Highly Selective Synthesis of Catalytically Active Monodisperse Rhodium Nanocubes

Do Rh-Hydride Phases Contribute to the Catalytic Activity of Rh Catalysts under Reductive Conditions?

Rh-hydride phases were believed to be key causes of the exceptional catalytic ability of Rh catalysts under H2 reductive conditions. Here, we utilize the large-scale machine-learning-based global optimization to explore millions of Rh bulk, surface, and nanoparticle structures in contact with H2, which rules out the presence of subsurface/interstitial H in Rh and Rh-hydride phases as thermodynamically stable phases under ambient conditions. Instead, an exceptional Rh-H affinity is identified for surface Rh atoms in Rh nanoparticles that can accommodate a high concentration of adsorbed H, with the surface Rh to H ratio reaching ∼2.5, featuring stable six-H-coordinated Rh, [RhH6]. Such [RhH6] species forming at edged Rh sites are found to be the key intermediates in the electrochemical hydrogen evolution reaction (HER) on Rh. Guided by theory, our synthesized Rh concave nanocubes with a high density of edged Rh sites achieve a Tafel slope of 28.4 mV dec-1 and a low overpotential of 36.1 mV at jECSA = 1 mA cm-2, which outperforms commercial Pt/C and other morphologies of Rh catalysts. Our results clarify the active phase in Rh-H nanosystems and guide the catalyst design by precise morphology control of nanocatalysts.

Kinetics-Driven Crystal Facet Evolution Mechanism of Atomically Ordered Intermetallic PtFe Nanocubes toward Electrochemical Catalysis

Crystal structure engineering in nanoparticles has been regarded as a vital method in catalyst development and design. Herein, PtFe nanocubes, manufactured with ordered PtFe intermetallic structure and a desired facet of {202}, have been successfully prepared via the combination of selective deposition strategy and spatial barrier effect. In-situ X-ray photoelectron spectroscopy found that the growth of the high-index facet and formation of the nanocube for o-PtFe-202 materials arise from the surface Fe2+ modification stabilized effect and the selective deposition of Cl-, respectively. Moreover, density functional theory calculations and X-ray adsorption spectroscopies further proved that the improved oxygen reduction reaction activity and stability of o-PtFe-202 mainly originate from the synergistic effect of the desired high-index facet, ordered crystal structure, and resulting optimal d-band center of Pt. As expected, the o-PtFe-202 exhibits excellent mass activity (2.48 mA·ugPt-1) and specific activity (7.78 mA·cm-2), with only a 7.3% decrease in mass activity after 30 000 cycles.

Impact of quantum size effects to the band gap of catalytic materials: a computational perspective

The evolution of nanotechnology has facilitated the development of catalytic materials with controllable composition and size, reaching the sub-nanometer limit. Nowadays, a viable strategy for tailoring and optimizing the catalytic activity involves controlling the size of the catalyst. This strategy is underpinned by the fact that the properties and reactivity of objects with dimensions on the order of nanometers can differ from those of the corresponding bulk material, due to the emergence of quantum size effects. Quantum size effects have a deep influence on the band gap of semiconducting catalytic materials. Computational studies are valuable for predicting and estimating the impact of quantum size effects. This perspective emphasizes the crucial role of modeling quantum size effects when simulating nanostructured catalytic materials. It provides a comprehensive overview of the fundamental principles governing the physics of quantum confinement in various experimentally observable nanostructures. Furthermore, this work may serve as a tutorial for modeling the electronic gap of simple nanostructures, highlighting that when working at the nanoscale, the finite dimensions of the material lead to an increase of the band gap because of the emergence of quantum confinement. This aspect is sometimes overlooked in computational chemistry studies focused on surfaces and nanostructures.

Two-Dimensional Metal Nanostructures: From Theoretical Understanding to Experiment

This paper reviews recent studies on the preparation of two-dimensional (2D) metal nanostructures, particularly nanosheets. As metal often exists in the high-symmetry crystal phase, such as face centered cubic structures, reducing the symmetry is often needed for the formation of low-dimensional nanostructures. Recent advances in characterization and theory allow for a deeper understanding of the formation of 2D nanostructures. This Review firstly describes the relevant theoretical framework to help the experimentalists understand chemical driving forces for the synthesis of 2D metal nanostructures, followed by examples on the shape control of different metals. Recent applications of 2D metal nanostructures, including catalysis, bioimaging, plasmonics, and sensing, are discussed. We end the Review with a summary and outlook of the challenges and opportunities in the design, synthesis, and application of 2D metal nanostructures.

