Hollow Metal Nanocrystals with Ultrathin, Porous Walls and Well-Controlled Surface Structures

Bipyramidal Nanocrystals of Noble Metals: From Synthesis to Applications

Shape control has been a major theme of nanocrystal research in terms of synthesis, property tailoring, and optimization of performance in a variety of applications. Among the possible shapes, bipyramids are unique owing to their symmetry, planar defects, and exposed facets. In this article, we focus on the colloidal synthesis of noble-metal nanocrystals featuring a triangular bipyramidal shape, together with highlights of their properties and applications. We start with a brief discussion of the general classification and requirements for the nucleation and growth of bipyramidal nanocrystals, followed by specific aspects regarding the synthetic methods with a focus on the roles of reduction, etching, and capping, as well as controls of facet, size, aspect ratio, and corner truncation. In the end, we illustrate how these aspects affect the properties of bipyramidal nanocrystals for plasmonic and catalytic applications, together with future perspectives.

Rhombic dodecahedron nanoframes of PtIrCu with high-index faceted hyperbranched nanodendrites for efficient electrochemical ammonia oxidation via preferred NHx dimerization pathways

Ammonia has been emerging as a sustainable and environmentally friendly fuel. However, direct electrochemical ammonia oxidation reaction (AOR) in low-temperature fuel cells seriously suffers from high overpotential and deficient durability. Herein, rhombic dodecahedron nanoframe of platinum iridium copper (PtIrCu) with high-index faceted hyperbranched nanodendrites (RDNF-HNDs) was developed using a one-step self-etching solvothermal method. The framework structure with the high-index facets enables the PtIrCu nanocrystals to expose more effective active sites. They exhibit an ultra-low onset potential of 0.33 V vs. RHE and high mass activity of 26.1 A gPtIr-1 at 0.50 V, which is 140 mV lower and 7.5 times higher than that of commercial Pt/C in the AOR. In situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy verifies that AOR on PtIrCu RDNF-HNDs prefers to the NHx dimerization pathways, effectively alleviating the poison of Nads and NOx. The theoretical calculation also shows that both introducing Cu atoms into PtIr alloy and increasing the content of Ir in PtIrCu alloy can reduce the reaction energy barrier of electrochemical dehydrogenation from *NH2 to *NH. The specific structure of PtIrCu RDNF-NDs provides a new inspiration to solve the critical issue of electrocatalysts for AOR with low activity and durability.

Facile Synthesis of Ru Nanoboxes with a Hexagonal Close-Packed Structure by Templating with Ag Nanocubes and Their Catalytic Properties

Noble-metal nanoboxes offer an attractive form of nanomaterials for catalytic applications owing to their open structure and highly efficient use of atoms. Herein, we report the facile synthesis of Ag-Ru core-shell nanocubes and then Ru nanoboxes with a hexagonal close-packed (hcp) structure, as well as evaluation of their catalytic activity toward a model hydrogenation reaction. By adding a solution of Ru(acac)3 in ethylene glycol (EG) dropwise to a suspension of silver nanocubes in EG at 170 °C, Ru atoms are generated and deposited onto the entire surface of a nanocube. As the volume of the RuIII precursor is increased, Ru atoms are also produced through a galvanic replacement reaction, generating Ag-Ru nanocubes with a hollow interior. The released Ag+ ions are then reduced by EG and deposited back onto the nanocubes. By selectively etching away the remaining Ag with aqueous HNO3 , the as-obtained Ag-Ru nanocubes are transformed into Ru nanoboxes, whose walls are characterized by an hcp structure and an ultrathin thickness of a few nanometers. Finally, we evaluated the catalytic properties of the Ru nanoboxes with two different wall thicknesses by using a model hydrogenation reaction; both samples showed excellent performance.

Atomically Dispersed Ruthenium Catalysts with Open Hollow Structure for Lithium-Oxygen Batteries

Lithium-oxygen battery with ultra-high theoretical energy density is considered a highly competitive next-generation energy storage device, but its practical application is severely hindered by issues such as difficult decomposition of discharge products at present. Here, we have developed N-doped carbon anchored atomically dispersed Ru sites cathode catalyst with open hollow structure (h-RuNC) for Lithium-oxygen battery. On one hand, the abundance of atomically dispersed Ru sites can effectively catalyze the formation and decomposition of discharge products, thereby greatly enhancing the redox kinetics. On the other hand, the open hollow structure not only enhances the mass activity of atomically dispersed Ru sites but also improves the diffusion efficiency of catalytic molecules. Therefore, the excellent activity from atomically dispersed Ru sites and the enhanced diffusion from open hollow structure respectively improve the redox kinetics and cycling stability, ultimately achieving a high-performance lithium-oxygen battery.

