Composition-dependent ordering transformations in Pt-Fe nanoalloys.

SignificanceDynamically understanding the microscopic processes governing ordering transformations has rarely been attained. The situation becomes even more challenging for nanoscale alloys, where the significantly increased surface-area-to-volume ratio not only opens up a variety of additional freedoms to initiate an ordering transformation but also allows for kinetic interplay between the surface and bulk due to their close proximity. We provide direct evidence of the microscopic processes controlling the ordering transformation through the surface-bulk interplay in Pt-Fe nanoalloys and new features rendered by variations in alloy composition and chemical stimuli. These results provide a mechanistic detail of ordering transformation phenomena which are widely relevant to nanoalloys as chemical ordering occurs in most multicomponent materials under suitable environmental bias.

Randomly Layered Superstructure of In2O3 Truncated Nano-Octahedra and Its High-Pressure Behavior.

This study outlines the preparation and characterization of a unique superlattice composed of indium oxide (In2O3) vertex-truncated nano-octahedra, along with an exploration of its response to high-pressure conditions. Transmission electron microscopy and scanning transmission electron microscopy were employed to determine the average circumradius (15.2 nm) of these vertex-truncated building blocks and their planar superstructure. The resilience and response of the superlattice to pressure variations, peaking at 18.01 GPa, were examined using synchrotron-based wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) techniques. The WAXS data revealed no phase transitions, reinforcing the stability of the 2D superlattice composed of random layers in alignment with a p31m planar symmetry as discerned by SAXS. Notably, the SAXS data also unveiled a pressure-induced, irreversible translation of octahedra and ligand interaction occurring within the random layer. Through our examination of these pressure-sensitive behaviors, we identified a distinctive translation model inherent to octahedra and observed modulation of the superlattice cell parameter induced by pressure. This research signifies a noteworthy advancement in deciphering the intricate behaviors of 2D superlattices under a high pressure.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies.

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Improving Oxygen Reduction Performance of Surface-Layer-Controlled Pt-Ni Nano-Octahedra via Gaseous Etching.

This study demonstrates an atomic composition manipulation on Pt-Ni nano-octahedra to enhance their electrocatalytic performance. By selectively extracting Ni atoms from the {111} facets of the Pt-Ni nano-octahedra using gaseous carbon monoxide at an elevated temperature, a Pt-rich shell is formed, resulting in an ∼2 atomic layer Pt-skin. The surface-engineered octahedral nanocatalyst exhibits a significant enhancement in both mass activity (∼1.8-fold) and specific activity (∼2.2-fold) toward the oxygen reduction reaction compared with its unmodified counterpart. After 20,000 potential cycles of durability tests, the surface-etched Pt-Ni nano-octahedral sample shows a mass activity of 1.50 A/mgPt, exceeding the initial mass activity of the unetched counterpart (1.40 A/mgPt) and outperforming the benchmark Pt/C (0.18 A/mgPt) by a factor of 8. DFT calculations predict this improvement with the Pt surface layers and support these experimental observations. This surface-engineering protocol provides a promising strategy for developing novel electrocatalysts with improved catalytic features.

Noble-Metal Based Random Alloy and Intermetallic Nanocrystals: Syntheses and Applications.

Precise control over the size, shape, composition, structure, and crystal phase of random alloy and intermetallic nanocrystals has been intensively explored in technologically important applications in recent years. Different from the monometallic nanocrystals and other types of structural nanocrystals such as core-shell and heterostructured nanocrystals, well-defined multimetallic random alloy and intermetallic nanocrystals exhibit unique and intriguing physicochemical properties, serving as ideal models for benefiting the structure-to-property studies. As such, random alloy and intermetallic nanocrystals have attracted extensive attention and interest in scientific research and shown huge potential in various fields. In this review, we focus specifically on summarizing the synthetic principles and strategies developed to form random alloy and intermetallic nanocrystals with enhanced performance. Some representative examples are purposely selected for emphasizing basic concepts and mechanistic understanding. We then highlight the fascinating properties and widespread applications of random alloy and intermetallic nanocrystals in electrocatalysis, heterogeneous catalysis, optical and photocatalysis, as well as magnetism and conclude the review by addressing the prospects and current challenges for the controlled synthesis of random alloy and intermetallic nanocrystals.