Microwave-assisted synthesis of octahedral Rh nanocrystals and their performance for electrocatalytic oxidation of formic acid

Uniform and well-defined octahedral Rh nanocrystals were rapidly synthesized in a domestic microwave oven for only 140 s of irradiation by reducing Rh(acac)₃ with tetraethylene glycol (TEG) as both a solvent and a reducing agent in the presence of an appropriate amount of KI, didecyl dimethyl ammonium chloride (DDAC), ethylene diamine (EDA) and polyvinylpyrrolidone (PVP). KI, DDAC and EDA were essential for the creation of octahedral Rh nanocrystals. Electrochemical measurements showed a significantly enhanced electrocatalytic activity and stability for the as-prepared octahedral Rh nanocrystals compared with commercial Rh black.

Chemocatalytic Conversion of Lignocellulosic Biomass to Ethanol: A Mini-Review

Ethanol has been widely used as a clean fuel, solvent, and hydrogen carrier. Currently, ethanol is generally produced through fermentation of starch- and sugarcane-derived sugars (e.g., glucose and sucrose) or ethylene hydration. Its production from abundant and inexpensive lignocellulosic biomass would facilitate the development of green and sustainable society. Biomass-derived carbohydrates and syngas can serve as important feedstocks for ethanol synthesis via biological and chemical pathways. Nevertheless, the biological pathway for producing ethanol through biomass-derived glucose fermentation has the disadvantages of long production period and carbon loss. These issues can be effectively mitigated by chemocatalytic methods, which can readily convert biomass to ethanol in high yields and high atomic efficiency. In this article, we review the recent advances in chemocatalytic conversion of lignocellulosic biomass to ethanol, with a focus on analyzing the mechanism of chemocatalytic pathways and discussing the issues related to these methods. We hope this mini-review can provide new insights into the development of direct ethanol synthesis from renewable lignocellulosic biomass.

Prediction and Design of Nanozymes using Explainable Machine Learning

An abundant number of nanomaterials have been discovered to possess enzyme-like catalytic activity, termed nanozymes. It is identified that a variety of internal and external factors influence the catalytic activity of nanozymes. However, there is a lack of essential methodologies to uncover the hidden mechanisms between nanozyme features and enzyme-like activity. Here, a data-driven approach is demonstrated that utilizes machine-learning algorithms to understand particle-property relationships, allowing for classification and quantitative predictions of enzyme-like activity exhibited by nanozymes. High consistency between predicted outputs and the observations is confirmed by accuracy (90.6%) and R² (up to 0.80). Furthermore, sensitive analysis of the models reveals the central roles of transition metals in determining nanozyme activity. As an example, the models are successfully applied to predict or design desirable nanozymes by uncovering the hidden relationship between different periods of transition metals and their enzyme-like performance. This study offers a promising strategy to develop nanozymes with desirable catalytic activity and demonstrates the potential of machine learning within the field of material science.

Formation and selected catalytic properties of ruthenium, rhodium, osmium and iridium nanoparticles

The synthesis and applications in catalysis of nanoparticles formed from ruthenium, rhodium, osmium and iridium have been reviewed.

Kinetically Controlled Synthesis of Rhodium Nanocrystals with Different Shapes and a Comparison Study of Their Thermal and Catalytic Properties

We report the synthesis of Rh nanocrystals with different shapes by controlling the kinetics involved in the growth of preformed Rh cubic seeds. Specifically, Rh nanocrystals with cubic, cuboctahedral, and octahedral shapes can all be obtained from the same cubic seeds under suitable reduction kinetics for the precursor. The success of such a synthesis also relies on the use of a halide-free precursor to avoid oxidative etching, as well as the involvement of a sufficiently high temperature to remove Br^- ions from the seeds while ensuring adequate surface diffusion. The availability of Rh nanocrystals with cubic and octahedral shapes allows for an evaluation of the facet dependences of their thermal and catalytic properties. The data from in situ electron microscopy studies indicate that the cubic and octahedral Rh nanocrystals can keep their original shapes up to 700 and 500 °C, respectively. When tested as catalysts for hydrazine decomposition, the octahedral nanocrystals exhibit almost 4-fold enhancement in terms of H₂ selectivity relative to the cubic counterpart. As for ethanol oxidation, the order is reversed, with the cubic nanocrystals being about three times more active than the octahedral sample.

