Planar tripods of platinum: formation and self-assembly

Plate-Like Colloidal Metal Nanoparticles

The pseudo-two-dimensional (2D) morphology of plate-like metal nanoparticles makes them one of the most anisotropic, mechanistically understood, and tunable structures available. Although well-known for their superior plasmonic properties, recent progress in the 2D growth of various other materials has led to an increasingly diverse family of plate-like metal nanoparticles, giving rise to numerous appealing properties and applications. In this review, we summarize recent progress on the solution-phase growth of colloidal plate-like metal nanoparticles, including plasmonic and other metals, with an emphasis on mechanistic insights for different synthetic strategies, the crystallographic habits of different metals, and the use of nanoplates as scaffolds for the synthesis of other derivative structures. We additionally highlight representative self-assembly techniques and provide a brief overview on the attractive properties and unique versatility benefiting from the 2D morphology. Finally, we share our opinions on the existing challenges and future perspectives for plate-like metal nanomaterials.

Etching suppression as a means to Pt dendritic ultrathin nanosheets by seeded growth

2D ultrathin metal nanostructures are emerging materials displaying distinct physical and chemical properties compared to their analogues of different dimensionalities. Nanosheets of fcc metals are intriguing, as their crystal structure does not favour a 2D configuration. Thanks to their increased surface-to-volume ratios and the optimal exposure of low-coordinated sites, 2D metal nanostructures can be advantageously exploited in catalysis. Synthesis approaches to ultrathin nanosheets of pure platinum are scarce compared to other noble metals and to Pt-based alloys. Here, we present the selective synthesis of Pt ultrathin nansosheets by a simple seeded-growth method. The most crucial point in our approach is the selective synthesis of Pt seeds comprising planar defects, a main driving force for the 2D growth of metals with fcc structure. Defect engineering is employed here, not in order to disintegrate, but for conserving the defect comprising seeds. This is achieved by in situ elimination of the principal etching agent, chloride, which is present in the PtCl₂ precursor. As a result of etching suppression, twinned nuclei, that are selectively formed during the early stage of nucleation, survive and grow to multipods comprising planar defects. Using the twinned multipods as seeds for the subsequent 2D overgrowth of Pt from Pt(acac)₂ yields ultrathin dendritic nanosheets, in which the planar defects are conserved. Using phenylacetylene hydrogenation as a model reaction of selective hydrogenation, we compared the performance of Pt nanosheets to that of a commercial Pt/C catalyst. The Pt nanosheets show better stability and much higher selectivity to styrene than the commercial Pt/C catalyst for comparable activity.

Ni@Ru and NiCo@Ru Core-Shell Hexagonal Nanosandwiches with a Compositionally Tunable Core and a Regioselectively Grown Shell

The development of highly active electrocatalysts is crucial for the advancement of renewable energy conversion devices. The design of core-shell nanoparticle catalysts represents a promising approach to boost catalytic activity as well as save the use of expensive precious metals. Here, a simple, one-step synthetic route is reported to prepare hexagonal nanosandwich-shaped Ni@Ru core-shell nanoparticles (Ni@Ru HNS), in which Ru shell layers are overgrown in a regioselective manner on the top and bottom, and around the center section of a hexagonal Ni nanoplate core. Notably, the synthesis can be extended to NiCo@Ru core-shell nanoparticles with tunable core compositions (Ni₃ Co_x @Ru HNS). Core-shell HNS structures show superior electrocatalytic activity for the oxygen evolution reaction (OER) to a commercial RuO₂ black catalyst, with their OER activity being dependent on their core compositions. The observed trend in OER activity is correlated to the population of Ru oxide (Ru⁴⁺ ) species, which can be modulated by the core compositions.

