Co@C Nanoparticle Embedded Hierarchically Porous N-Doped Hollow Carbon for Efficient Oxygen Reduction

The rational construction of highly active and stable non-noble metal electrocatalysts for the oxygen reduction reaction (ORR) is an ongoing challenge for practical applications of catalysts. Here, we report a novel nanostructured hollow N-doped carbon hybrid through pyrolysis of silica@CoZn-coordinated zeolitic imidazolate frameworks. The carbon layer encased cobalt nanoparticles were embedded in the hierarchically porous carbon catalyst (Co@C-HN-hC). Profiting from the synergistic effect between highly active Co@C NPs and HN-hC, the Co@C-HN-hC catalyst exhibited remarkable catalytic performances as compared to porous N-doped hollow carbon (N-hC) and N-doped carbon encased Co NPs (Co@N-C). The electrochemical measurements show that the performances of the Co@C-HN-hC catalyst is close to that of the Pt/C catalysts, along with an excellent stability and durability in the ORR process. This study provides a guideline for controllable design of carbon-based ORR catalysts for substituting noble metal catalysts.

Scalable Preparation of the Chemically Ordered Pt-Fe-Au Nanocatalysts with High Catalytic Reactivity and Stability for Oxygen Reduction Reactions

Carbon-supported Au-Pt _xFe _y nanoparticles were synthesized via microwave heating polyol process, followed by annealing for the formation of the ordered structure. The structure characterizations indicate that Au is alloyed with intermetallic Pt-Fe nanoparticles and therefore the surface electronic properties are tuned. The electrochemical tests show that the microwave heating polyol process is more effective than oil bath heating polyol process for synthesizing the highly active catalysts. The introduction of trace Au (0.2 wt % Au) significantly improves the oxygen reduction reaction (ORR) catalytic activity of Pt _xFe _y catalysts. Au-PtFe/C-H (0.66 A/mg_Pt) and Au-PtFe₃/C-H (0.63 A/mg_Pt) prepared in a batch of 10.0 g show significantly improved catalytic activities than their counterparts (PtFe/C-H and PtFe₃/C-H) as well as commercial Johnson Matthey Pt/C (0.17 A/mg_Pt). In addition, the as-prepared Au-PtFe/C-H and Au-PtFe₃/C-H display highly enhanced stability toward the ORR compared to the commercial Pt/C. The superior catalytic performance is attributed to the synergistic effect of chemically ordered intermetallic structure and Au. This work provides a scalable synthesis of the multimetallic chemically ordered Au-Pt _xFe _y catalysts with high ORR catalytic performance in acidic condition.

Synthesis of Hierarchical 4H/fcc Ru Nanotubes for Highly Efficient Hydrogen Evolution in Alkaline Media

Hierarchical metal nanostructures containing 1D nanobuilding blocks have stimulated great interest due to their abundant active sites for catalysis. Herein, hierarchical 4H/face-centered cubic (fcc) Ru nanotubes (NTs) are synthesized by a hard template-mediated method, in which 4H/fcc Au nanowires (NWs) serve as sacrificial templates which are then etched by copper ions (Cu²⁺ ) in dimethylformamide. The obtained hierarchical 4H/fcc Ru NTs contain ultrathin Ru shells (5-9 atomic layers) and tiny Ru nanorods with length of 4.2 ± 1.1 nm and diameter of 2.2 ± 0.5 nm vertically decorated on the surface of Ru shells. As an electrocatalyst for the hydrogen evolution reaction in alkaline media, the hierarchical 4H/fcc Ru NTs exhibit excellent electrocatalytic performance, which is better than 4H/fcc Au-Ru NWs, commercial Pt/C, Ru/C, and most of the reported electrocatalysts.

'Painting' nanostructured metals-playing with liquid metal

Materials scientists always dream to 'paint' nanostructured metal on a metallic foil, just as artists paint a painting on a canvas. Herein, we have, for the first time, realized this dream using liquid gallium (Ga) as the paint. Through a liquid Ga stimulated painting-alloying-dealloying strategy, seven kinds of nanostructured metallic films including Au, Ag, Pd, Pt, Cu, Co, and Ni were generally fabricated and supported on the corresponding metallic foils. Owing to the painting-like operation, nanostructured films with complicated patterns and large sizes (up to meters) were successfully produced without any shape/size limitations. The nanostructured metallic films possess advantages like their unique nanoporous structures, self-supporting nature, good continuity, flexibility and high specific surface areas, and could serve as robust electrodes in devices like batteries, fuel cells, water splitting electrolyzers, etc. Moreover, the proposed strategy shows great potential in the fabrication of other self-supporting, flexible, advanced nanomaterials.

