Catalysis on faceted noble-metal nanocrystals: both shape and size matter

Development of Highly-Active Catalysts toward Oxygen Reduction by Controlling the Shape and Composition of Pt-Ni Nanocrystals

Electrocatalysts comprised of Pt-Ni alloy nanocrystals have garnered substantial attention due to their outstanding performance in catalyzing the oxygen reduction reaction (ORR). Herein, we present the synthesis of Pt-Ni nanocrystals with a variety of controlled shapes and compositions in an effort to investigate the impact of the Ni content on the formation of {111} facets and thereby the ORR activity. By completely excluding O2 from the reaction system, we could prevent the generation of Ni(OH)2 on the surface of the nanocrystals and thereby achieve a linear relationship between the atomic ratio of Pt to Ni in the nanocrystals and the feeding ratio of the precursors. The atomic ratio of Pt to Ni in the Pt-Ni nanocrystals was tunable within the range of 1.2-7.2, while their average sizes were kept around 9 nm in terms of edge length. In addition, a quantitative correlation between the area ratio of {111} to {100} facets and the feeding ratio of Pt(II) to Ni(II) was obtained by adjusting the mole fraction of the Ni(II) precursor in the reaction mixture. For the catalysts comprising octahedral nanocrystals, their specific ORR activities exhibited a positive correlation with the Pt/Ni atomic ratio. After the accelerated durability test, both specific and mass activity displayed a volcano-type trend with a peak value at a Pt/Ni atomic ratio of 1.6.

Chloride enables the growth of Ag nanocubes and nanowires by making PVP binding facet-selective

Solution-phase synthesis of metal nanocrystals with multiple additives is a common strategy for control over nanocrystal shape, and thus control over their properties. However, few rules are available to predict the effect of multiple capping agents on metal nanocrystal shapes, making it hard to rationally design synthetic conditions. This work uses a combination of seed-mediated growth, single-crystal electrochemistry, and DFT calculations to determine the roles of PVP and Cl^- in the anisotropic growth of single-crystal and penta-twinned silver nanocrystals. Single-crystal seeds grow into truncated octahedra bounded by a mixture of {111} and {100} facets in the presence of 0.03-30 mM PVP, but when 3-6 μM Cl^- is added with PVP, the single-crystal seeds grow into cubes bounded by {100} facets. Electrochemical measurements on Ag(100) and Ag(111) single-crystal electrodes show PVP is a capping agent but it exhibits no selectivity for a particular facet. Addition of Cl^- to PVP further passivates Ag(100) but not Ag(111), leading to conditions that favor formation of nanocubes. DFT calculations indicate the preferential binding of Cl^- to Ag(100) causes preferential binding of PVP to Ag(100). The combined results indicate the presence or absence of Cl^- modulates binding of PVP to (100) facets, leading to the formation of nanocubes with Cl^-, or truncated octahedra without it.

Modulating Dominant Facets of Pt through Multistep Selective Anchored on WC for Enhanced Hydrogen Evolution Catalysis

Facilitating the exposure of the active crystal facets on the surfaces of composite catalysts is a representative route to promote catalytic activity. Based on a tailored galvanic replacement reaction, herein, a self-assembly route is reported to prepare Pt-WC/CNT with Pt (200) preferential orientation and well-dispersed structure, which are capable of substantially boosting electrocatalysis in hydrogen evolution reaction (HER). Formation mechanism reveals that the (200)-dominated Pt-based catalysts form in galvanic replacement reaction through selective anchored on WC, and the multistep galvanic replacement process plays a critical role to realize the Pt (200)-dominated growth in higher Pt loading catalyst. These unique structural features endow the Pt-WC/CNT with a high turnover frequency of 94.18 H2·s-1 at 100 mV overpotential, 7-fold higher than that of commercial Pt/C (13.55 H2·s-1), ranking it among the most active catalysts. In addition, this method, which combines with gas-solid reaction and galvanic replacement reaction, paves the way to scalable synthesis as Pt facets-controllable composite catalysts to challenge commercial Pt/C.