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.

Surface engineering of Rh-modified Pd nanocrystals by colloidal underpotential deposition for electrocatalytic methanol oxidation

The development of methods to control the surface structures of metallic nanocatalysts is of vital importance for their application as heterogeneous catalysts in chemical conversions of energy and environmental and chemical engineering. The underpotential deposition (UPD) phenomenon has received considerable interest as a tool for the controllable synthesis of metal nanocrystals and engineering their catalytic performances. Herein, the discovery of UPD of Rh on Pd nanocrystals is reported. More importantly, the UPD of Rh is explored as a strategy to direct the synthesis of Rh-modified Pd nanocrystals with controllable shapes and surface structures. The mechanism of the UPD of Rh on Pd is elucidated in terms of electronegativity difference considerations. Compared with pristine Pd octahedral nanocrystals and commercial carbon-supported Pd catalysts, the Rh-modified Pd octahedral nanocrystals exhibit remarkable electrocatalytic performances during the methanol oxidation reaction in alkaline media. Our discovery heralds a new paradigm for UPD-mediated growth of metal nanocrystals and may provide a mechanistic understanding for the guided design of other colloidal UPD systems in the synthesis and surface engineering of metal nanocrystals.

A review of the role and mechanism of surfactants in the morphology control of metal nanoparticles

Although great progress has been made in the synthesis of metal nanoparticles, good repeatability and accurate predictability are still difficult to achieve. This difficulty can be attributed to the synthetic method based primarily on observation and subjective experience, and the role of many surfactants remains unclear. It should be noted that surfactants play an important role in the synthetic process. Understanding their function and mechanism in the synthetic process is a prerequisite for the rational design of nanocatalysts with ideal morphology and performance. In this review article, the function of surfactants is introduced first, and then the mechanism of action of surfactants in controlling the morphology of nanoparticles is discussed according to the types of surfactants, and the promoting and sealing effects of surfactants on the crystal surface is revealed. The relationship between surfactants and the morphology structure of nanoparticles is studied. The removal methods of surfactants are discussed, and the existing problems in the current development strategy are summarized. Finally, the application of surfactants in controlling the morphology of metal nanocrystals is prospected. It is hoped that the review can open up new avenues for the synthesis of nanocrystals.

Electron-Rich Gold Clusters Stabilized by Poly(vinylpyridines) as Robust and Active Oxidation Catalysts

In this report, we introduced poly(n-vinylpyridine) (PnVP, n = 2, 4) as an electron-donating stabilizer for small (<2 nm) Au clusters and elucidated how coordinating pyridines affect the physical, optical, chemical, and catalytic properties of Au clusters. Spectroscopic measurements and theoretical calculation suggested the high electron-donating ability of PnVP. PnVP-stabilized Au clusters improved robustness in aerobic oxidation of alcohols compared to poly(N-vinyl-2-pyrrolidone)-stabilized ones, while retaining catalytic activities.

Controlled Synthesis of Palladium Nanocubes as an Efficient Nanocatalyst for Suzuki-Miyaura Cross-Coupling and Reduction of p-Nitrophenol