High performance layer-by-layer Pt3Ni(Pt-skin)-modified Pd/C for the oxygen reduction reaction

A core (Pd)/shell (Pt₃Ni(Pt-skin)) ORR catalyst was obtained, and displayed eminently superior catalytic performance to state-of-the-art Pt–Ni catalysts.

Bimetallic Pt–Ni with Pt on the outermost layer and an innermost layer enriched in Ni, referred to as Pt₃Ni(Pt-skin), is a promising configuration of an electrocatalyst for the oxygen reduction reaction (ORR) in fuel cells. We prepare a core (Pd)/shell (Pt₃Ni(Pt-skin)) catalyst (Pt₃Ni(Pt-skin)/Pd/C) from Zn underpotential deposition (UPD) on a Ni UPD modified Pd/C catalyst, facilitating Pt atomic layer-by-layer growth on the Ni surface through the galvanic replacement process. Pt₃Ni(Pt-skin)/Pd/C shows the best ORR performance, with a Pt specific activity of 16.7 mA cm^–2 and Pt mass activity of 14.2 A mg_Pt^–1, which are 90- and 156- fold improvements over commercial Pt/C catalysts. The Pt₃Ni(Pt-skin) structure effectively inhibits Ni leaching to improve the durability in two accelerated durability test modes mimicking the catalyst lifetime and start-up/shut-down cycles.

Two-Dimensional Metal Nanomaterials: Synthesis, Properties, and Applications

As one unique group of two-dimensional (2D) nanomaterials, 2D metal nanomaterials have drawn increasing attention owing to their intriguing physiochemical properties and broad range of promising applications. In this Review, we briefly introduce the general synthetic strategies applied to 2D metal nanomaterials, followed by describing in detail the various synthetic methods classified in two categories, i.e. bottom-up methods and top-down methods. After introducing the unique physical and chemical properties of 2D metal nanomaterials, the potential applications of 2D metal nanomaterials in catalysis, surface enhanced Raman scattering, sensing, bioimaging, solar cells, and photothermal therapy are discussed in detail. Finally, the challenges and opportunities in this promising research area are proposed.

Emerging Hierarchical Aerogels: Self-Assembly of Metal and Semiconductor Nanocrystals

Aerogels assembled from colloidal metal or semiconductor nanocrystals (NCs) feature large surface area, ultralow density, and high porosity, thus rendering them attractive in various applications, such as catalysis, sensors, energy storage, and electronic devices. Morphological and structural modification of the aerogel backbones while maintaining the aerogel properties enables a second stage of the aerogel research, which is defined as hierarchical aerogels. Different from the conventional aerogels with nanowire-like backbones, those hierarchical aerogels are generally comprised of at least two levels of architectures, i.e., an interconnected porous structure on the macroscale and a specially designed configuration at local backbones at the nanoscale. This combination "locks in" the inherent properties of the NCs, so that the beneficial genes obtained by nanoengineering are retained in the resulting monolithic hierarchical aerogels. Herein, groundbreaking advances in the design, synthesis, and physicochemical properties of the hierarchical aerogels are reviewed and organized in three sections: i) pure metallic hierarchical aerogels, ii) semiconductor hierarchical aerogels, and iii) metal/semiconductor hybrid hierarchical aerogels. This report aims to define and demonstrate the concept, potential, and challenges of the hierarchical aerogels, thereby providing a perspective on the further development of these materials.

Shape-Controlled Synthesis of Colloidal Metal Nanocrystals by Replicating the Surface Atomic Structure on the Seed

Controlling the surface structure of metal nanocrystals while maximizing the utilization efficiency of the atoms is a subject of great importance. An emerging strategy that has captured the attention of many research groups involves the conformal deposition of one metal as an ultrathin shell (typically 1-6 atomic layers) onto the surface of a seed made of another metal and covered by a set of well-defined facets. This approach forces the deposited metal to faithfully replicate the surface atomic structure of the seed while at the same time serving to minimize the usage of the deposited metal. Here, the recent progress in this area is discussed and analyzed by focusing on the synthetic and mechanistic requisites necessary for achieving surface atomic replication of precious metals. Other related methods are discussed, including the one-pot synthesis, electrochemical deposition, and skin-layer formation through thermal annealing. To close, some of the synergies that arise when the thickness of the deposited shell is decreased controllably down to a few atomic layers are highlighted, along with how the control of thickness can be used to uncover the optimal physicochemical properties necessary for boosting the performance toward a range of catalytic reactions.