Colloidal synthesis of monodisperse trimetallic Pt-Fe-Ni nanocrystals and their enhanced electrochemical performances.

Among the multi-metallic nanocatalysts, Pt-based alloy nanocrystals (NCs) have demonstrated promising performance in fuel cells and water electrolyzers. Herein, we demonstrate a facile colloidal synthesis of monodisperse trimetallic Pt-Fe-Ni alloy NCs through a co-reduction of metal precursors. The as-synthesized ternary NCs exhibit superior mass and specific activities toward oxygen reduction reaction (ORR), which are ∼2.8 and 5.6 times as high as those of the benchmark Pt/C catalyst, respectively. The ORR activity of the carbon-supported Pt-Fe-Ni nanocatalyst is persistently retained after the durability test. Owing to the incorporation of Fe and Ni atoms into the Pt lattice, the as-prepared trimetallic Pt-alloy electrocatalyst also manifestly enhances the electrochemical activity and durability toward the oxygen evolution reaction with a reduced overpotential when compared with that of the benchmark Pt/C (△η= 0.20 V, at 10 mA cm-2). This synthetic strategy paves the way for improving the reactivity for a broad range of electrocatalytic applications.

Improvement of Oxygen Reduction Performance in Alkaline Media by Tuning Phase Structure of Pd-Bi Nanocatalysts.

Tuning the crystal phase of bimetallic nanocrystals offers an alternative avenue to improving their electrocatalytic performance. Herein, we present a facile and one-pot synthesis approach that is used to enhance the catalytic activity and stability toward oxygen reduction reaction (ORR) in alkaline media via control of the crystal structure of Pd-Bi nanocrystals. By merely altering the types of Pd precursors under the same conditions, the monoclinic structured Pd5Bi2 and conventional face-centered cubic (fcc) structured Pd3Bi nanocrystals with comparable size and morphology can be precisely synthesized, respectively. Interestingly, the carbon-supported monoclinic Pd5Bi2 nanocrystals exhibit superior ORR activity in alkaline media, delivering a mass activity (MA) as high as 2.05 A/mgPd. After 10,000 cycles of ORR durability test, the monoclinic structured Pd5Bi2/C nanocatalysts still remain a MA of 1.52 A/mgPd, which is 3.6 times, 16.9 times, and 21.7 times as high as those of the fcc Pd3Bi/C counterpart, commercial Pd/C, and Pt/C electrocatalysts, respectively. Moreover, structural characterizations of the monoclinic Pd5Bi2/C nanocrystals after the durability test demonstrate the excellent retention of the original size, morphology, composition, and crystal phase, greatly alleviating the leaching of the Bi component. This work provides new insight for the synthesis of multimetallic catalysts with a metastable phase and demonstrates phase-dependent catalytic performance.

Construction of Lattice Strain in Bimetallic Nanostructures and Its Effectiveness in Electrochemical Applications.

Bimetallic nanocrystals (NCs), associated with various surface functions such as ligand effect, ensemble effect, and strain effect, exhibit superior electrocatalytic properties. The stress-induced surface strain effect can alter binding strength between the surface active sites and reactants as well as their intermediates, and the electrochemical performance of bimetallic NCs can be significantly facilitated by the lattice-strain modification via their morphologies, sizes, shell-thickness, surface defectiveness as well as compositions. In this review, an overview of fundamental principles, characterization techniques, and quantitative determination of the surface lattice strain is provided. Various strategies and synthesis efforts on creating lattice-strain-engineered bimetallic NCs, including the de-alloying process, atomic layer-by-layer deposition, thermal treatment evolution, one-pot synthesis, and other efforts are also discussed. It is further outlined how the lattice strain effect promotes electrochemical catalysis through the selected case studies. The reactions on oxygen reduction reaction, small molecular oxidation, water splitting reaction, and electrochemical carbon dioxide reduction reactions are focused. In particular, studies of lattice strain arisen from core-shell nanostructure and defectiveness are highlighted. Lastly, the potential challenges are summarized and the prospects of lattice-strain-based engineering on bimetallic nanocatalysts with suggestion and guidance of the future electrocatalyst design are envisioned.