Anisotropic nanocatalysts have attracted considerable attention in comparison to bulk/nanocatalysts for their enhanced activity and reactivity. The demand toward anisotropic palladium (Pd) nanostructures has increased rapidly in the field of catalysis. Pd is a well-known active catalyst for several carbon-carbon (C-C) cross-coupling reactions; among them, the Suzuki-Miyaura cross-coupling reaction is one of the most versatile and dominant methods for constructing the extraordinarily useful unsymmetrical biaryls and also for hydrogenation of organic contaminants like p-nitrophenol (p-NP). This paper provides a brief explanation about the controlled synthesis, characterization, and catalytic activity of well-defined palladium nanocubes (Pd NCs) prepared by a seed-mediated method. The synthesized monodispersed Pd NCs were characterized by spectroscopic and microscopic tools such as UV-visible, XRD, FESEM, HRTEM, and EDS analyses. Pd NCs proved as an efficient catalyst for Suzuki-Miyaura cross-coupling reactions and p-NP reduction. The catalyst shows enhanced activity, greater stability, and higher selectivity with remarkable recyclability up to 92% for five consecutive cycles. The catalytic performance of the synthesized Pd NCs was also studied in the reduction of the organic contaminant p-NP, which showed an excellent performance screening of 99% conversion in 6 min.

Preference for low-coordination sites by adsorbed CO on small platinum nanoparticles

FTIR spectra of ¹²CO adsorbed on poly(vinylpyrrolidone) (PVP)-stabilized colloidal platinum at room temperature were acquired and studied. Two new bands, at 2021 cm^-1 and 1994 cm^-1, were observed for the first time and were assigned to the stretching vibrations of CO linearly adsorbed on the Pt surface at edge and corner sites, respectively. The relative intensities of these two bands were found to vary with the coverage of CO, where the smallest particles showed the highest intensity, corresponding to the relative quantities of edge and corner sites per unit surface. The vibrational spectra signals reported for terrace sites on colloidal Pt red-shifted as the particle size was decreased, which showed the electronic interactions between the Pt surface and PVP, with PVP acting as an electron donor.

Synthesis of low- and high-index faceted metal (Pt, Pd, Ru, Ir, Rh) nanoparticles for improved activity and stability in electrocatalysis

Driven by the quest for future energy solution, faceted metal nanoparticles are being pursued as the next generation electrocatalysts for renewable energy applications. Thanks to recent advancement in solution phase synthesis, different low- and high-index facets on metal nanocrystals become accessible and are tested for specific electrocatalytic reactions. This minireview summarises the key approaches to prepare nanocrystals containing the most catalytically active platinum group metals (Pt, Pd, Ru, Ir and Rh) exposed with low- and high-index facets using solution phase synthesis. Electrocatalytic studies related to the different facets are highlighted to emphasise the importance of exposing facets for catalysing these reactions, namely oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), alcohol oxidation including methanol (MOR) and ethanol oxidation reactions (EOR), formic acid oxidation reaction (FAOR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The future outlook discusses the challenges and opportunities for making electrocatalysts that are even more active and stable.

Anisotropic nanomaterials for shape-dependent physicochemical and biomedical applications

This review contributes towards a systematic understanding of the mechanism of shape-dependent effects on nanoparticles (NPs) for elaborating and predicting their properties and applications based on the past two decades of research. Recently, the significance of shape-dependent physical chemistry and biomedicine has drawn ever increasing attention. While there has been a great deal of effort to utilize NPs with different morphologies in these fields, so far research studies are largely localized in particular materials, synthetic methods, or biomedical applications, and have ignored the interactional and interdependent relationships of these areas. This review is a comprehensive description of the NP shapes from theory, synthesis, property to application. We figure out the roles that shape plays in the properties of different kinds of nanomaterials together with physicochemical and biomedical applications. Through systematic elaboration of these shape-dependent impacts, better utilization of nanomaterials with diverse morphologies would be realized and definite strategies would be expected for breakthroughs in these fields. In addition, we have proposed some critical challenges and open problems that need to be addressed in nanotechnology.

Role of Gluten in Surface Chemistry: Nanometallic Bioconjugation of Hard, Medium, and Soft Wheat Protein