Transient modes of zeolite surface growth from 3D gel-like islands to 2D single layers

Zeolite crystallization occurs by multifaceted processes involving molecule attachment and nonclassical pathways governed by the addition of amorphous precursors. Here, we use scanning probe microscopy to monitor zeolite LTA crystallization in situ with a spatiotemporal resolution that captures dynamic processes in real time. We report a distinctive pathway involving the formation of gel-like islands from supersaturated solutions comprised of (alumino)silicate molecules. Three-dimensional assembly and evolution of these islands constitutes a unique mode of growth that differs from classical theories. Time-resolved imaging also reveals that growth can occur by (nearly) oriented attachment. At later stages of crystallization, a progressive transition to lower supersaturation shifts growth to a layered mechanism involving two-dimensional nucleation and spreading of layers. Here, we show that LTA crystallization occurs by multiple pathways, thereby reconciling putative hypotheses of growth mechanisms while also highlighting new modes of nonclassical crystallization that may prove relevant to other zeolites and related materials.

On-Chip in Situ Monitoring of Competitive Interfacial Anionic Chemisorption as a Descriptor for Oxygen Reduction Kinetics

The development of future sustainable energy technologies relies critically on our understanding of electrocatalytic reactions occurring at the electrode-electrolyte interfaces, and the identification of key reaction promoters and inhibitors. Here we present a systematic in situ nanoelectronic measurement of anionic surface adsorptions (sulfates, halides, and cyanides) on ultrathin platinum nanowires during active electrochemical processes, probing their competitive adsorption behavior with oxygenated species and correlating them to the electrokinetics of the oxygen reduction reaction (ORR). The competitive anionic adsorption features obtained from our studies provide fundamental insight into the surface poisoning of Pt-catalyzed ORR kinetics by various anionic species. Particularly, the unique nanoelectronic approach enables highly sensitive characterization of anionic adsorption and opens an efficient pathway to address the practical poisoning issue (at trace level contaminations) from a fundamental perspective. Through the identified nanoelectronic indicators, we further demonstrate that rationally designed competitive anionic adsorption may provide improved poisoning resistance, leading to performance (activity and lifetime) enhancement of energy conversion devices.

Ultrathin Wall (1 nm) and Superlong Pt Nanotubes with Enhanced Oxygen Reduction Reaction Performance

The synthesis of Pt nanotubes catalysts remains a substantial challenge, especially for those with both sub-nanometer wall thickness and micrometer-scale length characteristics. Combining techniques of insulin fibril template with Pd nanowire template, numerous Pt nanotubes with diameter of 5.5 nm, tube-length of several micrometers, and ultrathin wall thickness of 1 nm are assembled. These tubular catalysts with both open ends deliver electrochemical active surface area (ECSA) of 91.43 m² g_pt^-1 which results from multiple Pt atoms exposed on the inner and outer surfaces that doubled Pt atoms can participate in catalytic reactions, further with enhanced electrocatalytic performance for oxygen reduction reaction (ORR). The ultrafine Pt nanotubes represent a class of hollow nanostructure with increased Pt-utilization and large ECSA, which is regarded as a type of cost-effective catalysts for ORR.

Pt-Pd Bimetal Popcorn Nanocrystals: Enhancing the Catalytic Performance by Combination Effect of Stable Multipetals Nanostructure and Highly Accessible Active Sites

Exploration of highly efficient electrocatalysts is significantly urgent for the extensive adoption of the fuel cells. Because of their high activity and super stability, Pt-Pd bimetal nanocrystals have been widely recognized as one class of promising electrocatalysts for oxygen reduction. This article presents the synthesis of popcorn-shaped Pt-Pd bimetal nanoparticles with a wide composition range through a facile hydrothermal strategy. The hollow-centered nanoparticles are surrounded by several petals and concave surfaces. By exploring the oxygen reduction reaction on the carbon supported Pt-Pd popcorns in perchloric acid solution, it is found that compared with the commercial Pt/C catalyst the present catalysts display superior catalytic performances in aspects of catalytic activity and stability. More importantly, the Pt-Pd popcorns display minor performance degradations through prolonged potential cycling. The enhanced performances can be mainly attributed to the unique popcorn structure of the Pt-Pd components, which allows the appearance and long existence of the high active sites with more accessibility. The present work highlights the key roles of accessible high active sites in the oxygen reduction reaction, which will ultimately guide the design of highly durable Pt-Pd catalysts.