Leveraging Pd(100)/SnO2 interfaces for highly efficient electrochemical formic acid oxidation

The electrocatalytic formic acid oxidation (FAO) is the crucial anodic reaction of direct formic acid fuel cells (DFAFCs), but its activity remains to be largely improved in order to be practically viable. The rational development of enhanced catalysts requires thorough consideration of various contributing factors that are possibly integrated in composite systems. Here, we demonstrate that, Pd(100)/SnO₂ interfaces, provided being efficiently exploited, can significantly boost FAO activity by a factor of ∼10, compared with pure Pd(100) facets, with the mass activity reaching a record of 14.55 A mg_Pd^-1 at a 40 mV-lower peak potential. Unique Pd/SnO₂ nanocomposites with a myriad of Pd(100)/SnO₂ interfaces were obtained by a newly developed successive seeded growth strategy, wherein pre-formed SnO₂ nanospheres are used as seeds for two-round overgrowth of multitudinous Pd nanocubes. Using electron microscopic, electrochemical, spectroscopic and computational analyses, we found that the Pd(100)/SnO₂ interfaces induce lattice contraction and electron loss on Pd nanocubes, which optimize intermediate binding during FAO. Moreover, we showed that the good cubicity of the Pd nanocubes and the presence of SnO₂ nearby further promote the activity by facilitating the potential-determining step and the elimination of the poisoning CO intermediate, respectively. As such, the combined high intrinsic activity and number density of Pd(100)/SnO₂ interfaces enabled the superior activity of the Pd/SnO₂ nanocomposites. The composite material presented here holds promise for application in DFAFCs, but equally importantly, the insights regarding the structure-performance relationship would be beneficial for designing efficient metal/oxide composite catalysts for diverse electro- and photo-catalytic reactions.

Concave gold nano-arrows (AuCNAs) for efficient catalytic reduction of 4-nitrophenol

Anisotropic gold nanostructures have attracted great attention in different fields including catalysis. Thermodynamically driven selective surface growth offers a reliable and reproducible method for anisotropic gold nanoparticle synthesis with specific morphologies. Herein, monocrystalline concave gold nano-arrows (AuCNAs) are prepared by the over-growth method using Au nanorods (AuNRs) as seeds. The as-prepared AuCNAs consist of a biconical head and four concave structures. Interestingly, silver ions (Ag⁺) concentration significantly affects the product morphology by tuning the peak positions of surface plasmon resonance (SPR), aspect ratio, arrow, and concave morphology of AuCNAs. The position of longitudinal SPR peaks is observed at 810, 805 and 782 nm at [Ag⁺]/[Au³⁺] molar ratios of 1:2, 1:1, and 2:1, respectively. Diameters and lengths of AuCNAs varied from 25 nm to 36 nm; 104 nm, 78 nm, and 120 nm, respectively. Additionally, the AuCNAs are applied for the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in presence of excess NaBH₄. Compared to gold nanorods (AuNRs), the prepared AuCNAs catalyst shows excellent catalytic activity, demonstrating that concave structures and sharp corners significantly enhance the catalytic activity. The value of pseudo-first-order reaction kinetic constants (k_app) increased from 0.0051 to 0.0195 s^-1 with increasing catalyst valume from 7.5 to 37.5 μL. The highest normalized reaction rate constant (K_nor) and turnover frequency (TOF) reach 5.84 × 10⁴ min^-1 mmol^-1 and 443.47 h^-1, respectively, at [Ag⁺]/[Au³⁺] ratio of 1:1 in AuCNAs catalyst. This study expands catalytic applications of anisotropic gold nanostructures and widens their potential application areas, such as surface plasmon exciton photonics, biomedical photonics, and photocatalysis.

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.

Strain Engineering: A Boosting Strategy for Photocatalysis

Whilst the photocatalytic technique is considered to be one of the most significant routes to address the energy crisis and global environmental challenges, the solar-to-chemical conversion efficiency is still far from satisfying practical industrial requirements, which can be traced to the suboptimal bandgap and electronic structure of photocatalysts. Strain engineering is a universal scheme that can finely tailor the bandgap and electronic structure of materials, hence supplying a novel avenue to boost their photocatalytic performance. Accordingly, to explore promising directions for certain breakthroughs in strained photocatalysts, an overview on the recent advances of strain engineering from the basics of strain effect, creations of strained materials, as well as characterizations and simulations of strain level is provided. Besides, the potential applications of strain engineering in photocatalysis are summarized, and a vision for the future controllable-electronic-structure photocatalysts by strain engineering is also given. Finally, perspectives on the challenges for future strain-promoted photocatalysis are discussed, placing emphasis on the creation and decoupling of strain effect, and the modification of theoretical frameworks.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Biotechnological synthesis of Pd-based nanoparticle catalysts