Synthesis of Core@Shell Cu-Ni@Pt-Cu Nano-Octahedra and Their Improved MOR Activity.

Fabrication of 3d metal-based core@shell nanocatalysts with engineered Pt-surfaces provides an effective approach for improving the catalytic performance. The challenges in such preparation include shape control of the 3d metallic cores and thickness control of the Pt-based shells. Herein, we report a colloidal seed-mediated method to prepare octahedral CuNi@Pt-Cu core@shell nanocrystals using CuNi octahedral cores as the template. By precisely controlling the synthesis conditions including the deposition rate and diffusion rate of the shell-formation through tuning the capping ligand, reaction temperature, and heating rate, uniform Pt-based shells can be achieved with a thickness of <1 nm. The resultant carbon-supported CuNi@Pt-Cu core@shell nano-octahedra showed superior activity in electrochemical methanol oxidation reaction (MOR) compared with the commercial Pt/C catalysts and carbon-supported CuNi@Pt-Cu nano-polyhedron counterparts.

Pressure-Engineered Structural and Optical Properties of Two-Dimensional (C4H9NH3)2PbI4 Perovskite Exfoliated nm-Thin Flakes.

Resolving the structure-property relationships of two-dimensional (2D) organic-inorganic hybrid perovskites is essential for the development of photovoltaic and photoelectronic devices. Here, pressure (0-10 GPa) was applied to 2D hybrid perovskite flakes mechanically exfoliated from butylammonium lead halide single crystals, (C4H9NH3)2PbI4, from which we observed a series of changes of the strong excitonic emissions in the photoluminescence spectra. By correlating with in situ high-pressure X-ray diffraction results, we examine successfully the relationship between structural modifications in the inorganic PbI42- layer and their excitonic properties. During the transition between Pbca (1b) phase and Pbca (1a) phase at around 0.1 GPa, the decrease in ⟨Pb-I-Pb⟩ bond angle and increase in Pb-I bond length lead to an abrupt blue shift of the excitonic bandgap. The presence of the P21/a phase above 1.4 GPa increases the ⟨Pb-I-Pb⟩ bond angle and decreases the Pb-I bond length, leading to a deep red shift of the excitonic bandgap. The total band gap narrowing of ∼350 meV to 2.03 eV at 5.3 GPa before amorphization, facilitates (C4H9NH3)2PbI4 as a much better solar absorber. Moreover, phase transitions inevitably modify the carrier lifetime of (C4H9NH3)2PbI4, where an initial 150 ps at ambient phase is prolongated to 190 ps in the Pbca (1a) phase along with enhanced photoluminescence (PL), originating from pressure-induced strong radiative recombination of trapped excitons.The onset of P21/a phase shortens significantly the carrier lifetime to 53 ps along with a weak PL emission due to pressure-induced severe lattice distortion and amorphization. High-pressure study on (C4H9NH3)2PbI4 nm-thin flakes may provide insights into the mechanisms for synthetically designing novel 2D hybrid perovskite based photoelectronic devices and solar cells.

Phase Transitions of Formamidinium Lead Iodide Perovskite under Pressure.