Bimetallic core-shell nanocrystals: opportunities and challenges

With mastery over the colloidal synthesis of monometallic nanocrystals, a combination of two distinct metals with intricate architectures has emerged as a new direction of innovation. Among the diverse architectures, the one with a core-shell structure has attracted the most scientific endeavors owing to its merits of high controllability and variability. Along with the new hopes arising from the addition of a shell composed of a different metal, there comes unexpected complications for the surface composition, hindering both structural understanding and application performance. In this Focus article, we present a brief overview of the opportunities provided by the bimetallic core-shell nanocrystals, followed by a discussion of the technical challenge to elucidate the true composition of the outermost surface. Some of the promising solutions are then highlighted as well, aiming to inspire future efforts toward this frontier of research.

Preventing the Galvanic Replacement Reaction toward Unconventional Bimetallic Core-Shell Nanostructures

A bimetallic core-shell nanostructure is a versatile platform for achieving intriguing optical and catalytic properties. For a long time, this core-shell nanostructure has been limited to ones with noble metal cores. Otherwise, a galvanic replacement reaction easily occurs, leading to hollow nanostructures or completely disintegrated ones. In the past few years, great efforts have been devoted to preventing the galvanic replacement reaction, thus creating an unconventional class of core-shell nanostructures, each containing a less-stable-metal core and a noble metal shell. These new nanostructures have been demonstrated to show unique optical and catalytic properties. In this work, we first briefly summarize the strategies for synthesizing this type of unconventional core-shell nanostructures, such as the delicately designed thermodynamic control and kinetic control methods. Then, we discuss the effects of the core-shell nanostructure on the stabilization of the core nanocrystals and the emerging optical and catalytic properties. The use of the nanostructure for creating hollow/porous nanostructures is also discussed. At the end of this review, we discuss the remaining challenges associated with this unique core-shell nanostructure and provide our perspectives on the future development of the field.

Enhancement Mechanism of Pt/Pd-Based Catalysts for Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is one of the key catalytic reactions for hydrogen fuel cells, biofuel cells and metal-air cells. However, due to the complex four-electron catalytic process, the kinetics of the oxygen reduction reaction are sluggish. Platinum group metal (PGM) catalysts represented by platinum and palladium are considered to be the most active ORR catalysts. However, the price and reserves of Pt/Pd are major concerns and issues for their commercial application. Improving the catalytic performance of PGM catalysts can effectively reduce their loading and material cost in a catalytic system, and they will be more economical and practical. In this review, we introduce the kinetics and mechanisms of Pt/Pd-based catalysts for the ORR, summarize the main factors affecting the catalytic performance of PGMs, and discuss the recent progress of Pt/Pd-based catalysts. In addition, the remaining challenges and future prospects in the design and improvement of Pt/Pd-based catalysts of the ORR are also discussed.

Mesoporous Pt@Pt-skin Pt3Ni core-shell framework nanowire electrocatalyst for efficient oxygen reduction

The design of Pt-based nanoarchitectures with controllable compositions and morphologies is necessary to enhance their electrocatalytic activity. Herein, we report a rational design and synthesis of anisotropic mesoporous Pt@Pt-skin Pt₃Ni core-shell framework nanowires for high-efficient electrocatalysis. The catalyst has a uniform core-shell structure with an ultrathin atomic-jagged Pt nanowire core and a mesoporous Pt-skin Pt₃Ni framework shell, possessing high electrocatalytic activity, stability and Pt utilisation efficiency. For the oxygen reduction reaction, the anisotropic mesoporous Pt@Pt-skin Pt₃Ni core-shell framework nanowires demonstrated exceptional mass and specific activities of 6.69 A/mg_pt and 8.42 mA/cm² (at 0.9 V versus reversible hydrogen electrode), and the catalyst exhibited high stability with negligible activity decay after 50,000 cycles. The mesoporous Pt@Pt-skin Pt₃Ni core-shell framework nanowire configuration combines the advantages of three-dimensional open mesopore molecular accessibility and compressive Pt-skin surface strains, which results in more catalytically active sites and weakened chemisorption of oxygenated species, thus boosting its catalytic activity and stability towards electrocatalysis.

Colloidal Synthesis of Metal Nanocrystals: From Asymmetrical Growth to Symmetry Breaking

Nanocrystals offer a unique platform for tailoring the physicochemical properties of solid materials to enhance their performances in various applications. While most work on controlling their shapes revolves around symmetrical growth, the introduction of asymmetrical growth and thus symmetry breaking has also emerged as a powerful route to enrich metal nanocrystals with new shapes and complex morphologies as well as unprecedented properties and functionalities. The success of this route critically relies on our ability to lift the confinement on symmetry by the underlying unit cell of the crystal structure and/or the initial seed in a systematic manner. This Review aims to provide an account of recent progress in understanding and controlling asymmetrical growth and symmetry breaking in a colloidal synthesis of noble-metal nanocrystals. With a touch on both the nucleation and growth steps, we discuss a number of methods capable of generating seeds with diverse symmetry while achieving asymmetrical growth for mono-, bi-, and multimetallic systems. We then showcase a variety of symmetry-broken nanocrystals that have been reported, together with insights into their growth mechanisms. We also highlight their properties and applications and conclude with perspectives on future directions in developing this class of nanomaterials. It is hoped that the concepts and existing challenges outlined in this Review will drive further research into understanding and controlling the symmetry breaking process.