Platinum nanoparticles in nanobiomedicine

Oxidative stress-dependent inflammatory diseases represent a major concern for the population's health worldwide. Biocompatible nanomaterials with enzymatic properties could play a crucial role in the treatment of such pathologies. In this respect, platinum nanoparticles (PtNPs) are promising candidates, showing remarkable catalytic activity, able to reduce the intracellular reactive oxygen species (ROS) levels and impair the downstream pathways leading to inflammation. This review reports a critical overview of the growing evidence revealing the anti-inflammatory ability of PtNPs and their potential applications in nanomedicine. It provides a detailed description of the wide variety of synthetic methods recently developed, with particular attention to the aspects influencing biocompatibility. Special attention has been paid to the studies describing the toxicological profile of PtNPs with an attempt to draw critical conclusions. The emerging picture suggests that the material per se is not causing cytotoxicity, while other physicochemical features related to the synthesis and surface functionalization may play a crucial role in determining the observed impairment of cellular functions. The enzymatic activity of PtNPs is also summarized, analyzing their action against ROS produced by pathological conditions within the cells. In particular, we extensively discuss the potential of these properties in nanomedicine to down-regulate inflammatory pathways or to be employed as diagnostic tools with colorimetric readout. A brief overview of other biomedical applications of nanoplatinum is also presented.

Catalyst design by scanning probe block copolymer lithography

Scanning probe block copolymer lithography (SPBCL), in combination with density-functional theory (DFT), has been used to design and synthesize hydrogen evolution catalysts. DFT was used to calculate the hydrogen adsorption energy on a series of single-element, bimetallic, and trimetallic (Au, Pt, Ni, and Cu) substrates to provide leads that could be synthesized in the form of alloy or phase-separated particles via SPBCL. PtAuCu (18 nm, ∼1:1:1 stoichiometry) has been identified as a homogeneous alloy phase that behaves as an effective hydrogen evolution catalyst in acidic aqueous media, even when it is made in bulk form via solution phase methods. Significantly, the bulk-prepared PtAuCu/C nanocatalyst discovered via this process exhibits an activity seven times higher than that of the state-of-the-art commercial Pt/C catalyst (based upon Pt content). The advantage of using SPBCL in the discovery process is that one can uniformly make particles, each consisting of a uniform phase combination (e.g., all alloy or all phase-segregated species) at a fixed elemental ratio, an important consideration when working with polyelemental species where multiple phases may exist.

Crystal Engineering for Catalysis

Crystal engineering relies upon the ability to predictively control intermolecular interactions during the assembly of crystalline materials in a manner that leads to a desired (and predetermined) set of properties. Economics, scalability, and ease of design must be leveraged with techniques that manipulate the thermodynamics and kinetics of crystal nucleation and growth. It is often challenging to exact simultaneous control over multiple physicochemical properties, such as crystal size, habit, chirality, polymorph, and composition. Engineered materials often rely upon postsynthesis (top-down) processes to introduce properties that would otherwise be challenging to attain through direct (bottom-up) approaches. We discuss the application of crystal engineering to heterogeneous catalysts with a focus on four general themes: ( a) tailored nanocrystal size, ( b) controlled environments surrounding active sites, ( c) tuned morphology with well-defined facets, and ( d) hierarchical materials with disparate pore size and active site distributions. We focus on nonporous materials, including metals and metal oxides, and two classes of porous materials: zeolites and metal organic frameworks. We review novel synthesis methods involving synergistic experimental and computational design approaches, the challenges facing catalyst development, and opportunities for future advancement in crystal engineering.

