatalysts include a high number density of catalytically active sites, high surface-to-volume ratio, inherently high catalytic activity, and robustness of the catalyst surface structure under harsh operating conditions, it is extremely difficult to synthesize nanocatalysts that could possess all these structural characteristics. Nanotemplate-mediated synthesis, namely, the coating or filling of a template with a desired material phase followed by the removal of the template, has captured the interest of researchers because of the ease of creating hollow-structured nanocatalysts with a high surface to volume ratio. Recent studies, however, have revealed that nanotemplates could be more than just passive supports because they greatly affect catalytic performance by creating an unusual synergy between the substrate and catalyst and by providing dopants to the actual catalyst phase owing to their reactive nature. In this review, we discuss the most notable recent advances in the nanotemplate-based synthesis of electrocatalysts as well as the unusual effects of nanotemplates on the performance of nanocatalysts. We also provide an outlook for this fledgling field so that future research efforts could be focused on the development of practically useful electrocatalysts that could shape the future of energy technologies.

Ternary core-shell PdM@Pt (M = Mn and Fe) nanoparticle electrocatalysts with enhanced ORR catalytic properties

In this work, we introduce composition-tunable core-shell-like PdM@Pt (M = Mn and Fe) nanoparticles (NPs) on carbon support (PdM@Pt/C) synthesized by one-pot sonochemical reactions using high-intensity ultrasonic probe (150 W, 20 kHz, with 13 mm solid probe) and investigate their electrocatalytic performance for oxygen reduction reaction (ORR). The core-shell-like structure of the NPs are evidenced by the elemental distribution maps obtained by energy dispersive X-ray spectroscopy equipped on scanning transmission electron microscopy. Based on the characterization data, PdM@Pt NPs were synthesized with variable elemental compositions (Pd₄₉Fe₂₁@Pt₃₀, Pd₁₇Fe₃₁@Pt₅₂, Pd₄₆Mn₆@Pt₄₈ and Pd₁₅Mn₅@Pt₈₀). All PdM@Pt samples are composed of large (~10 nm) and small (~3 nm) NPs, the large ones appear to be aggregates of the smaller ones, and the proportion of the larger NPs increases with the Pd content, which can be explained with the known mechanisms of sonochemical reactions of related systems. Electrochemical analyses on samples show that the ORR mass activity of PdM@Pt/C is 3-fold (normalized by Pt) and 1.7-fold (normalized by platinum group metal (PGM)) higher than those of Pt/C (commercial). All PdM@Pt/C sample show superior durability with the electrochemical surface area (ECSA) change of -4.4-+12.0% and half-wave potential change (ΔE_1/2) of 8-14 mV after 10 k cycles accelerated stress test (AST) to Pt/C with ECSA change of -25.6% and ΔE_1/2 of 19 mV.

Atomic Structure and Dynamics of Epitaxial Platinum Bilayers on Graphene

Platinum atomic layers grown on graphene were investigated by atomic resolution transmission electron microscopy (TEM). These TEM images reveal the epitaxial relationship between the atomically thin platinum layers and graphene, with two optimal epitaxies observed. The energetics of these epitaxies influences the grain structure of the platinum film, facilitating grain growth via in-plane rotation and assimilation of neighbor grains, rather than grain coarsening from the movement of grain boundaries. This growth process was enabled due to the availability of several possible low-energy intermediate states for the rotating grains, the Pt-Gr epitaxies, which are minima in surface energy, and coincident site lattice grain boundaries, which are minima in grain boundary energy. Density functional theory calculations reveal a complex interplay of considerations for minimizing the platinum grain energy, with free platinum edges also having an effect on the relative energetics. We thus find that the platinum atomic layer grains undergo significant reorientation to minimize interface energy (via epitaxy), grain boundary energy (via low-energy orientations), and free edge energy. These results will be important for the design of two-dimensional graphene-supported platinum catalysts and obtaining large-area uniform platinum atomic layer films and also provide fundamental experimental insight into the growth of heteroepitaxial thin films.

Facile synthesis of Pd@Pt core-shell nanocubes with low Pt content via direct seed-mediated growth and their enhanced activity for formic acid oxidation

Pd@Pt core-shell nanocubes with a partially covered Pt shell on the Pd nanocubes were synthesized by a direct seed-mediated growth method without a washing process. The FAO activity of Pd@Pt 0.4 at% was 4.3 times and 2.2 times higher than that of Pd cubes and commercial Pt/C, respectively.