Palladium metal nanoparticles are excellent catalysts used industrially for reactions such as hydrogenation and Heck and Suzuki C-C coupling reactions. However, the global demand for Pd far exceeds global supply, therefore the sustainable use and recycling of Pd is vital. Conventional chemical synthesis routes of Pd metal nanoparticles do not meet sustainability targets due to the use of toxic chemicals, such as organic solvents and capping agents. Microbes are capable of bioreducing soluble high oxidation state metal ions to form metal nanoparticles at ambient temperature and pressure, without the need for toxic chemicals. Microbes can also reduce metal from waste solutions, revalorising these waste streams and allowing the reuse of precious metals. Pd nanoparticles supported on microbial cells (bio-Pd) can catalyse a wide array of reactions, even outperforming commercial heterogeneous Pd catalysts in several studies. However, to be considered a viable commercial option, the intrinsic activity and selectivity of bio-Pd must be enhanced. Many types of microorganisms can produce bio-Pd, although most studies so far have been performed using bacteria, with metal reduction mediated by hydrogenase or formate dehydrogenase enzymes. Dissimilatory metal-reducing bacteria (DMRB) possess additional enzymes adapted for extracellular electron transport that potentially offer greater control over the properties of the nanoparticles produced. A recent and important addition to the field are bio-bimetallic nanoparticles, which significantly enhance the catalytic properties of bio-Pd. In addition, systems biology can integrate bio-Pd into biocatalytic processes, and processing techniques may enhance the catalytic properties further, such as incorporating additional functional nanomaterials. This review aims to highlight aspects of enzymatic metal reduction processes that can be bioengineered to control the size, shape, and cellular location of bio-Pd in order to optimise its catalytic properties.

Recent Advances in Amino-Based Molecules Assisted Control of Noble-Metal Electrocatalysts

Morphology-control synthesis is an effective means to tailor surface structure of noble-metal nanocrystals, which offers a sensitive knob for tuning their electrocatalytic properties. The functional molecules are often indispensable in the morphology-control synthesis through preferential adsorption on specific crystal facets, or controlling certain crystal growth directions. In this review, the recent progress in morphology-control synthesis of noble-metal nanocrystals assisted by amino-based functional molecules for electrocatalytic applications are focused on. Although a mass of noble-metal nanocrystals with different morphologies have been reported, few review studies have been published related to amino-based molecules assisted control strategy. A full understanding for the key roles of amino-based molecules in the morphology-control synthesis is still necessary. As a result, the explicit roles and mechanisms of various types of amino-based molecules, including amino-based small molecules and amino-based polymers, in morphology-control of noble-metal nanocrystals are summarized and discussed in detail. Also presented in this progress are unique electrocatalytic properties of various shaped noble-metal nanocrystals. Particularly, the optimization of electrocatalytic selectivity induced by specific amino-based functional molecules (e.g., polyallylamine and polyethyleneimine) is highlighted. At the end, some critical prospects, and challenges in terms of amino-based molecules-controlled synthesis and electrocatalytic applications are proposed.

Facet Engineering of Pd Nanocrystals for Enhancing Photocatalytic Hydrogenation: Modulation of the Schottky Barrier Height and Enrichment of Surface Reactants

Metal cocatalyst loading is one of the most widely explored strategies in promoting photocatalytic solar energy conversion. Engineering surface-active facets of metal cocatalyst and exploring how they modulate the reactivity is crucial for the further development of advanced photocatalysts. In this work, through controlled hybridization of two-dimensional (2D) TiO₂ nanosheets with well-designed Pd nanocube (Pd NC) with exposed {100} facet and Pd nano-octahedron (NO) with exposed {111} facet, we unravel the distinct crystal facet effect of Pd cocatalyst in promoting the selective hydrogenation of nitroarenes to amines of TiO₂ photocatalyst. The activity tests show that the Pd NO with {111} facet is a more efficient cocatalyst than the Pd NC with exposed {100} facet. The prepared TiO₂-Pd NO composite displays a 900% enhancement of photocatalytic hydrogenation rate in comparison with bare TiO₂, while the TiO₂-Pd NC sample only shows a 200% photoactivity enhancement. Microscopic mechanism study discloses that the distinctive photoactivity improvement of Pd NO is ascribed to the concurrent modulation of the Schottky barrier height and enrichment of surface reactants: (i) the Pd NO with a lower Fermi level could result in steeper band bending of TiO₂ (i.e., higher Schottky barrier) than the Pd NC, which is more efficient in boosting interfacial separation and inhibiting the recombination of photoexcited charge pairs; and (ii) the {111} facet of Pd has higher nitroarenes adsorption ability and especially stronger hydrogen enrichment capability, thus accelerating the surface hydrogenation process and contributing to a higher reaction rate. This work emphasizes the rational facet control of cocatalysts for enhancing the photocatalytic hydrogenation performance.