The pressure-induced structural evolution of formamidinium-based perovskite FAPbI3 was investigated using in situ synchrotron X-ray diffraction and laser-excited photoluminescence methods. Cubic α-FAPbI3 ( Pm3̅ m) partially and irreversibly transformed to hexagonal δ-FAPbI3 ( P63 mc) at a pressure less than 0.1 GPa. Structural transitions of α-FAPbI3 followed the sequence of Pm3̅ m → P4/ mbm → Im3̅ → partial amorphous during compression to 6.59 GPa, whereas the δ-phase converted to an orthorhombic Cmc21 structure between 1.26 and 1.73 GPa. During decompression, FAPbI3 recovered the P63 mc structure of the δ-phase as a minor component (∼18 wt %) from 2.41-1.40 GPa and the Pm3̅ m structure of the α-phase becomes dominant (∼82 wt %) at 0.10 GPa but with an increased fraction of δ-FAPbI3. The photoluminescence behaviors from both the α- and δ-forms were likely controlled by radiative recombination at the defect levels rather than band-edge emission during pressure cycling. FAPbI3 polymorphism is exquisitely sensitive to pressure. While modest pressures can engineer FAPbI3-based photovoltaic devices, irreversible δ-phase crystallization may be a limiting factor and should be taken into account.

Entropy-Driven Pt3Co Nanocube Assembles and Thermally Mediated Electrical Conductivity with Anisotropic Variation of the Rhombohedral Superlattice.

Understanding the shape-dependent superlattices and resultant anisotropies of both structure and property allows for rational design of materials processing and engineering to fabricate transformative materials with useful properties for applications. This work shows the structural evolution from square lattice of two-dimensional (2D) thin film to rhombic lattice of large three-dimensional (3D) assembles of Pt3Co nanocubes (NCs). Synchrotron-based X-ray supercrystallography determines the superlattice of large 3D supercrystal into an obtuse rhombohedral (Rh) symmetry, which holds a long-range coherence of both NC translation and atomic crystallographic orientation. The Rh superlattice has a trigonal cell angle of 104°, and the constitute NCs orient their atomic Pt3Co(111) planes to the superlattice Rh[111] direction. The temperature-dependent in situ small and wide-angle X-ray scattering (SAXS/WAXS) measurements reveal a thermally induced superlattice contraction of supercrystal, which maintains translational ordering but slightly develops orientational disordering. The observed increases of both the packing density and the rotation magnitude of NCs indicate a rational compromise between configurational and rotational entropies of NCs. The resultant minimization of the total free energy is responsible for the formation and stability of the obtuse Rh superlattice. The temperature-dependent in situ measurements of SAXS and electrical resistance reveal that, in conjunction with the thermally induced sharp contraction of superlattice at 160 °C, the supercrystal becomes measurable of electrical resistance, which was followed by a temperature-dependent linear increase. Upon rapid annealing from 250 °C, the supercrystal remains almost constant in both structure and electrical resistance. The heating-enabled electrical conductivity of the supercrystal at high temperature implies the formation of a NC-interconnected architecture. The experiments and overall analysis provide solid evidence and essential information for the use of shape-dependent structural anisotropies of supercrystal to create nanobased novel architecture with desired properties.

High-Indexed Pt3Ni Alloy Tetrahexahedral Nanoframes Evolved through Preferential CO Etching.

Chemically controlling crystal structures in nanoscale is challenging, yet provides an effective way to improve catalytic performances. Pt-based nanoframes are a new class of nanomaterials that have great potential as high-performance catalysts. To date, these nanoframes are formed through acid etching in aqueous solutions, which demands long reaction time and often yields ill-defined surface structures. Herein we demonstrate a robust and unprecedented protocol for facile development of high-performance nanoframe catalysts using size and crystallographic facet-controlled PtNi4 tetrahexahedral nanocrystals prepared through a colloidal synthesis approach as precursors. This new protocol employs the Mond process to preferentially dealloy nickel component in the ⟨100⟩ direction through carbon monoxide etching of carbon-supported PtNi4 tetrahexahedral nanocrystals at an elevated temperature. The resultant Pt3Ni alloy tetrahexahedral nanoframes possess an open, stable, and high-indexed microstructure, containing a segregated Pt thin layer strained to the Pt-Ni alloy surfaces and featuring a down-shift d-band center as revealed by the density functional theory calculations. These nanoframes exhibit much improved catalytic performance, such as high stability under prolonged electrochemical potential cycles, promoting direct electro-oxidation of formic acid to carbon dioxide and enhancing oxygen reduction reaction activities. Because carbon monoxide can be generated from the carbon support through thermal annealing in air, a common process for pretreating supported catalysts, the developed approach can be easily adopted for preparing industrial scale catalysts that are made of Pt-Ni and other alloy nanoframes.