Platinum tripods as nanometric frequency multiplexing devices

Electrical and structural characterization studies of nano-particles are very important steps to determine their potential applications in microelectronics. In this paper, we address the crystallographic and electric transport properties of soft-chemistry-grown nanometric Pt tribranches. We report that Pt nanostars grown from the reduction of H₂PtCl₆ salt in pure oleylamine present a remarkable crystalline structure and deeply metallic character despite being grown under mild conditions. We demonstrate that such devices are able to operate at current densities surpassing 200 MA cm^-2, actuating as highly compact frequency multiplexers in the non-ohmic regime.

Integration of Kinetic Control and Lattice Mismatch To Synthesize Pd@AuCu Core-Shell Planar Tetrapods with Size-Dependent Optical Properties

Planar nanocrystals with multiple branches exhibit unique localized surface plasmon resonance properties and great promise in optical applications. Here, we report an aqueous synthesis of Pd@AuCu core-shell planar tetrapods through preferential overgrowth on Pd cubic seeds. The large lattice mismatch between the Pd core and the AuCu shell is the key to induce the formation of branches under sluggish reduction kinetics. Meanwhile, the capping effect of cetyltrimethylammonium chloride on the {100} facets of Pd cubes with an aspect ratio of 1.2 can determine the growth direction of AuCu branches to form a planar structure. Through simply varying the amounts of Pd cubic seeds, the sizes of products can be well-controlled in the range from 33 to 70 nm. With the manipulation of sizes, the peak position of in-plane dipole resonance can be adjusted from visible to near-infrared region. Due to the presence of tips and edges in the branches, planar tetrapods exhibited excellent surface-enhanced Raman scattering performance with an enhancement factor up to 9.0 × 10(3) for 70 nm Pd@AuCu planar tetrapods.

Crystallinity-induced shape evolution of Pt–Ag nanosheets from branched nanocrystals

2D Pt–Ag nanosheets and 3D Pt–Ag–Cu tetrapods have been selectively synthesized, which were determined by the crystallinity of the nanoseeds.

Facile synthesis of three-dimensional Pt-Pd alloyed multipods with enhanced electrocatalytic activity and stability for ethylene glycol oxidation

A facile one-pot solvothermal method was developed for the fabrication of well-defined three-dimensional highly branched Pt-Pd alloyed multipods, using ethylene glycol as a solvent and a reducing agent, along with N-methylimidazole as a structure-directing agent, without any seed, template, or surfactant. The as-prepared nanocrystals exhibited a relatively large electrochemically active surface area, improved electrocatalytic activity and superior stability for ethylene glycol oxidation in alkaline media, compared with commercial Pt black and Pd black, making them promising electrocatalysts in fuel cells.

Using structural diversity to tune the catalytic performance of Pt nanoparticle ensembles

While reducing the size, and restricting shape of nanocatalysts can improve performance, monodispersed samples are not necessarily ideal.

A rational biomimetic approach to structure defect generation in colloidal nanocrystals

Controlling the morphology of nanocrystals (NCs) is of paramount importance for both fundamental studies and practical applications. The morphology of NCs is determined by the seed structure and the following facet growth. While means for directing facet formation in NC growth have been extensively studied, rational strategies for the production of NCs bearing structure defects in seeds have been much less explored. Here, we report mechanistic investigations of high density twin formation induced by specific peptides in platinum (Pt) NC growth, on the basis of which we derive principles that can serve as guidelines for the rational design of molecular surfactants to introduce high yield twinning in noble metal NC syntheses. Two synergistic factors are identified in producing twinned Pt NCs with the peptide: (1) the altered reduction kinetics and crystal growth pathway as a result of the complex formation between the histidine residue on the peptide and Pt ions, and (2) the preferential stabilization of {111} planes upon the formation of twinned seeds. We further apply the discovered principles to the design of small organic molecules bearing similar binding motifs as ligands/surfactants to create single and multiple twinned Pd and Rh NCs. Our studies demonstrate the rich information derived from biomimetic synthesis and the broad applicability of biomimetic principles to NC synthesis for diverse property tailoring.