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.

Engineering ultrathin PdAu nanoring via a facile process for electrocatalytic ethanol oxidation

Ultrathin nanoframes with more available electrocatalytic active sites on both internal and external surfaces have attracted great attention especially in the field of electrocatalysis. Herein, we report a facile process to prepare PdAu nanorings (NRs) in aqueous solution without adding any organic ligands. The growth mechanism of PdAu NRs was explored in detail. The Au precursors were reduced into Au clusters around the edges of Pd nanosheets (NSs) via galvanic replacement, then the center of Pd NSs was oxidatively etched by Cl^-/O₂, and finally the Pd and Au atoms on the edge sites were rearranged to form uniform PdAu alloy. PdAu NRs with different ratios and ternary PdAuPt NRs could be easily prepared using this strategy. Owing to the synergistically structural and compositional advantages, Pd₇₉Au₂₁ NRs exhibited higher electrocatalytic activity and stability, as well as low activation energy (E_a) for the ethanol electrooxidation reaction.

Molecular Exchange of Dynamic Imine Bond for the Etching of Polymer Particles

The underlying trend of colloidal synthesis has focused on extending the structure and composition complexity of colloidal particles. Hollow and yolk-shell particles are successful examples that have potential applications in frontier fields. In this paper, we develop a facile and controllable etching method based on the molecular exchange of the dynamic imine bond to generate cavities in polymer particles. Starting from boronate ester polymer particles and inorganic@boronate core-shell particles with the imine bonds incorporated in the polymer networks, our etching method easily affords hollow and yolk-shell particles with tunable shell thicknesses. The molecular exchange dynamics analysis indicates that guest amine molecules cause the reconstruction of imine bonds and the leakage of molecular and oligomer fragments, resulting in the formation of the hollow structure. This molecular exchange-based etching method may be of interest in the construction of polymer architectures with increased composition and structure complexities. This article is protected by copyright. All rights reserved.

Noble-Metal-Based Hollow Mesoporous Nanoparticles: Synthesis Strategies and Applications

As second-generation mesoporous materials, mesoporous noble metals (NMs) are of significant interest for their wide applications in catalysis, sensing, bioimaging, and biotherapy owing to their structural and metallic features. The introduction of interior hollow cavity into NM-based mesoporous nanoparticles (MNs), which subtly integrate hierarchical hollow and mesoporous structure into one nanoparticle, produces a new type of hollow MNs (HMNs). Benefiting from their higher active surface, better electron/mass transfer, optimum electronic structure, and nanoconfinement space, NM-based HMNs exhibit their high efficiency in enhancing catalytic activity and stability and tuning catalytic selectivity. In this review, recent progress in the design, synthesis, and catalytic applications of NM-based HMNs is summarized, including the findings of the groups. Five main strategies for synthesizing NM-based HMNs, namely silica-assisted surfactant-templated nucleation, surfactant-templated sequential nucleation, soft "dual"-template, Kirkendall effect in synergistic template, and galvanic-replacement-assisted surfactant template, are described in detail. In addition, the applications in ethanol oxidation electrocatalysis and hydrogenation reactions are discussed to highlight the high activity, enhanced stability, and optimal selectivity of NM-based HMNs in (electro)catalysis. Finally, the further outlook that may lead the directions of synthesis and applications of NM-based HMNs is prospected.

Phase-Controlled Synthesis of Ru Nanocrystals via Template-Directed Growth: Surface Energy versus Bulk Energy

Despite the successful control of crystal phase using template-directed growth, much remains unknown about the underlying mechanisms. Here, we demonstrate that the crystal phase taken by the deposited metal depends on the lateral size of face-centered cubic (fcc)-Pd nanoplate templates with 12 nm plates giving fcc-Ru while 18-26 nm plates result in hexagonal closed-packed (hcp)-Ru. Although Ru overlayers with a metastable fcc- (high in bulk energy) or stable hcp-phase (low in bulk energy) can be epitaxially deposited on the basal planes, the lattice mismatch will lead to jagged hcp- (high in surface energy) and smooth fcc-facets (low in surface energy), respectively, on the side faces. As the proportion of basal and side faces on the nanoplates varies with lateral size, the crystal phase will change depending on the relative contributions from the surface and bulk energies. The Pd@fcc-Ru outperforms the Pd@hcp-Ru nanoplates toward ethylene glycol and glycerol oxidation reactions.