An Obtuse Rhombohedral Superlattice Assembled by Pt Nanocubes.

We grew large single three-dimensional supercrystals from colloidal Pt nanocubes (NCs) suspended in hexane. A synchrotron-based two circle diffractometer was used to obtain an unprecedented level of detail from full sets of small/wide-angle X-ray scattering (SAXS/WAXS) patterns. Automatic indexing and simulations of X-ray patterns enabled detailed reconstruction of NC translation and shape orientation within the supercrystals from atomic to mesometric levels. The supercrystal has an obtuse rhombohedral (Rh) superlattice with space group R3m and a trigonal cell angle of 106.2°. Individual NCs orient themselves in a manner of atomic Pt[111] parallel to superlattice Rh[111]. We analyzed the superlattice structure in context of three spatial relationships of proximate NCs including face-to-face, edge-to-edge, and corner-to-corner configurations. Detailed analysis of supercrystal structure reveals nearly direct corner-to-corner contacts and a tight interlocking NC structure. We employed the correlations between strain and lattice distortion and established the first structural correlating mechanism between five superlattice polymorphs to elucidate the superlattice transformations and associated developing pathways. Together, the experimental and modeling results provide comprehensive structural information toward controlling design and efficient materials-processing for large fabrication of nanobased functional materials with tailored structures and desired properties.

Pressure-Dependent Polymorphism and Band-Gap Tuning of Methylammonium Lead Iodide Perovskite.

We report the pressure-induced crystallographic transitions and optical behavior of MAPbI3 (MA=methylammonium) using in situ synchrotron X-ray diffraction and laser-excited photoluminescence spectroscopy, supported by density functional theory (DFT) calculations using the hybrid functional B3PW91 with spin-orbit coupling. The tetragonal polymorph determined at ambient pressure transforms to a ReO3 -type cubic phase at 0.3 GPa. Upon continuous compression to 2.7 GPa this cubic polymorph converts into a putative orthorhombic structure. Beyond 4.7 GPa it separates into crystalline and amorphous fractions. During decompression, this phase-mixed material undergoes distinct restoration pathways depending on the peak pressure. In situ pressure photoluminescence investigation suggests a reduction in band gap with increasing pressure up to ≈0.3 GPa and then an increase in band gap up to a pressure of 2.7 GPa, in excellent agreement with our DFT calculation prediction.

Solvent-mediated self-assembly of nanocube superlattices.

Self-organization of colloidal Pt nanocubes into two types of distinct ordered superlattices, simple-cubic and body-centered-tetragonal structures, has been achieved using a home-built setup. Detailed translational and orientational characteristics of these superstructures were determined using a transmission electron microscopy tomographic technique with 3D reconstruction analysis. The formation of these distinct superlattices is the result of a delicate choice of solvent (i.e., aliphatic hexane or aromatic toluene hydrocarbons), which serves as a dispersion medium to fine-tune the relative strengths of ligand-ligand and ligand-solvent interactions during the self-assembly process. This work provides important insights into the effects of ligand-solvent interactions on superlattice formation from nonspherical nanoparticles.

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.

Shape-control and electrocatalytic activity-enhancement of Pt-based bimetallic nanocrystals.