Catalytically active and thermally stable core-shell gold-silica nanorods for CO oxidation

Deactivation based on sintering phenomena is one of the most costly issues for the industrial application of metal nanoparticle catalysts. To address this drawback, mesoporous silica encapsulation is reported as a promising strategy to stabilize metallic nanoparticles towards use in high temperature catalytic applications. These protective shells provide significant structural support to the nanoparticles, while the mesoporosity allows for efficient transport of the reactants to the catalytically active surface of the metallic nanoparticle in the core. Here, we extend the use of gold nanorods with mesoporous silica shells by investigating their stability in the CO oxidation reaction as an example of high temperature gas phase catalysis. Gold nanorods were chosen as the model system due to the availability of a simple, high yield synthesis method for both the metallic nanorods and the mesoporous silica shells. We demonstrate the catalytic activity of gold nanorods with mesoporous silica shells at temperatures up to 350 °C over several cycles, as well as the thermal stability up to 500 °C, and compare these results to surfactant-stabilized gold nanorods of similar size, which degrade, and lose most of their catalytic activity, before reaching 150 °C. These results show that the gold nanorods protected by the mesoporous silica shells have a significantly higher thermal stability than surfactant-stabilized gold nanorods and that the mesoporous silica shell allows for stable catalytic activity with little degradation at high temperatures.

Using Reduction Kinetics to Control and Predict the Outcome of a Colloidal Synthesis of Noble-Metal Nanocrystals

Improving the performance of noble-metal nanocrystals in various applications critically depends on our ability to manipulate their synthesis in a rational, robust, and controllable fashion. Different from a conventional trial-and-error approach, the reduction kinetics of a colloidal synthesis has recently been demonstrated as a reliable knob for controlling the synthesis of noble-metal nanocrystals in a deterministic and predictable manner. Here we present a brief Viewpoint on the recent progress in leveraging reduction kinetics for controlling and predicting the outcome of a synthesis of noble-metal nanocrystals. With a focus on Pd nanocrystals, we first offer a discussion on the correlation between the initial reduction rate and the internal structure of the resultant seeds. The kinetic approaches for controlling both nucleation and growth in a one-pot setting are then introduced with an emphasis on manipulation of the reduction pathways taken by the precursor. We then illustrate how to extend the strategy into a bimetallic system for the preparation of nanocrystals with different shapes and elemental distributions. Finally, the influence of speciation of the precursor on reduction kinetics is highlighted, followed by our perspectives on the challenges and future endeavors in achieving a controllable and predictable synthesis of noble-metal nanocrystals.

Developing a CNT-SPE Sensing Platform Based on Green Synthesized AuNPs, Using Sargassum sp

Detection and quantification of diverse analytes such as molecules, cells receptor and even particles and nanoparticles, play an important role in biomedical research, particularly in electrochemical sensing platform technologies. In this study, gold nanoparticles (AuNPs) prepared by green synthesis from Sargassum sp. were characterized using ultraviolet-visible (UV-Vis) and Fourier transform-infrared (FT-IR) spectroscopies, X-ray diffraction (XRD), scanning electron microscopy (SEM), dynamic light scattering (DLS) and zeta potential (ζ) obtaining organic capped face-centered cubic 80-100 nm AuNPs with an excellent stability in a wide range of pH. The AuNPs were used to modify a carbon nanotubes-screen printed electrode (CNT-SPE), through the drop-casting method, to assemble a novel portable electrochemical sensing platform for glucose, using a novel combination of components, which together have not been employed. The ability to sense and measure glucose was demonstrated, and its electrochemical fundamentals was studied using cyclic voltammetry (CV). The limits of detection (LOD) and quantification (LOQ) to glucose were 50 μM and 98 μM, respectively, and these were compared to those of other sensing platforms.