Design and Synthesis of Hollow Nanostructures for Electrochemical Water Splitting

Electrocatalytic water splitting using renewable energy is widely considered as a clean and sustainable way to produce hydrogen as an ideal energy fuel for the future. Electrocatalysts are indispensable elements for large-scale water electrolysis, which can efficiently accelerate electrochemical reactions occurring at both ends. Benefitting from high specific surface area, well-defined void space, and tunable chemical compositions, hollow nanostructures can be applied as promising candidates of direct electrocatalysts or supports for loading internal or external electrocatalysts. Herein, some recent progress in the structural design of micro-/nanostructured hollow materials as advanced electrocatalysts for water splitting is summarized. First, the design principles and corresponding strategies toward highly effective hollow electrocatalysts for oxygen/hydrogen evolution reactions are highlighted. Afterward, an overview of current reports about hollow electrocatalysts with diverse architectural designs and functionalities is given, including direct hollow electrocatalysts with single-shelled, multi-shelled, or open features and heterostructured electrocatalysts based on hollow hosts. Finally, some future research directions of hollow electrocatalysts for water splitting are discussed based on personal perspectives.

Bimetallic Janus Nanocrystals: Syntheses and Applications

Bimetallic Janus nanocrystals have received considerable interest in recent years owing to their unique properties and niche applications. The side-by-side distribution of two distinct metals provides a flexible platform for tailoring the optical and catalytic properties of nanocrystals. First, a brief introduction to the structural features of bimetallic Janus nanocrystals, followed by an extensive discussion of the synthetic approaches, is given. The strategies and experimental controls for achieving the Janus structure, as well as the mechanistic understandings, are specifically discussed. Then, a number of intriguing properties and applications enabled by the Janus nanocrystals are highlighted. Finally, this article is concluded with future directions and outlooks with respect to both syntheses and applications of this new class of functional nanomaterials.

Modulation of the Al/Cu2O Schottky Barrier Height for p-Type Oxide TFTs Using a Polyethylenimine Interlayer

We introduced an organic interlayer into the Schottky contact interface to control the contact property. After inserting an 11-nm-thick polyethylenimine (PEI) interlayer between the aluminum (Al) source/drain electrode and the cuprous oxide (Cu₂O) channel layer, the Cu₂O thin-film transistors (TFTs) exhibited improved electrical characteristics compared with Cu₂O TFTs without a PEI interlayer; the field-effect mobility improved from 0.02 to 0.12 cm²/V s, the subthreshold swing decreased from 14.82 to 7.34 V/dec, and the on/off current ratio increased from 2.43 × 10² to 1.47 × 10³, respectively. Careful investigation of the contact interface between the source/drain electrode and the channel layer established that the performance improvements were caused by the formation of electric dipoles in the PEI interlayer. These electric dipoles reduced the Schottky barrier height by neutralizing the charges at the metal/oxide semiconductor interface, and the holes passed the reduced Schottky barrier by means of tunneling or thermionic injection. In this way, p-type oxide TFTs, which generally need a noble metal having a high work function as an electrode, were demonstrated with a low-work-function metal. As a basic application for logic circuits, a complementary inverter based on n-type indium-gallium-zinc oxide and p-type Cu₂O TFTs was fabricated using only Al source/drain electrodes. This research achieved advances in low-cost circuit design by broadening the electrode metals available for the manufacture of p-type oxide semiconductor-based electronics.

Janus Nanocages of Platinum-Group Metals and Their Use as Effective Dual-Electrocatalysts

Janus nanocages with distinctive platinum-group metals on the outer and inner surfaces can naturally catalyze at least two different reactions. Here we report a general method based on successive deposition and then selective etching for the facile synthesis of such nanocages. We have fabricated 11 different types of Janus nanocages characterized by a uniform size and well-defined {100} facets, together with porous, ultrathin, asymmetric walls up to 1.6 nm thick. When tested as dual-electrocatalysts toward oxygen reduction and evolution reactions, the Janus nanocages based on Pt and Ir exhibited superior activities depending on the thickness and relative position of the metal layer. Density functional theory studies suggest that the alloy composition and surface structure of the nanocages both play important roles in enhancing the electrocatalytic activities by modulating the stability of key reaction intermediates.

Magical Mathematical Formulas for Nanoboxes

Hollow nanostructures are at the forefront of many scientific endeavors. These consist of nanoboxes, nanocages, nanoframes, and nanotubes. We examine the mathematics of atomic coordination in nanoboxes. Such structures consist of a hollow box with n shells and t outer layers. The magical formulas we derive depend on both n and t. We find that nanoboxes with t = 2  or  3, or walls with only a few layers generally have bulk coordinated atoms. The benefits of low-coordination in nanostructures is shown to only occur when the wall thickness is much thinner than normally synthesized. The case where t = 1 is unique, and has distinct magic formulas. Such low-coordinated nanoboxes are of interest for a myriad variety of applications, including batteries, fuel cells, plasmonic, catalytic and biomedical uses. Given these formulas, it is possible to determine the surface dispersion of the nanoboxes. We expect these formulas to be useful in understanding how the atomic coordination varies with n and t within a nanobox.