Hard, medium, and soft wheat proteins, based on gluten content, were studied for their important roles in nanometallic surface chemistry. In situ synthesis of Au nanoparticles (NPs) was followed to determine the surface adsorption behavior of wheat protein based on the gluten contents. A greater amount of gluten contents facilitated the nucleation to produce Au NPs. X-ray photoelectron spectroscopy (XPS) surface analysis clearly showed the surface adsorption of protein on nanometallic surfaces which was almost equally prevalent for the hard, medium, and soft wheat proteins. Wheat protein conjugated NPs were highly susceptible to phase transfer from aqueous to organic phase that was entirely related to the amount of gluten contents. The presence of higher gluten content in hard wheat protein readily enabled the hard wheat protein conjugated NPs to move across the aqueous-organic interface followed by medium and soft wheat protein conjugated NPs. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS page) analysis allowed us to determine molar masses of nanometallic surface adsorbed protein fractions. Only two protein fractions of high molar masses (74 and 85 kDa) from SDS solubilized hard, medium, and soft wheat proteins preferred to adsorb on nanometallic surfaces out of more than 15 protein fractions of pure wheat protein. This made the surface adsorption of wheat protein highly selective and closely related to gluten content. Cetyltrimethylammonium bromide (CTAB) solubilized wheat protein conjugated NPs demonstrated their strong antimicrobial activities against both Gram negative and Gram positive bacteria making them suitable for their applications in food industry.

Ru Octahedral Nanocrystals with a Face-Centered Cubic Structure, {111} Facets, Thermal Stability up to 400 °C, and Enhanced Catalytic Activity

Ruthenium nanocrystals with both a face-centered cubic ( fcc) structure and well-controlled facets are attractive catalytic materials for various reactions. Here we report a simple method for the synthesis of Ru octahedral nanocrystals with an fcc structure and an edge length of 9 nm. The success of this synthesis relies on the use of 4.5 nm Rh cubes as seeds to facilitate the heterogeneous nucleation and overgrowth of Ru atoms. We choose Rh because it can resist oxidative etching under the harsh conditions for Ru overgrowth, it can be readily prepared as nanocubes with edge lengths less than 5 nm, and its atoms have a size close to that of Ru atoms. During the seed-mediated growth, the atomic packing of Ru overlayers follows an fcc lattice, in contrast to the conventional hexagonal close-packed ( hcp) lattice associated with bulk Ru. The final product takes an octahedral shape, with the surface enclosed by {111} facets. Our in situ measurements suggest that both the octahedral shape and the fcc crystal structure can be well preserved up to 400 °C, which is more than 100 °C higher than what was reported for Ru octahedral nanocages. When utilized as catalysts, the Ru octahedral nanocrystals exhibited 4.4-fold enhancement in terms of specific activity toward oxygen evolution relative to hcp-Ru nanoparticles. We also demonstrate that Ru{111} facets are more active than Ru{100} facets in catalyzing the oxygen evolution reaction. Altogether, this work offers an effective method for the synthesis of Ru nanocrystals with an fcc structure and well-defined {111} facets, as well as enhanced thermal stability and catalytic activity. We believe these nanocrystals will find use in various catalytic applications.

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.

Microwave-assisted synthesis of mutually embedded Rh concave nanocubes with enhanced electrocatalytic activity

Novel mutually embedded Rh concave nanocubes were synthesized by reducing Rh(acac)₃ in tetraethylene glycol in the presence of benzyldimethylhexadecylammonium, KI and polyvinylpyrrolidone under microwave irradiation for 120 s. KI and HDBAC were crucial to the formation of mutually embedded nanostructures. The as-prepared Rh nanocrystals exhibited higher electrocatalytic activity and stability.

Mutually embedded Rh concave nanocubes were synthesized by reducing Rh(acac)₃ with tetraethylene glycol (TEG) as both a solvent and a reducing agent under microwave irradiation for 120 s.

Nanofibrous rhodium with a new morphology for the hydrogenation of CO2 to formate

Herein, fibrous rhodium (Rh) was engineered using a microemulsion system.

Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the Reductants

There is a growing interest in controlling the synthesis of colloidal metal nanocrystals and thus tailoring their properties toward various applications. In this context, choosing an appropriate combination of reagents (e.g., salt precursor, reductant, capping agent, and stabilizer) plays a pivotal role in enabling the synthesis of metal nanocrystals with diversified sizes, shapes, and structures. Here we present a comprehensive review that highlights one of the key reagents for the synthesis of metal nanocrystals via chemical reduction: the reductants. We start with a brief introduction to the compounds commonly employed as reductants in the colloidal synthesis of metal nanocrystals by showing their oxidation half-reactions and the corresponding oxidation potentials. Then we offer specific examples pertaining to the controlled synthesis of metal nanocrystals, followed by some fundamental aspects covering the general mechanisms of metal ion reduction based on the Marcus Theory. Afterwards, we present a case-by-case discussion on a wide variety of reductants, including their major properties, reduction mechanisms, and additional effects on the final products. We illustrate these aspects by selecting key examples from the literature and paying close attention to the underlying mechanism in each case. At the end, we conclude by summarizing the highlights of the review and providing some perspectives on future directions.