Edge Enrichment of Ultrathin 2D PdPtCu Trimetallic Nanostructures Effectuates Top-Ranked Ethanol Electrooxidation

Atomic edge sites on two-dimensional (2D) nanomaterials display striking catalytic behavior, whereas edge engineering for 2D metal nanocatalysts remains an insurmountable challenge. Here we advance a one-pot synthesis of ultrathin 2D PdPtCu trimetallic nanosheets and nanorings with escalating low-coordinated edge proportions from 11.74% and 23.11% to 45.85% as cutting-edge ethanol oxidation reaction (EOR) electrocatalysts. This in situ edge enrichment hinges on a competitive surface capping and etching strategy with integrated manipulation of the reaction kinetics. Electrocatalysis tests demystify an edge-relied EOR performance, where the edge-richest 9.0 nm-Pd₆₁Pt₂₂Cu₁₇ nanorings attain an exceptional activity (12.42 A mg^-1_Pt+Pd, 20.2 times that of commercial Pt/C) with substantially improved durability. Molecularly mechanistic studies certify that the unsaturated edge sites on these 2D catalysts prevail, triggering the C-C bond scission and succeeding CO removal to facilitate a 12-electron-transferring EOR process. This study introduces the "metal-edge-driven" concept and enables the "edge sites on 2D multimetallic nanocatalysts" technique to design versatile heterocatalysts.

Recent Advances on the Rational Design of Non-Precious Metal Oxide Catalysts Exemplified by CuOx/CeO2 Binary System: Implications of Size, Shape and Electronic Effects on Intrinsic Reactivity and Metal-Support Interactions

Catalysis is an indispensable part of our society, massively involved in numerous energy and environmental applications. Although, noble metals (NMs)-based catalysts are routinely employed in catalysis, their limited resources and high cost hinder the widespread practical application. In this regard, the development of NMs-free metal oxides (MOs) with improved catalytic activity, selectivity and durability is currently one of the main research pillars in the area of heterogeneous catalysis. The present review, involving our recent efforts in the field, aims to provide the latest advances—mainly in the last 10 years—on the rational design of MOs, i.e., the general optimization framework followed to fine-tune non-precious metal oxide sites and their surrounding environment by means of appropriate synthetic and promotional/modification routes, exemplified by CuOx/CeO2 binary system. The fine-tuning of size, shape and electronic/chemical state (e.g., through advanced synthetic routes, special pretreatment protocols, alkali promotion, chemical/structural modification by reduced graphene oxide (rGO)) can exert a profound influence not only to the reactivity of metal sites in its own right, but also to metal-support interfacial activity, offering highly active and stable materials for real-life energy and environmental applications. The main implications of size-, shape- and electronic/chemical-adjustment on the catalytic performance of CuOx/CeO2 binary system during some of the most relevant applications in heterogeneous catalysis, such as CO oxidation, N2O decomposition, preferential oxidation of CO (CO-PROX), water gas shift reaction (WGSR), and CO2 hydrogenation to value-added products, are thoroughly discussed. It is clearly revealed that the rational design and tailoring of NMs-free metal oxides can lead to extremely active composites, with comparable or even superior reactivity than that of NMs-based catalysts. The obtained conclusions could provide rationales and design principles towards the development of cost-effective, highly active NMs-free MOs, paving also the way for the decrease of noble metals content in NMs-based catalysts.

Orthogonal deposition of Au on different facets of Ag cuboctahedra for the fabrication of nanoboxes with complementary surfaces

We report the fabrication of Ag-Au cuboctahedral nanoboxes enclosed by {100} and {111} facets, respectively, through the orthogonal deposition of Au on two different facets of Ag cuboctahedra. Specifically, we titrate aqueous HAuCl₄ into an aqueous mixture containing Ag cuboctahedra, ascorbic acid, and NaOH (under basic conditions), in the presence of poly(vinylpyrrolidone) (PVP) and cetyltrimethylammonium chloride (CTAC), respectively. In the case of PVP, the oxidation of Ag was initiated from the {111} facets of the cuboctahedra through the galvanic replacement reaction between Au(iii) and Ag, accompanied by the deposition of Au onto the {100} facets. Because the dissolved Ag(i) ions could react with NaOH to form Ag₂O on the {111} facets and thus terminate the galvanic reaction, the Au(iii) ions would be further reduced by the ascorbate monoanion (HAsc^-) to generate Au atoms for their continuing deposition on the {100} facets, converting Ag cuboctahedra to Ag@Au_{100} cuboctahedra. Upon the etching of Ag from the core, we obtained Ag-Au cuboctahedral nanoboxes enclosed by {100} facets. In contrast, when CTAC was present, the oxidation of Ag through a galvanic reaction could continuously proceed on {100} facets as the dissolved Ag(i) ions would react with the excessive amount of Cl^- ions derived from CTAC to produce soluble AgCl₂^- ions rather than insoluble Ag₂O. As a result, the dissolved Ag(i) and Au(iii) ions would be co-reduced by HAsc^- for the generation of Ag and Au atoms, followed by their co-deposition onto {111} facets for the generation of Ag@Au_{111} concave cuboctahedra. After the removal of Ag from the core by etching, we obtained Ag-Au_{111} cuboctahedral nanoboxes enclosed by {111} facets. Both samples of cuboctahedral nanoboxes exhibited strong optical absorption in the infrared region. Interestingly, the cuboctahedral nanoboxes enclosed by {111} facets showed significantly enhanced catalytic activity toward the reduction of 4-nitrophenol by NaBH₄ relative to their counterparts encased by {100} facets.