Physical Transformations of Noble-Metal Nanocrystals upon Thermal Activation

ConspectusThe last two decades have witnessed the successful development of noble-metal nanocrystals with well-controlled properties for a variety of applications in catalysis, plasmonics, electronics, and biomedicine. Most of these nanocrystals are kinetically controlled products greatly deviated from the equilibrium state defined by thermodynamics. When subjected to elevated temperatures, their arrangements of atoms are expected to undergo various physical transformations, inducing changes to the shape, morphology (hollow vs solid), spatial distribution of elements (segregated vs alloyed/intermetallic), internal structure (twinned vs single-crystal), and crystal phase. In order to optimize the performance of these nanocrystals in various applications, there is a pressing need to understand and improve their thermal stability.By integrating in situ heating with transmission electron microscopy or X-ray diffraction, we have investigated the physical transformations of various types of noble-metal nanocrystals in real time. We have also explored the atomistic detail responsible for a physical transformation using first-principles calculations, providing insightful guidance for the development of noble-metal nanocrystals with augmented thermal stability. Specifically, solid nanocrystals were observed to transform into pseudospherical particles favored by thermodynamics by reducing the surface area while eliminating the facets high in surface energy. For nanocrystals of relatively large in size, a single-crystal lattice was more favorable than a twinned structure. When switching to core-shell nanocrystals, the elevation in temperature caused changes to the elemental distribution in addition to shape transformation. The compositional stability of a core-shell nanocrystal was found to be strongly dependent on the shape and thus the type of facet expressed on the surface. For hollow nanocrystals such as nanocages and nanoframes, their thermal stabilities were typically inferior to the solid counterparts, albeit their unique structure and large specific surface area are highly desired in applications such as catalysis. When a metastable crystal structure was involved, phase transition was also observed at a temperature close to that responsible for shape or compositional change. We hope the principles, methodologies, and mechanistic insights presented in this Account will help the readers achieve a good understanding of the physical transformations that are expected to take place in noble-metal nanocrystals when they are subjected to thermal activation. Such an understanding may eventually lead to the development of effective methods for retarding or even preventing some of the transformations.

The impact of synthetic method on the catalytic application of intermetallic nanoparticles

Intermetallic alloy nanocrystals have emerged as a promising next generation of nanocatalyst, largely due to their promise of surface tunability. Atomic control of the geometric and electronic structure of the nanoparticle surface offers a precise command of the catalytic surface, with the potential for creating homogeneous active sites that extend over the entire nanoparticle. Realizing this promise, however, has been limited by synthetic difficulties, imparted by differences in parent metal crystal structure, reduction potential, and atomic size. Further, little attention has been paid to the impact of synthetic method on catalytic application. In this review, we seek to connect the two, organizing the current synthesis methods and catalytic scope of intermetallic nanoparticles and suggesting areas where more work is needed. Such analysis should help to guide future intermetallic nanoparticle development, with the ultimate goal of generating precisely controlled nanocatalysts tailored to catalysis.

Engineering the electronic and strained interface for high activity of PdMcore@Ptmonolayer electrocatalysts for oxygen reduction reaction

Alloyed nanoparticles with core-shell structures provide a favorable model to modulate interfacial interaction and surface structures at the atomic level, which is important for designing electrocatalysts with high activity and durability. Herein, core-shell structured Pd₃M@Pt/C nanoparticles with binary PdM alloy cores (M = Fe, Ni, and Co) and a monolayer Pt shell were successfully synthesized with diverse interfaces. Among these, Pd₃Fe@Pt/C exhibited the best oxygen reduction reaction catalytic performance, roughly 5.4 times more than that of the commercial Pt/C catalyst used as reference. The significantly enhanced activity is attributed to the combined effects of strain engineering, interfacial electron transfer, and improved Pt utilization. Density functional theory simulations and extended X-ray absorption fine structure analysis revealed that engineering the alloy core with moderate lattice mismatch and alloy composition (Pd₃Fe) optimizes the surface oxygen adsorption energy, thereby rendering excellent electrocatalytic activity. Future researches may use this study as a guide on the construction of highly effective core-shell electrocatalysts for various energy conversions and other applications.