The Many "Facets" of Halide Ions in the Chemistry of Colloidal Inorganic Nanocrystals

Over the years, scientists have identified various synthetic "handles" while developing wet chemical protocols for achieving a high level of shape and compositional complexity in colloidal nanomaterials. Halide ions have emerged as one such handle which serve as important surface active species that regulate nanocrystal (NC) growth and concomitant physicochemical properties. Halide ions affect the NC growth kinetics through several means, including selective binding on crystal facets, complexation with the precursors, and oxidative etching. On the other hand, their presence on the surfaces of semiconducting NCs stimulates interesting changes in the intrinsic electronic structure and interparticle communication in the NC solids eventually assembled from them. Then again, halide ions also induce optoelectronic tunability in NCs where they form part of the core, through sheer composition variation. In this review, we describe these roles of halide ions in the growth of nanostructures and the physical changes introduced by them and thereafter demonstrate the commonality of these effects across different classes of nanomaterials.

Morphology-Controlled Synthesis of Rhodium Nanoparticles for Cancer Phototherapy

Rhodium nanoparticles are promising transition metal nanocatalysts for electrochemical and synthetic organic chemistry applications. However, notwithstanding their potential, to date, Rh nanoparticles have not been utilized for biological applications; there has been no cytotoxicity study of Rh reported in the literature. In this regard, the absence of a facile and controllable synthetic strategy of Rh nanostructures with various sizes and morphologies might be responsible for the lack of progress in this field. Herein, we have developed a synthetic strategy for Rh nanostructures with controllable morphology through an inverse-directional galvanic replacement reaction. Three types of Rh-based nanostructures-nanoshells, nanoframes, and porous nanoplates-were successfully synthesized. A plausible synthetic mechanism based on thermodynamic considerations has also been proposed. The cytotoxicity, surface functionalization, and photothermal therapeutic effect of manufactured Rh nanostructures were systematically investigated to reveal their potential for in vitro and in vivo biological applications. Considering the comparable behavior of porous Rh nanoplates to that of gold nanostructures that are widely used in nanomedicine, the present study introduces Rh-based nanostructures into the field of biological research.

The polyol process: a unique method for easy access to metal nanoparticles with tailored sizes, shapes and compositions

After about three decades of development, the polyol process is now widely recognized and practised as a unique soft chemical method for the preparation of a large variety of nanoparticles which can be used in important technological fields. It offers many advantages: low cost, ease of use and, very importantly, already proven scalability for industrial applications. Among the different classes of inorganic nanoparticles which can be prepared in liquid polyols, metals were the first reported. This review aims to give a comprehensive account of the strategies used to prepare monometallic nanoparticles and multimetallic materials with tailored size and shape. As regards monometallic materials, while the preparation of noble as well as ferromagnetic metals is now clearly established, the scope of the polyol process has been extended to the preparation of more electropositive metals, such as post-transition metals and semi-metals. The potential of this method is also clearly displayed for the preparation of alloys, intermetallics and core-shell nanostructures with a very large diversity of compositions and architectures.

Plasmon-Enhanced Catalysis: Distinguishing Thermal and Nonthermal Effects

In plasmon-enhanced heterogeneous catalysis, illumination accelerates reaction rates by generating hot carriers and hot surfaces in the constituent nanostructured metals. In order to understand how photogenerated carriers enhance the nonthermal reaction rate, the effects of photothermal heating and thermal gradients in the catalyst bed must be confidently and quantitatively characterized. This is a challenging task considering the conflating effects of light absorption, heat transport, and reaction energetics. Here, we introduce a methodology to distinguish the thermal and nonthermal contributions from plasmon-enhanced catalysts, demonstrated by illuminated rhodium nanoparticles on oxide supports to catalyze the CO₂ methanation reaction. By simultaneously measuring the total reaction rate and the temperature gradient of the catalyst bed, the effective thermal reaction rate may be extracted. The residual nonthermal rate of the plasmon-enhanced reaction is found to grow with a superlinear dependence on illumination intensity, and its apparent quantum efficiency reaches ∼46% on a Rh/TiO₂ catalyst at a surface temperature of 350 °C. Heat and light are shown to work synergistically in these reactions: the higher the temperature, the higher the overall nonthermal efficiency in plasmon-enhanced catalysis.