Rational Design and Synthesis of Low-Temperature Fuel Cell Electrocatalysts

Abstract

Recent progresses in proton exchange membrane fuel cell electrocatalysts are reviewed in this article in terms of cathodic and anodic reactions with a focus on rational design. These designs are based around gaining active sites using model surface studies and include high-index faceted Pt and Pt-alloy nanocrystals for anodic electrooxidation reactions as well as Pt-based alloy/core–shell structures and carbon-based non-precious metal catalysts for cathodic oxygen reduction reactions (ORR). High-index nanocrystals, alloy nanoparticles, and support effects are highlighted for anodic catalysts, and current developments in ORR electrocatalysts with novel structures and different compositions are emphasized for cathodic catalysts. Active site structures, catalytic performances, and stability in fuel cells are also reviewed for carbon-based non-precious metal catalysts. In addition, further developmental perspectives and the current status of advanced fuel cell electrocatalysts are provided.

Graphical Abstract

Monocrystalline platinum-nickel branched nanocages with enhanced catalytic performance towards the hydrogen evolution reaction

Single crystalline noble metal nanocages are the most promising candidates for heterogeneous catalysis due to their large specific surface area, well-defined structure and enhanced structural stability. Herein, based on the observation of an unexpected phenomenon that the alloying of Pt and transition metals by co-reduction is more preferential than the formation of pure Pt NCs, we propose a feasible one-pot strategy to synthesize a uniformly epitaxial core-shell Pt-Ni structure with a Ni-rich alloy as the core and a Pt-rich alloy as the shell. The as-prepared Pt-Ni core-shell structures are subsequently etched into monocrystalline Pt-Ni branched nanocages with the wall thickness being 2.8 nm. This unique structure exhibits excellent catalytic performance and stability for the hydrogen evolution reaction (HER) in alkaline solution which is of great significance for the energy-intensive water-alkali and chlor-alkali industry.

Nanoscale kinetics of asymmetrical corrosion in core-shell nanoparticles

Designing new materials and structure to sustain the corrosion during operation requires better understanding on the corrosion dynamics. Observation on how the corrosion proceeds in atomic scale is thus critical. Here, using a liquid cell, we studied the real-time corrosion process of palladium@platinum (Pd@Pt) core-shell nanocubes via transmission electron microscopy (TEM). The results revealed that multiple etching pathways operatively contribute to the morphology evolution during corrosion, including galvanic etching on non-defected sites with slow kinetics and halogen-induced etching at defected sites at faster rates. Corners are the preferential corrosion sites; both etching pathways are mutually restricted during corrosion. Those insights on the interaction of nanostructures with reactive liquid environments can help better engineer the surface structure to improve the stability of electrocatalysts as well as design a new porous structure that may provide more active sites for catalysis.

Combination of hollow fluorescent carbon and gold nanoparticles: A super-catalyst

Hollow fluorescent carbon nanoparticles (HFCNs) have been combined with gold nanoparticles (AuNPs) to produce a special catalyst. The catalytic properties of HFCNs and AuNPs were exploited to conceptualize the new catalytic functionality. The AuNP-embedded-HFCNs produced in situ were found to massively enhance the rate of reduction of 4-nitrophenol (a model reaction) in presence of sodium borohydride. Comparison with functioning of other nanoparticulate catalysts on the same reaction proved our product to be an extremely efficient catalyst.

Albumin-coordinated assembly of clearable platinum nanodots for photo-induced cancer theranostics

Photoactive noble metal nanoparticles are of increasing importance toward personalized cancer therapy in the field of precision nanomedicine. A critical challenge remains in the exploration of clinically potential noble metal nanoparticles for highly efficient cancer theranostics. Here, we introduce albumin-coordinated assembly of clearable Pt nanodots (Pt-NDs) with monodisperse nanostructure as high-performance theranostic agents for imaging-guided photothermal tumor ablation. We precisely manipulate the reduction and growth of tetravalent Pt ions into ultrasmall nanodots through albumin-directed growth kinetics, thereby leading to the synthesis of monodisperse 6.7 nm Pt-NDs with albumin molecules as the corona. Pt-NDs exhibit the surface plasmon resonance at 225 nm with enhanced near-infrared (NIR) absorbance, ideal resistance to photo-bleaching, distinct photoacoustic and X-ray signals, as well as remarkable photothermal effect through non-radiative relaxation under NIR light irradiation. In particular, Pt-NDs possess preferable tumor accumulation, and effective in vivo excretory capability. Thus, these nanodots promote preferable in vivo microscopic photoacoustics and spatially anatomic CT imaging with enhanced contrast, as well as potent hyperthermia-mediated tumor ablation. These findings represent a facile and general approach to fabricate high-performance noble metal nanostructures with clinical potential for cancer theranostics.