Catalytic Nanoframes and Beyond

The ever-increasing need for the production and expenditure of sustainable energy is a result of the astonishing rate of consumption of fossil fuels and the accompanying environmental problems. Emphasis is being directed to the generation of sustainable energy by the fuel cell and water splitting technologies. Accordingly, the development of highly efficient electrocatalysts has attracted significant interest, as the fuel cell and water splitting technologies are critically dependent on their performance. Among numerous catalyst designs under investigation, nanoframe catalysts have an intrinsically large surface area per volume and a tunable composition, which impacts the number of catalytically active sites and their intrinsic catalytic activity, respectively. Nevertheless, the structural integrity of the nanoframe during electrochemical operation is an ongoing concern. Some significant advances in the field of nanoframe catalysts have been recently accomplished, specifically geared to resolving the catalytic stability concerns and significantly boosting the intrinsic catalytic activity of the active sites. Herein, general synthetic concepts of nanoframe structures and their structure-dependent catalytic performance are summarized, along with recent notable advances in this field. A discussion on the remaining challenges and future directions, addressing the limitations of nanoframe catalysts, are also provided.

Integration mesoporous surface and hollow cavity into PtPdRh nano-octahedra for enhanced oxygen reduction electrocatalysis

Design and synthesis of Pt-based nanocrystals with controlled structural diversity and complexity can potentially bring about multifunctional properties. In this work, we present a facile two-step strategy for the construction of the PtPdRh mesoporous octahedral nanocages (PtPdRh MONCs). This unique nanoarchitectonics rationally integrates multiple advantages (i.e. the octahedral shape, hollow cavity and mesoporous surface) into one catalyst, which facilitates the efficient utilization of noble metal atoms at both of the interior and exterior surfaces. As expected, the resultant PtPdRh MONCs could effectively catalyze the oxygen reduction reaction (ORR) under acidic conditions. The demonstrated ORR activity and catalytic durability are superior to the commercial Pt/C catalyst. The present study would provide a general guidance for architectural and compositional engineering of noble metal nanocrystals with desired functionalities and properties.

Concave PtCo nanooctahedra with high-energy {110} facets for the oxygen reduction reaction

For the first time, iminodiacetic acid serves as a morphology control agent for the synthesis of concave PtCo nanooctahedra.

Rapid synthesis of hollow PtPdCu trimetallic octahedrons at room temperature for oxygen reduction reactions in acid media

Hollow PtPdCu trimetallic octahedrons were prepared under mild conditions, exhibiting enhanced activity toward the oxygen reduction reaction in acid media.

Hollow PtFe Alloy Nanoparticles Derived from Pt-Fe3 O4 Dimers through a Silica-Protection Reduction Strategy as Efficient Oxygen Reduction Electrocatalysts

The development of efficient and stable electrocatalysts for the oxygen reduction reaction (ORR) is critical for the large-scale production of fuel cells. Platinum (Pt) nanoparticle catalysts show excellent performance for ORR, though the high cost of Pt is a limiting factor that directly impacts fuel cell production costs. Alloying Pt with other transition metals is an effective strategy to reduce Pt utilization whilst maintaining good ORR performance. In this work, novel hollow PtFe alloy catalysts were successfully synthesized by high-temperature pyrolysis of SiO₂ -coated Pt-Fe₃ O₄ nanoparticle dimers supported on carbon at 900 °C, followed by SiO₂ shell removal and partial dealloying of the PtFe nanoparticles formed using HF. The obtained hollow PtFe nanoparticle catalysts (denoted herein as PtFe-900) showed a 2.3-fold enhancement in ORR mass activity compared to PtFe nanoparticles synthesized without SiO₂ protection, and a remarkable 7.8-fold enhancement relative to a commercial Pt/C catalyst. Further, after 10 000 potential cycles, the ORR mass activity of PtFe-900 remained very high (90.9 % of the initial mass activity). The outstanding ORR performance of PtFe-900 can be attributed to the modification of Pt lattice and electronic structure by alloying with Fe at high temperature under the protection of the SiO₂ coating. This work guides the development of improved, highly dispersed Pt-based alloy nanoparticle catalysts for ORR and fuel cell applications.

Insights into Compositional and Structural Effects of Bimetallic Hollow Mesoporous Nanospheres toward Ethanol Oxidation Electrocatalysis

A one-pot soft-templating method is reported to fabricate nanosized bimetallic PdAg hollow mesoporous nanospheres (HMSs) for electrocatalytic ethanol oxidation reaction (EOR). The synthesis relies on the "dual-template" surfactant of dioctadecyldimethylammonium chloride that drives in situ growth of mesoporous frameworks on the surface of vesicles into the HMSs with radially opened mesochannels. The synthetic protocol is extendable to engineer elemental compositions and hierarchical nanostructures of PdAg nanoalloys. This system thus provides a direct yet solid platform to understand catalytic add-in synergies of PdAg HMSs toward electrochemical EOR. By evaluating compositional and structural features separately, bimetallic Pd₆₅Ag₃₅ HMSs display the highest EOR activity with a mass activity of 4.61 A mg_Pd^-1. Mechanism studies indicate that synergistically electronic and bifunctional effects as well as structural advantages of Pd₆₅Ag₃₅ HMSs kinetically optimize the removal of poisoning carbonaceous intermediates and accelerate the diffusion processes (the rate-determining step), and thus promote the EOR performance accordingly.