Branched Pd@Rh core@shell nanocrystals with exposed Rh {100} facets: an effective electrocatalyst for hydrazine electro-oxidation

Shape control of noble metal (NM) nanocrystals (NCs) is of great importance for improving their electrocatalytic performance. In this report, branched Pd@Rh core@shell NCs that have right square prism-like arms with preferential exposure of Rh {100} facets (denoted as b-Pd@Rh-NCs thereafter) are synthesized and utilized as an electrocatalyst for the hydrazine electrooxidation (HEO) in acidic and alkaline electrolytes. The b-Pd@Rh-NCs are obtained by the heteroepitaxial growth of Rh on the pre-formed branched Pd NCs (denoted as b-Pd-NCs thereafter) core in the presence of poly(vinyl pyrrolidone) (PVP) and bromide ions. A comparative analysis of the voltammetric data for the HEO shows a higher activity on the b-Pd@Rh-NCs exposed with Rh {100} faces than on Rh black, the b-Pd-NCs, and Pd black in acid and alkaline solutions, indicating a structure sensitivity of the reaction. Analysis of the products from the b-Pd@Rh-NCs catalysed HEO reveals a very high hydrazine fuel efficiency, as determined by on-line differential electrochemical mass spectrometry (DEMS).

Mesoporous metallic rhodium nanoparticles

Mesoporous noble metals are an emerging class of cutting-edge nanostructured catalysts due to their abundant exposed active sites and highly accessible surfaces. Although various noble metal (e.g. Pt, Pd and Au) structures have been synthesized by hard- and soft-templating methods, mesoporous rhodium (Rh) nanoparticles have never been generated via chemical reduction, in part due to the relatively high surface energy of rhodium (Rh) metal. Here we describe a simple, scalable route to generate mesoporous Rh by chemical reduction on polymeric micelle templates [poly(ethylene oxide)-b-poly(methyl methacrylate) (PEO-b-PMMA)]. The mesoporous Rh nanoparticles exhibited a ∼2.6 times enhancement for the electrocatalytic oxidation of methanol compared to commercially available Rh catalyst. Surprisingly, the high surface area mesoporous structure of the Rh catalyst was thermally stable up to 400 °C. The combination of high surface area and thermal stability also enables superior catalytic activity for the remediation of nitric oxide (NO) in lean-burn exhaust containing high concentrations of O₂.

Mesoporous noble metal nanostructures offer great promise in catalytic applications. Here, Yamauchi and co-workers synthesize mesoporous rhodium nanoparticles using polymeric micelle templates, and report appreciable activities for methanol oxidation and NO remediation.

Electrodeposition of Rhodium Nanowires Arrays and Their Morphology-Dependent Hydrogen Evolution Activity

This work reports on the electrodeposition of rhodium (Rh) nanowires with a controlled surface morphology synthesized using an anodic aluminum oxide (AAO) template. Vertically aligned Rh nanowires with a smooth and coarse morphology were successfully deposited by adjusting the electrode potential and the concentration of precursor ions and by involving a complexing reagent in the electrolyte solution. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses were used to follow the morphological evolution of Rh nanowires. As a heterogeneous electrocatalyst for hydrogen evolution reactions (HER), the coarse Rh nanowire array exhibited an enhanced catalytic performance respect to smooth ones due to the larger surface area to mass ratio and the higher density of catalytically active defects, as evidenced by voltammetric measurements and TEM. Results suggest that the morphology of metallic nanomaterials could be readily engineered by electrodeposition. The controlled electrodeposition offers great potential for the development of an effective synthesis tool for heterogeneous catalysts with a superior performance for wide applications.