Mechanism and kinetics of the electrocatalytic reaction responsible for the high cost of hydrogen fuel cells

The sluggish oxygen reduction reaction (ORR) is a major impediment to the economic use of hydrogen fuel cells in transportation. In this work, we report the full ORR reaction mechanism for Pt(111) based on Quantum Mechanics (QM) based Reactive metadynamics (RμD) simulations including explicit water to obtain free energy reaction barriers at 298 K. The lowest energy pathway for 4 e^- water formation is: first, *OOH formation; second, *OOH reduction to H₂O and O*; third, O* hydrolysis using surface water to produce two *OH and finally *OH hydration to water. Water formation is the rate-determining step (RDS) for potentials above 0.87 Volt, the normal operating range. Considering the Eley-Rideal (ER) mechanism involving protons from the solvent, we predict the free energy reaction barrier at 298 K for water formation to be 0.25 eV for an external potential below U = 0.87 V and 0.41 eV at U = 1.23 V, in good agreement with experimental values of 0.22 eV and 0.44 eV, respectively. With the mechanism now fully understood, we can use this now validated methodology to examine the changes upon alloying and surface modifications to increase the rate by reducing the barrier for water formation.

Ag3PO4 electrocatalyst for oxygen reduction reaction: enhancement from positive charge

We have demonstrated Ag3PO4 as an active non-Pt electrocatalyst with enhanced activity compared with Ag for oxygen reduction reaction (ORR). Density functional theory reveals that better ORR performance of Ag atoms on Ag3PO4 surface than that on pure silver surface originates from more appropriate oxygen adsorption on positively charged Ag atoms. Further study of the surface geometry of Ag3PO4 including tetrahedron, rhombic dodecahedron and cube indicates that the highest density of Ag and appropriate oxygen adsorption on {110} surface of rhombic dodecahedral Ag3PO4 lead to the highest ORR activity, which is about 12 times that of Pt catalysts from a commercial perspective. It may be applicable for developing low-cost and highly active non-Pt catalytic materials from a broader range of material systems.

Phase pure α-Mn2O3 prisms and their bifunctional electrocatalytic activity in oxygen evolution and reduction reactions

Synthesizing manganese oxide materials with exact control of the nanoparticle shape and phase is difficult, making it challenging to understand the influence of the surface structure on electrocatalysis. Here we describe an inexpensive, low-temperature method to synthesize single-crystal orthorhombic phase α-Mn₂O₃ prisms bound by the {100} facets. The synthesis is the first method to use the cation bridging effect to assist in the creation of α-Mn₂O₃ prisms. According to structural analysis using X-ray diffraction, X-ray pair-distribution function (PDF) measurements and high resolution transmission electron microscopy, the material is composed exclusively of α-Mn₂O₃ prisms, and no additional amorphous or nanocrystalline phases are present. Heating the prisms transformed the material to a more symmetrical, cubic phase α-Mn₂O₃ that exhibited strong bifunctional electrocatalytic activity for the oxygen evolution (OER) and oxygen reduction (ORR) reactions. We compared the oxygen electrode activities (OEA) and found that the α-Mn₂O₃ prisms performed 79% better than commercially-produced α-Mn₂O₃ powders, indicating that these α-Mn₂O₃ prisms perform well as inexpensive, earth-abundant materials for reversible electrodes.

Synthesis of ultrathin platinum nanoplates for enhanced oxygen reduction activity

Ultrathin Pt nanostructures exposing controlled crystal facets are highly desirable for their superior activity and cost-effectiveness in the electrocatalytic oxygen reduction reaction (ORR), and they are conventionally synthesized by epitaxial growth of Pt on a limited range of templates, such as Pd nanocrystals, resulting in a high cost and less structural diversity of the ultrathin Pt nanostructures. To solve this problem, we demonstrate that ultrathin Pt nanostructures can be synthesized by templating conveniently available Ag nanocrystals without involving galvanic replacement, which enables a much-reduced cost and controllable new morphologies, such as ultrathin Pt nanoplates that expose the {111} facets. The resulting ultrathin Pt nanoplates are ∼1-2 nm in thickness, which show an ∼22-fold increase in specific activity (5.3 mA cm-2), an ∼9.5-fold increase in mass activity (1.62 A mg-1) and significantly enhanced catalytic stability in the ORR, compared with the commercial Pt/C catalyst. We believe this strategy opens a door to a highly extendable family of ultrathin noble metal nanostructures, thus promising excellent activity and stability in a broad range of catalytic applications.