Ruthenium Nanoframes in the Face-Centered Cubic Phase: Facile Synthesis and Their Enhanced Catalytic Performance

Owing to their highly open structure and a large number of low-coordination sites on the surface, noble-metal nanoframes are intriguing for catalytic applications. Here, we demonstrate the rational synthesis of Ru cuboctahedral nanoframes with enhanced catalytic performance toward hydrazine decomposition. The synthesis starts from Pd nanocubes, which quickly undergo truncation at the corners as a consequence of oxidative etching caused by Br^- ions. Afterward, the galvanic replacement reaction between Pd and Ru(III) ions dominates, leading to the selective deposition of Ru atoms on the corners and edges and thereby the fabrication of Pd@Ru core-frame cuboctahedra. Significantly, the deposited Ru atoms are crystallized in a face-centered cubic (fcc) phase instead of the hexagonal close-packed (hcp) structure typical of bulk Ru. Upon the removal of Pd remaining in the core via chemical etching, we obtain Ru cuboctahedral nanoframes. By varying the amount of the Ru(III) precursor, the ridge thickness of the nanoframes can be tuned from a few atomic layers up to 10. Both the frame structure and fcc crystal phase of the Ru cuboctahedral nanoframes can be well preserved up to 300 °C. When compared with hcp-Ru nanoparticles, the fcc-Ru nanoframes displayed substantial enhancement in terms of H₂ selectivity toward hydrazine decomposition. This work offers the opportunity to engineer both the morphology and crystal phase of Ru nanocrystals for catalytic applications.

Magnetically Hollow Pt Nanocages with Ultrathin Walls as a Highly Integrated Nanoreactor for Catalytic Transfer Hydrogenation Reaction

Fabricating efficient and stable nanocatalysts for chemoselective hydrogenation of nitroaromatics is highly desirable because the amines hold tremendous promise for the synthesis of nitrogen containing chemicals. Here, a highly reactive and stable porous carbon nitride encapsulated magnetically hollow platinum nanocage is developed with subnanometer thick walls (Fe3O4@snPt@PCN) for this transformation. This well-controlled nanoreactor is prepared via the following procedures: the preparation of core template, the deposition of platinum nanocage with subnanometer thick walls, oxidative etching, and calcination. This highly integrated catalyst demonstrates excellent performance for the catalytic transfer hydrogenation of various nitroaromatics and the reaction can reach >99% conversion and >99% selectivity. With the ultrathin wall structure, the atom utilization of platinum atoms is highly efficient. The X-ray photoelectron spectroscopy results indicate that partial electrons transfer from the iron oxides to Pt nanowalls, and this increases the electron density of snPt nanoparticles, thus promoting the catalytic activity for the transfer hydrogenation of nitroaromatics. For the reduction of 4-nitrophenol, the reaction rate constant K app is 0.23 min-1 and the turnover frequency (TOF) is up to 3062 h-1. Additional reaction results illustrate that this magnetic nanoreactor can be reused more than eight times and it is a promising catalytic nanoplatform in heterogeneous catalysis.

One-pot synthesis of hollow hydrangea Au nanoparticles as a dual catalyst with SERS activity for in situ monitoring of a reduction reaction

The controlled synthesis of metallic nanomaterials has attracted the interest of many researchers due to their shape-dependent physical and chemical properties. However, most of the synthesized nanocrystals cannot be combined with spectroscopy to measure the reaction kinetics, thus limiting their use in monitoring the catalytic reaction process to elucidate its mechanism. As a powerful analytical tool, surface-enhanced Raman spectroscopy (SERS) can be used to achieve in situ monitoring of catalytic reactions by developing bifunctional metal nanocrystals with both SERS and catalytic activities. Herein, we have developed a simple one-pot synthesis method for the large-scale and size-controllable preparation of highly rough hydrangea Au hollow nanoparticles. The growth mechanism of flower-like Au hollow nanostructures was also discussed. The hollow nanostructure with a 3D hierarchical flower shell combines the advantages of hollow nanostructure and hierarchical nanostructure, which possess high SERS activity and good catalytic activity simultaneously. Furthermore, the hydrangea Au hollow crystals were used as a bifunctional nanocatalyst for in situ monitoring of the reduction reaction of 4-nitrothiophenol to the 4-aminothiophenol.