Size and shape controlled synthesis of rhodium nanoparticles

Controlling of the size and/or shape of noble metal nanoparticles (NMNPs) is crucial to make use of their unique properties and to optimize their performance for a given application. Within the past decades, the development of wet-chemistry methods enables fine tailoring of the size and morphology of NMNPs. We herein devote this review to introduce the wet-chemistry-based methods for the size and shape-controlled synthesis of rhodium (Rh) NPs. We start with a summarization of the wet-chemistry-based approaches developed for producing Rh NPs and then focus on recent fascinating advances in their size- and shape-control in the aspects of kinetic and thermodynamic regimes depending on the synthetic conditions. Then, we use several typical examples to showcase the applications of Rh NPs with tunable sizes and shapes. Finally, we make some perspectives for the further research trends and development of Rh NPs. We hope through this reviewing effort, one can easily understand the technical bases for effectively designing and producing Rh NPs with desired properties.

Recent advances in electrocatalysts toward the oxygen reduction reaction: the case of PtNi octahedra

Designing highly efficient and durable electrocatalysts for the oxygen reduction reaction (ORR), the key step for the operation of polymer electrolyte membrane fuel cells (PEMFCs), is of a pivotal importance for advancing PEMFC technology. Since the most significant progress has been made on Pt3Ni(111) alloy surfaces, nanoscale PtNi alloy octahedra enclosed by (111) facets have emerged as promising electrocatalysts toward the ORR. However, because their practical uses have been hampered by the cost, sluggish reaction kinetics, and poor durability, recent advances have engendered a wide variety of structure-, size-, and composition-controlled bimetallic PtNi octahedra. Herein, we therefore review the important recent developments of PtNi octahedral electrocatalysts point by point to give an overview of the most promising strategies. Specifically, the present review article focuses on the synthetic methods for the PtNi octahedra, the core-shell and multi-metallic strategies for performance improvement, and their structure-, size-, and composition-control-based ORR activity. By considering the results achieved in this field, a prospect for this alloy nanocatalysts system for future sustainable energy applications is also proposed.

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

Recent developments of a novel class of catalytic materials built on hollow nanocrystals having ultrathin, porous walls, and well-controlled surface structures are discussed, with a focus on platinum and the oxygen reduction reaction (ORR). An introduction is given to the critical role of platinum in the proton exchange membrane fuel cells, and the pressing need to develop a strategy for achieving cost-effective and sustainable use of this precious metal. How to maximize the mass activity of ORR catalysts based on platinum by rationally engineering the surface structure while increasing the utilization efficiency of atoms is then discussed. After reporting on the synthetic methods involving galvanic replacement and seed-mediated growth followed by etching, respectively, a number of examples to demonstrate the enhancement in activity and durability for this new class of catalytic materials are showcased. The feasibility to have the methodology extended from platinum to other precious metals such as gold and ruthenium is highlighted. In conclusion, some of the remaining issues and emerging solutions are examined.

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.

Facet-selective deposition of Au and Pt on Ag nanocubes for the fabrication of bifunctional Ag@Au-Pt nanocubes and trimetallic nanoboxes