Prussian Blue Analogue Mesoframes for Enhanced Aqueous Sodium-ion Storage

Mesostructure engineering is a potential avenue towards the property control of coordination polymers in addition to the traditional structure design on an atomic/molecular scale. Mesoframes, as a class of mesostructures, have short diffusion pathways for guest species and thus can be an ideal platform for fast storage of guest ions. We report a synthesis of Prussian Blue analogue mesoframes by top-down etching of cubic crystals. Scanning and transmission electron microscopy revealed that the surfaces of the cubic crystals were selectively removed by HCl, leaving the corners, edges, and the cores connected together. The mesoframes were used as a host for the reversible insertion of sodium ions with the help of electrochemistry. The electrochemical intercalation/de-intercalation of Na+ ions in the mesoframes was highly reversible even at a high rate (166.7 C), suggesting that the mesoframes could be a promising cathode material for aqueous sodium ion batteries with excellent rate performance and cycling stability.

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

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

A General Strategy Assisted with Dual Reductants and Dual Protecting Agents for Preparing Pt-Based Alloys with High-Index Facets and Excellent Electrocatalytic Performance

Synthesizing noble metallic nanoparticles (NPs) enclosed by high-index facets (HIFs) is challenged as it involves the tuning of growth kinetics, the selective adsorption of certain chemical species, and the epitaxial growth from HIF enclosed seeds. Herein, a simple and general strategy is reported by using dual reduction agents and dual capping agents to prepare Pt-based alloy NPs with HIFs, in which both glycine and poly(vinylpyrrolidone) serve as the reductants and capping agents. Due to the facilely tunable growth/nucleation rates and protecting abilities of the reductants and capping agents, Pt concave nanocube (CNC), binary Pt-Ni CNC, ternary Pt-Mn-Cu CNC, and Pt-Mn-Cu ramiform polyhedron alloy NPs terminated by HIFs as well as other NPs with well-defined morphologies such as Pt-Mn-Cu nanocube and Pt-Mn-Cu nanoflower are obtained with this approach. Owing to the high density of low-coordinated Pt sites (HIF structure) and the unique electronic effect of Pt-Mn-Cu ternary alloys, the as-prepared Pt-Mn-Cu NPs show enhanced catalytic activity toward methanol and formic acid electro-oxidation reactions with excellent stability. This work provides a promising methodology for designing and fabricating Pt-based alloy NPs as efficient fuel cell catalyst.

Inflating hollow nanocrystals through a repeated Kirkendall cavitation process

The Kirkendall effect has been recently used to produce hollow nanostructures by taking advantage of the different diffusion rates of species involved in the chemical transformations of nanoscale objects. Here we demonstrate a nanoscale Kirkendall cavitation process that can transform solid palladium nanocrystals into hollow palladium nanocrystals through insertion and extraction of phosphorus. The key to success in producing monometallic hollow nanocrystals is the effective extraction of phosphorus through an oxidation reaction, which promotes the outward diffusion of phosphorus from the compound nanocrystals of palladium phosphide and consequently the inward diffusion of vacancies and their coalescence into larger voids. We further demonstrate that this Kirkendall cavitation process can be repeated a number of times to gradually inflate the hollow metal nanocrystals, producing nanoshells of increased diameters and decreased thicknesses. The resulting thin palladium nanoshells exhibit enhanced catalytic activity and high durability toward formic acid oxidation.

Owing to their unique properties, hollow metal nanocrystals demonstrate greater catalytic promise than their solid counterparts. Here the authors produce hollow and inflated palladium nanocrystals with thin shells via a repeated Kirkendall cavitation process, and demonstrate their activity for formic acid oxidation.