Due to the increasing worldwide energy demand and environ-mental concerns, the need for alternative energy sources is growing stronger, and platinum catalysts in fuel cells may help make the technologies a reality. However, the pursuit of highly active Pt-based electrocatalysts continues to be a challenge. Scientists developing electrocatalysts continue to focus on characterizing and directing the construction of nanocrystals and advancing their electrochemical applications. Although chemists have worked on Pt-based bimetallic (Pt-M) preparations in the past, more recent research shows that both shape-controlled Pt-M nanocrystals and the assembly of these nanocrystals into supercrystals are promising new directions. A solution-based synthesis approach is an effective technique for preparing crystallographic facet-directed nanocatalysts. This is aided by careful selection of the metal precursor, capping ligand, reducing agent, and solvent. Incorporating a secondary metal M into the Pt lattice and manipulating the crystal facets on the surface cooperatively alter the electrocatalytic behavior of these Pt-M bimetallic nanocrystals. Specifically, chemists have extensively studied the {111}- and {100}-terminated crystal facets because they show unique atomic arrangement on surfaces, exhibit different catalytic performance, and possess specific resistance to toxic adsorbed carbon monoxide (COads). For catalysts to have maximum efficiency, they need to have resistance to COads and other poisonous carbon-containing intermediates when the catalysts operate under harsh conditions. A necessary design to any synthesis is to clearly understand and utilize the role of each component in order to successfully induce shape-controlled growth. Since chemists began to understand Pt nanocrystal shape-dependent electrocatalytic activity, the main obstacles blocking proton exchange membrane fuel cells are anode poisoning, sluggish kinetics at the cathode, and low activity. In this Account, we discuss the basic concepts in preparation of Pt-M bimetallic nanocrystals, focusing on several immaculate examples of manipulation at the nanoscale. We briefly introduce the prospects for applying Pt-M nanocrystals as electrocatalysts based on the electronic and geometric standpoints. In addition, we discuss several key parameters in the solution-based synthesis approach commonly used to facilitate Pt-M nanocrystals, such as reaction temperature and time, the combination of organic amines and acids, gaseous adsorbates, anionic species, and solvent. Each example features various nanoscale morphologies, such as spheres, cubes, octahedrons, and tetrahedrons. Additionally, we outline and review the superior electrocatalytic performances of the recently developed high-index Pt-M nanostructures. Next, we give examples of the electrocatalytic capabilities from these shape-defined Pt-M architectures by highlighting significant accomplishments in specific systems. Then, using several typical cases, we summarize electrochemical evaluations on the Pt-based shape-/composition-dependent nanocatalysts toward reactions on both the anode and the cathode. Lastly, we provide an outlook of current challenges and promising directions for shape-controlled Pt-M bimetallic electrocatalysts.

Pt3Co concave nanocubes: synthesis, formation understanding, and enhanced catalytic activity toward hydrogenation of styrene.

We report a facile synthesis route to prepare high-quality Pt3Co nanocubes with a concave structure, and further demonstrate that these concave Pt3Co nanocubes are terminated with high-index crystal facets. The success of this preparation is highly dependent on an appropriate nucleation process with a successively anisotropic overgrowth and a preservation of the resultant high-index planes by control binding of oleyl-amine/oleic acid with a fine-tuned composition. Using a hydrogenation of styrene as a model reaction, these Pt3Co concave nanocubes as a new class of nanocatalysts with more open structure and active atomic sites located on their high-index crystallographic planes exhibit an enhanced catalytic activity in comparison with low-indexed surface terminated Pt3Co nanocubes in similar size.

Pressure-induced switching between amorphization and crystallization in PbTe nanoparticles.

Combining in situ high-pressure X-ray scattering with transmission electron microscopy, we investigated the pressure-induced structural switches between the rock salt and amorphous phases as well as the associated mechanisms of their crystallization and growth in 6 nm PbTe nanocrystal. It was observed that rock salt PbTe nanocrystal started to become amorphous above 10 GPa and then underwent a low-to-high density amorphous phase transformation at pressures over 15 GPa. The low-density amorphous phase exhibited a structural memory of the rock salt phase, as manifested by a backward transformation to the rock salt phase via single nucleation inside each nanoparticle upon the release of pressure. In contrast, the high-density amorphous phase remained stable and could be preserved at ambient conditions. In addition, electron beam-induced heating could drive a recrystallization of the rock salt phase on the recovered amorphous nanoparticles. These studies provide significant insights into structural mechanisms for pressure-induced switching between amorphous and crystalline phases as well as their associated growth processes.