Highly uniform platinum icosahedra made by hot injection-assisted GRAILS method

Highly uniform Pt icosahedral nanocrystals with an edge length of 8.8 nm were synthesized in nonhydrolytic systems using the hot injection-assisted GRAILS (gas reducing agent in liquid solution) method. The results show the key factors for the shape control include fast nucleation, kinetically controlled growth, and protection from oxidation by air. The effect of oxygen molecules on the Pt morphology was experimentally confirmed based on the study of shape evolution of icosahedral crystals upon exposure to oxygen gas. The Pt icosahedral catalysts obtained had an area-specific activity of 0.83 mA/cm(2) Pt, four times that of 0.20 mA/cm(2) Pt for typical Pt/C catalysts, in an oxygen reduction reaction (ORR).

Can polymorphism be used to form branched metal nanostructures?

Branched metal nanostructures are of great technological importance because of their unique size- and shape-dependent properties. A kinetically controlled synthesis that uses polymorphism to produce branched nickel nanoparticles is presented. These nanoparticles consist of a face-centred cubic (fcc) core and extended arms of alternating fcc and hexagonal close-packed (hcp) nickel phases.

High-index faceted noble metal nanocrystals

The formation of novel and complex structures with specific morphologies from nanocrystals via a direct assembly of atoms or ions remains challenging. In recent years, researchers have focused their attention on nanocrystals of noble metals and their controlled synthesis, characterization, and potential applications. Although the synthesis of various noble metal nanocrystals with different morphologies has been reported, most studies are limited to low-index facet-terminated nanocrystals. High-index facets, denoted by a set of Miller indices {hkl} with at least one index greater than unity, possess a high density of low-coordinated atoms, steps, edges, and kinks within these structures and serve as more active catalytic sites. With the potential for enhanced catalytic performance, researchers have used the insights from shape-controlled nanocrystal synthesis to construct noble metal nanocrystals bounded with high-index facets. Since the report of Pt tetrahexahedral nanocrystals, researchers have achieved significant progress and have prepared nanocrystals with various high-index facets. Because of the general order of surface energy for noble metals, high-index facets typically vanish faster in a crystal growth stage and are difficult to preserve on the surface of the final nanocrystals. Therefore researchers have had limited opportunities to examine high-indexed noble metal nanocrystals with a controlled morphology and investigate their resultant behaviors in depth. In this Account, we thoroughly discuss the basic concepts and state-of-the-art morphology control of some noble metal nanocrystals enclosed with high-index facets. We briefly introduce high-index facets from both crystallographic and geometrical points of view, both of which serve as methods to classify these high-index facets. Then, we summarize various typical noble metal nanocrystals terminated by different types of high-index facets, including {hk0} (h > k > 0), {hhl} (h > l > 0), {hkk} (h > k > 0), and {hkl} (h > k > l > 0). In each type, we describe several distinct morphologies including convex, concave, and other irregular shapes in detail. Based on these remarks, we discuss key factors that may induce the variations of Miller indices in each class, such as organic capping ligands and metallic cationic species. In a look at applications, we review several typical high-indexed noble metal nanocrystals showing enhanced electrocatalytic or chemical catalytic activities.

Heterogeneous catalysts need not be so "heterogeneous": monodisperse Pt nanocrystals by combining shape-controlled synthesis and purification by colloidal recrystallization