Improved LSPR Properties of Ag–Pt and Pt Nanoparticles: A Systematic Study on Various Configurations and Compositions of NPs via the Solid-State Dewetting of Ag–Pt Bilayers

The localized surface plasmon resonance (LSPR) of noble metal nanoparticles (NPs) has become an important research topic in various fields and can be systematically tuned to obtain the desired device performance through the appropriate structural and elemental modifications. In this research, the improved LSPR properties of Pt NPs and diverse configurations and compositions of Ag–Pt bimetallic alloy NPs were demonstrated on sapphire (0001) via the solid-state dewetting (SSD) of Ag–Pt bilayers. A strong and dynamic LSPR response in the ultraviolet (UV) and visible (VIS) regions was demonstrated depending on the elemental composition and surface morphology of the NPs, which is discussed along with finite difference time domain (FDTD) simulations. In comparison, the Ag–Pt NPs exhibited stronger LSPR excitation, whereas the Pt NPs showed a relatively weaker and broader response. Meanwhile, the Pt NPs fabricated in this study still demonstrated a much-enhanced LSPR response compared to previous studies on the solid-state dewetting of pure Pt films due to improvements in configuration, uniformity, and interparticle gaps. Various surface morphologies of NPs, such as connected nanoclusters, elongated NPs, and isolated spherical NPs, were obtained on the basis of alloying, diffusion, Rayleigh instability, and a surface minimization mechanism, which were different from those of pure Ag and Pt NPs in similar growth conditions. Particularly, one-step annealing of an Ag–Pt bilayer yielded Ag–Pt alloy NPs below 600 °C, which subsequently transformed into pure Pt NPs above 650 °C, in which the high diffusivity and high vapor pressure of Ag atoms significantly facilitated the overall growth process of the NPs.

Recent Advances on Controlled Synthesis and Engineering of Hollow Alloyed Nanotubes for Electrocatalysis

The past decade has witnessed great progress in the synthesis and electrocatalytic applications of 1D hollow alloy nanotubes with controllable compositions and fine structures. Hollow nanotubes have been explored as promising electrocatalysts in the fuel cell reactions due to their well-controlled surface structure, size, porosity, and compositions. In addition, owing to the self-supporting ability of 1D structure, hollow nanotubes are capable of avoiding catalyst aggregation and carbon corrosion during the catalytic process, which are two other issues for the widely investigated carbon-supported nanoparticle catalysts. It is currently a great challenge to achieve high activity and stability at a relatively low cost to realize commercialization of these catalysts. An overview of the structural and compositional properties of 1D hollow alloy nanotubes, which provide a large number of accessible active sites, void spaces for electrolytes/reactants impregnation, and structural stability for suppressing aggregation, is presented. The latest advances on several strategies such as hard template and self-templating methods for controllable synthesis of hollow alloyed nanotubes with controllable structures and compositions are then summarized. Benefiting from the advantages of the unique properties and facile synthesis approaches, the capability of 1D hollow nanotubes is then highlighted by discussing examples of their applications in fuel-cell-related electrocatalysis. Finally, the remaining challenges and potential solutions in the field are summarized to provide some useful clues for the future development of 1D hollow alloy nanotube materials.

Nanoscopic Porous Iridium/Iridium Dioxide Superstructures (15 nm): Synthesis and Thermal Conversion by In Situ Transmission Electron Microscopy

Porous particle superstructures of about 15 nm diameter, consisting of ultrasmall nanoparticles of iridium and iridium dioxide, are prepared through the reduction of sodium hexachloridoiridate(+IV) with sodium citrate/sodium borohydride in water. The water-dispersible porous particles contain about 20 wt % poly(N-vinylpyrrolidone) (PVP), which was added for colloidal stabilization. High-resolution transmission electron microscopy confirms the presence of both iridium and iridium dioxide primary particles (1-2 nm) in each porous superstructure. The internal porosity (≈58 vol%) is demonstrated by electron tomography. In situ transmission electron microscopy up to 1000 °C under oxygen, nitrogen, argon/hydrogen (all at 1 bar), and vacuum shows that the porous particles undergo sintering and subsequent compaction upon heating, a process that starts at around 250 °C and is completed at around 800 °C. Finally, well-crystalline iridium dioxide is obtained under all four environments. The catalytic activity of the as-prepared porous superstructures in electrochemical water splitting (oxygen evolution reaction; OER) is reduced considerably upon heating owing to sintering of the pores and loss of internal surface area.