Designing Robust Support for Pt Alloy Nanoframes with Durable Oxygen Reduction Reaction Activity

Polymer electrolyte membrane fuel cells are appealing to resolve the environmental and energy issues but are still heavily inhibited by the dilemma of fabricating effective and durable electrocatalysts for oxygen reduction reaction (ORR). In this work, highly open Pt₃Cu nanoframes and one-dimensional, hollow titanium nitride architectures are both successfully explored and implemented as the new system to replace the traditional Pt-carbon motif. The ORR performance of the obtained electrocatalyst shows specific and mass activities of 5.32 mA cm^-2 and 2.43 A mg_Pt^-1, respectively, which are 14.4 and 11.6 times higher than those of the commercial Pt/C. Notably, the novel catalyst also exhibits high stability and a much slower performance degradation than that of the benchmarked Pt₃Cu/C with the same durability testing procedures. The comprehensive data confirm that the new type of catalyst possesses a high charge transfer during the ORR process, and the unique structure and synergistic effects between anchored Pt and the support mainly contributes to the high stability. This work provides a strategic method for designing an effective ORR electrocatalyst with desirable stability.

Porous PdRh nanobowls: facile synthesis and activity for alkaline ethanol oxidation

Optimizing structure and composition with respect to electrocatalytic performance is critical to achieve outstanding Pd-based electrocatalysts. Herein, we have successfully developed a novel electrocatalyst of hollow and porous PdRh nanobowls (PdRh NBs) for the ethanol oxidation reaction (EOR) by using urea as a guiding surfactant. Under alkaline hydrothermal conditions, urea molecules can release bubbles (NH3 and CO2) that in turn guide the formation of PdRh nanobowls. The porous bowl-like structures of PdRh NBs expose abundant surface sites, which allows for increased collision frequency via confining reactants within open spaces. In regards to composition, the reason for introducing Rh is that not only is the redox potential of Rh approximate with that of Pd (beneficial to the formation of high PdRh alloy phase), but also it can effectively facilitate the breakage of C-C bond on the electrode surface (enhancing the total oxidation of ethanol to CO2). Benefiting from the compositional and structural advantages, the newly developed PdRh NBs exhibit significantly improved electrocatalytic activity for the EOR compared with those of the pure Pd NBs, PdRh nanoparticles (PdRh NPs) and commercial Pd black. These attributes might make them good anodic candidates for application in direct ethanol fuel cells.

Controlled-Size Hollow Magnesium Sulfide Nanocrystals Anchored on Graphene for Advanced Lithium Storage

Magnesium sulfide (MgS), representative of alkaline-earth metal chalcogenides (AEMCs), is a potential conversion/alloy-type electrode material for lithium ion batteries (LIBs), by virtue of its low potential, high theoretical capacity, and abundant magnesium resource. However, the limited capacity utilization and inferior rate performance still hinder its practical application, and the progress is rather slow due to the challenging fabrication technique for MgS. Herein, we report a series of controlled-size hollow MgS nanocrystals (NCs) homogeneously distributed on graphene (MgS@G), fabricated through a metal hydride framework (MHF) strategy, and its application as advanced electrode material for LIBs. The hollow structure of MgS NCs is mainly attributed to the Kirkendall effect and the escape of hydrogen atoms from metal hydride during sulfuration. The as-synthesized MgS@G demonstrates robust nanoarchitecture and admirable interactions, which ensure a spatially confined lithiation/delithiation process, optimize the dynamics of two-steps conversion/alloying reactions, and induce a synergetic pseudocapacitive storage contribution. As a result, a representative MgS@G composite delivers a largely enhanced capacity of >1208 mAh g^-1 at a current density of 100 mA g^-1 and a long-term cycle stability at a high current density of 5 A g^-1 with a capacity of 838 mAh g^-1 over 3000 cycles, indicating well-sustained structural integrity. This work presents an effective route toward the development of high-performance magnesium-based material for energy storage.

Multimetallic Hollow Mesoporous Nanospheres with Synergistically Structural and Compositional Effects for Highly Efficient Ethanol Electrooxidation

Controlling the nanostructures and chemical compositions of the electrochemical nanocatalysts has been recognized as two prominent means to kinetically promote the electrocatalytic performance. Herein, we report a general "dual"-template synthesis methodology for the formation of multimetallic hollow mesoporous nanospheres (HMSs) with an adjustable interior hollow cavity and cylindrically opened mesoporous shell as a highly efficient electrocatalyst for ethanol oxidation reaction. Three-dimensional trimetallic PdAgCu HMSs were synthesized via in situ coreduction of Pd, Ag, and Cu precursors on "dual"-template structural directing surfactant of dioctadecyldimethylammonium chloride in optimal synthesis conditions. Due to synergistic advantages on hollow mesoporous nanostructures and multimetallic compositions, the resultant PdAgCu HMSs exhibited significantly enhanced electrocatalytic performance toward ethanol oxidation reaction with a mass activity of 5.13 A mg_Pd ^-1 at a scan rate of 50 mV s^-1 and operation stability (retained 1.09 A mg_pd ^-1 after the electrocatalysis). The "dual"-template route will open a new avenue to rationally design multimetallic HMSs with controlled functions for broad applications.