We report a facile route to the synthesis of Ag@Au-Pt trimetallic nanocubes in which the Ag, Au, and Pt atoms are exposed at the corners, side faces, and edges, respectively. Our success relies on the use of Ag@Au nanocubes, with Ag2O patches at the corners and Au on the side faces and edges, as seeds for the site-selective deposition of Pt on the edges only in a reaction system containing ascorbic acid (H2Asc) and poly(vinylpyrrolidone). At an initial pH of 3.2, H2Asc can dissolve the Ag2O patches, exposing the Ag atoms at the corners of a nanocube. Upon the injection of the H2PtCl6 precursor, the Pt atoms derived from the reduction by both H2Asc and Ag are preferentially deposited on the edges, leading to the formation of Ag@Au-Pt trimetallic nanocubes. We demonstrate the use of 2,6-dimethylphenyl isocyanide as a molecular probe to confirm and monitor the deposition of Pt atoms on the edges of nanocubes through surface-enhanced Raman scattering (SERS). We further explore the use of these bifunctional trimetallic nanoparticles with integrated plasmonic and catalytic properties for in situ SERS monitoring the reduction of 4-nitrothiophenol by NaBH4. Upon the removal of Ag via H2O2 etching, the Ag@Au-Pt nanocubes evolve into trimetallic nanoboxes with a wall thickness of about 2 nm and well-defined openings at the corners. The trimetallic nanoboxes embrace plasmon resonance peaks in the near-infrared region with potential in biomedical applications.

The physical chemistry and materials science behind sinter-resistant catalysts

This tutorial review highlights recent progress in understanding the physical chemistry and materials science for developing sinter-resistant catalytic systems.

Synthesis of Cubic-Shaped Pt Particles with (100) Preferential Orientation by a Quick, One-Step and Clean Electrochemical Method

A new approach has been developed for in situ preparing cubic-shaped Pt particles with (100) preferential orientation on the surface of the conductive support by using a quick, one-step, and clean electrochemical method with periodic square-wave potential. The whole electrochemical deposition process is very quick (only 6 min is required to produce cubic Pt particles), without the use of particular capping agents. The shape and the surface structure of deposited Pt particles can be controlled by the lower and upper potential limits of the square-wave potential. For a frequency of 5 Hz and an upper potential limit of 1.0 V (vs saturated calomel electrode), as the lower potential limit decreases to the H adsorption potential region, the Pt deposits are changed from nearly spherical particles to cubic-shaped (100)-oriented Pt particles. High-resolution transmission electron microscopy and selected-area electron diffraction reveal that the formed cubic Pt particles are single-crystalline and enclosed by (100) facets. Cubic Pt particles exhibit characteristic H adsorption/desorption peaks corresponding to the (100) preferential orientation. Ge irreversible adsorption indicates that the fraction of wide Pt(100) surface domains is 47.8%. The electrocatalytic activities of different Pt particles are investigated by ammonia electro-oxidation, which is particularly sensitive to the amount of Pt(100) sites, especially larger (100) domains. The specific activity of cubic Pt particles is 3.6 times as high as that of polycrystalline spherical Pt particles, again confirming the (100) preferential orientation of Pt cubes. The formation of cubic-shaped Pt particles is related with the preferential electrochemical deposition and dissolution processes of Pt, which are coupled with the periodic desorption and adsorption processes of O-containing species and H adatoms.

Surface and interface engineering of FePt/C nanocatalysts for electro-catalytic methanol oxidation: enhanced activity and durability

A methodology by coupling a microfluidic-batch process with in situ carbon-black mixing, successive annealing and de-alloying post-treatment was developed for engineering surface and interface microstructures of FePt/C nanocomposites. Ultra-small angular FePt nanocrystals rich in vertexes/terraces/steps and with Pt contents gradually increasing from the inner to the outer part can be synthesized at certain Fe/Pt atomic ratios (2/1 or 1.1/1), which can directly grow on carbon-black for enhanced nanocrystal-carbon interface interaction by introducing the in situ carbon-black mixing process. Composition and structure characterization suggests that FePt@(Fe_1-xPt_x)O_y(OH)_z/C nanocomposites with FePt alloy cores and surface Pt-doping hydroxyl iron oxide shells are formed after annealing. After controlled de-alloying of Fe in annealed nanocrystals with a Fe/Pt ratio of 2/1, the finally formed nanocatalysts exhibited excellent electrochemical catalytic performance using the methanol oxidation reaction as a model, preserving an activity of 1610 mA mg^-1 Pt^-1 (12 times the commercial Pt/C catalysts, higher than the best result (7.9 times the commercial Pt/C catalysts) just published in Science (Science, 2016, 354, 1410-1414), enhanced durability and high tolerance to CO poisoning.

Seed-Mediated Growth of Colloidal Metal Nanocrystals

Seed-mediated growth is a powerful and versatile approach for the synthesis of colloidal metal nanocrystals. The vast allure of this approach mainly stems from the staggering degree of control one can achieve over the size, shape, composition, and structure of nanocrystals. These parameters not only control the properties of nanocrystals but also determine their relevance to, and performance in, various applications. The ingenuity and artistry inherent to seed-mediated growth offer extensive promise, enhancing a number of existing applications and opening the door to new developments. This Review demonstrates how the diversity of metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and reduction kinetics. New capabilities and future directions are also highlighted.