Fine Control over the Compositional Structure of Trimetallic Core-Shell Nanocrystals for Enhanced Electrocatalysis

Pt-based multimetallic nanocrystals (NCs) have attracted tremendous research interest because of their excellent catalytic properties in various electrocatalysis fields. However, the development of rational synthesis approaches that can give multimetallic NCs with desirable compositional structures is still a radical issue. In the present work, we devised an efficient strategy for the systematic control of the spatial distribution of constituent elements in Pt-based trimetallic core-shell NCs, through which NCs with distinctly different compositional structures, such as Au@PdPt, Au@Pd@Pt, AuPd@Pt, and AuPdPt@Pt core-shell NCs, could selectively be generated. The adjustment of the amount of a reducing agent, hydrazine, which can provide control over the relative reduction kinetics of multiple metals, is the key to the selective formation of NCs. Through extensive studies on the effect of the compositional structure of the trimetallic NCs on their catalytic function toward the methanol electro-oxidation reaction, we found that the Au@Pd@Pt NCs exhibited considerably enhanced catalytic performance in comparison to the other trimetallic NCs as well as to their binary counterparts, a commercial catalyst, and reported Pt-based nanocatalysts due to the optimized surface electronic structure. The present strategy will be useful to design and construct multicomponent catalytic systems for various energy and environmental applications.

Noble Metal Based Alloy Nanoframes: Syntheses and Applications in Fuel Cells

Noble metal nanostructures are being used broadly as catalysts for energy conversion in fuel cells. To overcome the future energy crises, fuel cells are anticipated as clean energy sources because they can be operated at low temperature, their energy conversion is high and their carbon release is almost zero. However, an active and stable electrocatalyst is essential for the electrochemical reactions in fuel cells. Therefore, properties of the nanostructures greatly depend on the shape of the nanostructures. Individual as well as interaction properties are greatly affected by changes in the surface area of the nanostructures. By shape controlled synthesis, properties of the nanostructures could be further enhanced by increasing the surface area or active sites for electrocatalysts. Therefore, an efficient approach is needed for the fabrication of nanostructures to increase their efficiency, activity, or durability in fuel cells by reducing the usage of noble metals. Different types of hollow nanostructures until now have been prepared including nanoboxes, nanocages, nanoshells, nanoframes (NFs), etc. NFs are the hollow unique three-dimensional structure which have no walls-they only contain corners or edges so they have large surface area. In electrocatalytic reactions, the molecules involved in the reaction can easily reach the inner surface of the nanoframes, thus noble metals' utilization efficiency increases. NFs usually have high surface area, greater morphological and compositional stabilities, allowing them to withstand harsh environmental conditions. By considering the current challenges in fabrication of noble metal based alloy NFs as electrocatalysts, this review paper will highlight recent progress, design, and fabrication of noble metal alloy NFs through different strategies-mainly photocatalytic template, electrodeposition, Kirkendall effect, galvanic replacement, chemical/oxidative etching, combination of both and other methods. Then, electrochemical applications of NFs in fuel cells toward formic acid, methanol, ethanol, oxygen reduction reaction as well as bifunctional catalyst will also be highlighted. Finally, we will summarize different challenges in the fabrication of highly proficient nanocatalysts for the fuel cells with low cost, high efficiency and high durability, which are the major issues for the highly commercial use of fuel cells in the future.

Ultrathin-Polyaniline-Coated Pt-Ni Alloy Nanooctahedra for the Electrochemical Methanol Oxidation Reaction

Controlling the morphology and composition of nanocatalysts constructed from metals and conductive polymers has attracted attention owing to their great potential for the development of high-efficiency catalysts for various catalytic applications. Herein, a facile synthetic approach for ultrathin-polyaniline-coated Pt-Ni nanooctahedra (Pt-Ni@PANI hybrids) with controllable PANI shell thicknesses is presented. Pt-Ni nanooctahedra/C catalysts enclosed by PANI shells with thicknesses from 0.6 to 2.4 nm were obtained by fine control over the amount of aniline. The various Pt-Ni@PANI hybrids exhibited electrocatalytic activity toward the methanol oxidation reaction that is highly dependent on the thickness of the PANI shell. Pt-Ni@PANI hybrids with the thinnest PANI shells (0.6 nm) showed markedly improved electrocatalytic performance for the methanol oxidation reaction compared with Pt-Ni@PANI hybrids with thicker PANI shells, Pt-Ni nanooctahedra/C, and commercial Pt/C due to synergistic benefits of ultrathin PANI shells and Pt-Ni alloy.