Rh nanoparticles supported on ultrathin carbon nanosheets for high-performance oxygen reduction reaction and catalytic hydrogenation

We reported a facile and scalable salt-templated approach to produce monodisperse Rh nanoparticles (NPs) on ultrathin carbon nanosheets with the assistance of calcination under inert gas. More importantly, in spite of the essentially poor ORR activity of Rh/C, the acquired Rh/C hybrid nanosheets display a comparable ORR activity to the optimal commercial Pt/C catalyst, which may be due to the extra-small size of Rh NPs and the 2D defect-rich amorphous carbon nanosheets that can facilitate the charge transfer and reactive surface exposure. Moreover, Rh/C nanosheets present the optimal current density and best durability with the minimum decline during the entire test, so that ∼93% activity after 20 000 s is achieved, indicating a good lifetime for ORR. In contrast, commercial Pt/C and commercial Rh/C exhibited worse durability, so that ∼74% and ∼85% activities after 20 000 s are maintained. What's more, in the model system of reduction of 4-nitrophenol (4-NP), the kinetic constant k for Rh/C nanosheets is 3.1 × 10^-3, which is 4.5 times than that of the commercial Rh/C catalyst, revealing that our Rh/C hybrid nanosheets can be potentially applied in industrial catalytic hydrogenation. This work opens a novel and facile way for the rest of the precious metal NPs to be supported on ultrathin carbon nanosheets for heterogeneous catalysis.

Surface Coordination Chemistry of Metal Nanomaterials

Surface coordination chemistry of nanomaterials deals with the chemistry on how ligands are coordinated on their surface metal atoms and influence their properties at the molecular level. This Perspective demonstrates that there is a strong link between surface coordination chemistry and the shape-controlled synthesis, and many intriguing surface properties of metal nanomaterials. While small adsorbates introduced in the synthesis can control the shapes of metal nanocrystals by minimizing their surface energy via preferential coordination on specific facets, surface ligands properly coordinated on metal nanoparticles readily promote their catalysis via steric interactions and electronic modifications. The difficulty in the research of surface coordination chemistry of nanomaterials mainly lies in the lack of effective tools to characterize their molecular surface coordination structures. Also highlighted are several model material systems that facilitate the characterizations of surface coordination structures, including ultrathin nanostructures, atomically precise metal nanoclusters, and atomically dispersed metal catalysts. With the understanding of surface coordination chemistry, the molecular mechanisms behind various important effects (e.g., promotional effect of surface ligands on catalysis, support effect in supported metal nanocatalysts) of metal nanomaterials are disclosed.

Environmentally friendly and facile synthesis of Rh nanoparticles at room temperature by alkaline ethanol solution and their application for ethanol electrooxidation

A facile, fact and green synthetic route was developed to prepare Rh nanoparticles at room temperature.

Synthesis of well-defined pentagonally prismatic silver nanoparticles: role of bromide in (100) stabilization and facet preservation

Use of bromide ions for (100) facet formation was demonstrated to produce silver pentagonal prism nanoparticles size and shape selectively.

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.

Noble-Metal Nanocrystals with Controlled Facets for Electrocatalysis

Noble-metal nanocrystals (NCs) show excellent catalytic performance for many important electrocatalysis reactions. The crystallographic properties of the facets by which the NCs are bound, closely associated with the shape of the NCs, have a profound influence on the electrocatalytic function of the NCs. To develop an efficient strategy for the synthesis of NCs with controlled facets as well as compositions, understanding of the growth mechanism of the NCs and their interaction with the chemical species involved in NC synthesis is quite important. Furthermore, understanding the facet-dependent catalytic properties of noble-metal NCs and the corresponding mechanisms for various electrocatalysis reactions will allow for the rational design of robust electrocatalysts. In this review, we summarize recently developed synthesis strategies for the preparation of mono- and bimetallic noble-metal NCs by classifying them by the type of facets through which they are enclosed and discuss the electrocatalytic applications of noble-metal NCs with controlled facets, especially for reactions associated with fuel-cell applications, such as the oxygen reduction reaction and fuel (methanol, ethanol, and formic acid) oxidation reactions.

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.