Formation of Ni-Fe Mixed Diselenide Nanocages as a Superior Oxygen Evolution Electrocatalyst

Exploring effective electrocatalysts is a crucial requirement for boosting the efficiency of water splitting to obtain clean fuels. Here, a self-templating strategy is reported to synthesize Ni-Fe mixed diselenide cubic nanocages for the electrocatalytic oxygen evolution reaction (OER). The diselenide nanocages are derived from corresponding Prussian-blue analog nanocages, which are first obtained by treating the nanocube precursor with a site-selective ammonia etchant. The resulting Ni-Fe mixed diselenide nanocages perform as a superior OER electrocatalyst, which affords a current density of 10 mA cm^-2 at a small overpotential of 240 mV; a high current density, mass activity, and turnover frequency of 100 mA cm^-2 , 1000 A g^-1 , and 0.58 s^-1 , respectively, at the overpotential of 270 mV; a Tafel slope as small as 24 mV dec^-1 ; and excellent stability in alkaline medium.

Unique Cu@CuPt Core-Shell Concave Octahedron with Enhanced Methanol Oxidation Activity

Although tremendous efforts have been devoted to the exploration of cost-effective, active, and stable electrochemical catalysts, only few significant breakthroughs have been achieved up to now. Therefore, exploring new catalysts and improving catalyst activity and stability are still major tasks at present. Controllable synthesis of Pt-based alloy nanocrystals with a uniform high-index surface and unique architecture has been regarded as an effective strategy to optimize their catalytic efficiency toward electrochemical reactions. Accordingly, here we present a one-pot facile solvothermal process to synthesize novel unique Cu@CuPt core-shell concave octahedron nanocrystals that exhibit both outstanding activity and long durability. By regulating temperatures during the synthesis process, we were able to control the reduction rate of Cu and Pt ions, which could subsequently lead to the sequential stacking of Cu and Pt atoms. Owing to the concave structure, the as-prepared core-shell nanoparticles hold a high-index surface of {312} and {413}. Such surfaces can provide a high density of atomic steps and terraces, which is suggested to be favorable for electrochemical catalysts. Specifically, the Cu@CuPt core-shell concave octahedron presents 8.6/13.1 times enhanced specific/mass activities toward the methanol oxidation reaction in comparison to those of a commercial Pt/C catalyst, respectively. Meanwhile, the as-prepared catalyst exhibits superior durability and antiaggregation properties under harsh electrochemical conditions. The facile method used here proposes a novel idea to the fabrication of nanocrystals with desired compositional distribution, and the as-prepared product offers exciting opportunities to be applied in direct methanol fuel cells.

Formation of the Protein Corona: The Interface between Nanoparticles and the Immune System

The interaction of inorganic nanoparticles and many biological fluids often withstands the formation of a Protein Corona enveloping the nanoparticle. This Protein Corona provides the biological identity to the nanoparticle that the immune system will detect. The formation of this Protein Corona depends not only on the composition of the nanoparticle, its size, shape, surface state and exposure time, but also on the type of media, nanoparticle to protein ratio and the presence of ions and other molecular species that interfere in the interaction between proteins and nanoparticles. This has important implications on immune safety, biocompatibility and the use of nanoparticles in medicine.

Engineering the internal surfaces of three-dimensional nanoporous catalysts by surfactant-modified dealloying

Tuning surface structures by bottom-up synthesis has been demonstrated as an effective strategy to improve the catalytic performances of nanoparticle catalysts. Nevertheless, the surface modification of three-dimensional nanoporous metals, fabricated by a top-down dealloying approach, has not been achieved despite great efforts devoted to improving the catalytic performance of three-dimensional nanoporous catalysts. Here we report a surfactant-modified dealloying method to tailor the surface structure of nanoporous gold for amplified electrocatalysis toward methanol oxidation and oxygen reduction reactions. With the assistance of surfactants, {111} or {100} faceted internal surfaces of nanoporous gold can be realized in a controllable manner by optimizing dealloying conditions. The surface modified nanoporous gold exhibits significantly enhanced electrocatalytic activities in comparison with conventional nanoporous gold. This study paves the way to develop high-performance three-dimensional nanoporous catalysts with a tunable surface structure by top-down dealloying for efficient chemical and electrochemical reactions.

Tuning the ratio of facets in materials can affect catalytic activity but methods to achieve this can be difficult to control. Here, using surfactants during dealloying, the authors prepare nanoporous gold with controlled facet ratios and show how it can significantly improve electrocatalytic activity.