Well-defined surfaces of Pt have been extensively studied for various catalytic processes. However, industrial catalysts are mostly composed of fine particles (e.g., nanocrystals), due to the desire for a high surface to volume ratio. Therefore, it is very important to explore and understand the catalytic processes both at nanoscale and on extended surfaces. In this report, a general synthetic method is described to prepare Pt nanocrystals with various morphologies. The synthesized Pt nanocrystals are further purified by exploiting the "self-cleaning" effect which results from the "colloidal recrystallization" of Pt supercrystals. The resulting high-purity nanocrystals enable the direct comparison of the reactivity of the {111} and {100} facets for important catalytic reactions. With these high-purity Pt nanocrystals, we have made several observations: Pt octahedra show higher poisoning tolerance in the electrooxidation of formic acid than Pt cubes; the oxidation of CO on Pt nanocrystals is structure insensitive when the partial pressure ratio p(O2)/p(CO) is close to or less than 0.5, while it is structure sensitive in the O(2)-rich environment; Pt octahedra have a lower activation energy than Pt cubes when catalyzing the electron transfer reaction between hexacyanoferrate (III) and thiosulfate ions. Through electrocatalysis, gas-phase-catalysis of CO oxidation, and a liquid-phase-catalysis of electron transfer reaction, we demonstrate that high quality Pt nanocrystals which have {111} and {100} facets selectively expose are ideal model materials to study catalysis at nanoscale.

Nanoparticle-controlled aggregation of colloidal tetrapods

Tetrapods are among the most promising building blocks for nanoscale self-assembly, offering various desirable features. Whereas these particles can be fabricated with remarkable precision, comparatively less is known about their aggregation behavior. Employing a novel, powerful simulation method, we demonstrate that charged nanoparticles offer considerable control over the assembly of tip-functionalized tetrapods. Extending these findings to tetrapods confined to a gas/liquid interface, we show that regular structures can be achieved even without functionalization.

Shape control of platinum and palladium nanoparticles for catalysis

Platinum and palladium are important catalysts for a wide variety of industrial processes. With the increasing demands of these materials, the development of high-performance catalysts is an important area of research, and as a result, shape control synthesis has become one of the leading research focuses. This minireview surveys the different approaches in solution-phase synthesis that have been successfully adopted for achieving shaped platinum and palladium nanoparticles that are enclosed with specific crystallographic facets. In addition, catalytic studies of the shaped nanoparticles are highlighted, in which promising results have been reported in terms of enhanced activity and selectivity. The future outlook discusses the aspects in synthesis and catalysis to be considered for the development of highly efficient and effective catalysts.

Nonspherical noble metal nanoparticles: colloid-chemical synthesis and morphology control

Metal nanoparticles have been the subject of widespread research over the past two decades. In recent years, noble metals have been the focus of numerous studies involving synthesis, characterization, and applications. Synthesis of an impressive range of noble metal nanoparticles with varied morphologies has been reported. Researchers have made a great progress in learning how to engineer materials on a nanometer length scale that has led to the understanding of the fundamental size- and shape-dependent properties of matter and to devising of new applications. In this article, we review the recent progress in the colloid-chemical synthesis of nonspherical nanoparticles of a few important noble metals (mainly Ag, Au, Pd, and Pt), highlighting the factors that influence the particle morphology and discussing the mechanisms behind the nonspherical shape evolution. The article attempts to present a thorough discussion of the basic principles as well as state-of-the-art morphology control in noble metal nanoparticles.

Electronic and magnetic properties of ultrathin Au/Pt nanowires

We have reported the synthesis of Au(25)Pt(75) and Au(48)Pt(52) alloyed ultrathin nanowires with average widths of less than 3 nm via a wet chemistry approach at room temperature. Using a combination of techniques, including scanning transmission electron microscopy equipped with X-ray energy dispersive spectroscopy, ultraviolet-visible spectroscopy, and X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopies, we identified the stoichiometry-dependent heterogeneous crystalline structures, as well as electronic structures with respect to the charge transfer between Pt and Au within both nanowires. In particular, we observed d-charge depletion at the Au site and the d-charge gain at the Pt site in Au(48)Pt(52) nanowires, which accounted for its ferromagnetic magnetic behavior, in contrast to the paramagnetism and diamagnetism appearing respectively in bulk Pt and Au.

Shape-controlled synthesis of metal nanocrystals: simple chemistry meets complex physics?

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.