Platinum Cubic Nanoframes with Enhanced Catalytic Activity and Durability Toward Oxygen Reduction

We report the synthesis and electrocatalytic properties of Pt cubic nanoframes with ultrathin ridges less than 2 nm in thickness. The nanoframes were synthesized through site-selected deposition of Pt onto the corner and edge sites of Pd nanocubes, followed by selective removal of the Pd cores via chemical etching. The Br^- ions chemisorbed on the side faces of a Pd nanocube played a critical role in enabling the siteselected deposition. In addition, the kinetics of deposition and the diffusion of Pt adatoms was optimized by carefully controlling the injection rate of the Pt precursor and the reaction temperature, respectively, to obtain the frame-like structure. When benchmarked against a commercial Pt/C comprised of Pt particles 2-3 nm in size, the Pt frame/C catalyst exhibited not only enhanced mass activity toward oxygen reduction, but also substantially improved catalytic durability. In an accelerated durability test, the Pt frame/C catalyst showed a mass activity more than 6× greater than for the Pt/C reference after 20 000 cycles of repeated potential sweeping. This improvement can be largely attributed to the frame-like structure, which is unique in suppressing both the detachment and aggregation of catalytic particles owing to the significantly enhanced interaction with carbon support.

Synthesis of octahedral, truncated octahedral, and cubic Rh2Ni nanocrystals and their structure-activity relationship for the decomposition of hydrazine in aqueous solution to hydrogen

We developed a co-reduction method to synthesize octahedral, truncated octahedral, and cubic Rh2Ni nanocrystals. The shape/size distribution, structural characteristics, and composition of the Rh2Ni nanocrystals are investigated, and their possible formation mechanism at high temperatures in margaric acid/1-aminoheptadecane solution in the presence of tetraethylgermanium and borane trimethylamine complexes is proposed. A preliminary probing of the structure-activity dependence of the surface "clean" Rh2Ni nanocrystals supported on carbon towards hydrazine (N2H4) in aqueous solution dehydrogenation revealed that the higher the percentage of {111} facets, the higher is the activity and H2 selectivity of the nanocrystals. This result was attributed to the {111} facets not only introducing more basic sites, but also weakening the interaction between the produced adspecies (including H2 and NHx) and surface metal atoms in comparison with those of {100} facets. Furthermore, the as-prepared Rh2Ni nanooctahedra exhibited 100% H2 selectivity and high activity at room temperature for H2 generation via N2H4 decomposition. The activation energy of the Rh2Ni nanooctahedra was 41.6 ± 1.2 kJ mol(-1). The Rh2Ni nanooctahedra were stable catalysts for the hydrolytic dehydrogenation of N2H4, providing 27 723 total turnovers in 30 h. Our work provides a new perspective concerning the possibility of constructing hydrogen-producing systems based on N2H4 and surface "clean" Rh2Ni nanocrystal catalysts with defined shapes supported on carbon that possess a competitive performance in comparison with NaBH4 and NH3BH3 hydrogen-producing systems for fuel cell applications.

Noble metal-based bimetallic nanoparticles: the effect of the structure on the optical, catalytic and photocatalytic properties

Nanoparticles composed of two different metal elements show novel electronic, optical, catalytic or photocatalytic properties from monometallic nanoparticles. Bimetallic nanoparticles could show not only the combination of the properties related to the presence of two individual metals, but also new properties due to a synergy between two metals. The structure of bimetallic nanoparticles can be oriented in random alloy, alloy with an intermetallic compound, cluster-in-cluster or core-shell structures and is strictly dependent on the relative strengths of metal-metal bond, surface energies of bulk elements, relative atomic sizes, preparation method and conditions, etc. In this review, selected properties, such as structure, optical, catalytic and photocatalytic of noble metals-based bimetallic nanoparticles, are discussed together with preparation routes. The effects of preparation method conditions as well as metal properties on the final structure of bimetallic nanoparticles (from alloy to core-shell structure) are followed. The role of bimetallic nanoparticles in heterogeneous catalysis and photocatalysis are discussed. Furthermore, structure and optical characteristics of bimetallic nanoparticles are described in relation to the some features of monometallic NPs. Such a complex approach allows to systematize knowledge and to identify the future direction of research.