Facile Synthesis of PtP2 Nanocrystals as Highly Active Electrocatalysts for Methanol Oxidation.

Although significant efforts have been made in the past few decades, the development of affordable, durable, and effective electrocatalysts for direct methanol fuel cells (DMFCs) remains a formidable challenge. Herein, we present a facile and efficient phosphorization approach for synthesizing PtP2 intermetallic nanocrystals and utilize them as electrocatalysts in the methanol oxidation reaction (MOR). Impressively, the synthesized PtP2 nanocatalysts exhibit a mass activity of 2.14 mA µg-1 and a specific activity of 6.28 mA cm-2, which are 5.1 and 9.5 times higher than those achieved by the current state-of-the-art commercial Pt/C catalyst, respectively. Moreover, the PtP2 nanocatalysts demonstrate improved stability toward acidic MOR by retaining 92.1% of its initial mass activity after undergoing 5000 potential cycles, far surpassing that of the commercial Pt/C (38%). Further DMFC tests present a 2.7 times higher power density than that of the commercial Pt/C, underscoring their potential for application in methanol fuel cells. Density functional theory calculations suggest that the accelerated MOR kinetics and improved CO tolerance on PtP2 can be attributed to the attenuated binding strength of CO intermediates and the enhanced stability due to strong Pt-P interaction. To our knowledge, this is the first report identifying the MOR performance on PtP2 intermetallic nanocrystals, highlighting their potential as highly active and stable nanocatalysts for DMFCs.

Synthesis of amorphous trimetallic PdCuNiP nanoparticles for enhanced OER.

Metal phosphides with multi-element components and amorphous structure represent a novel kind of electrocatalysts for promising activity and durability towards the oxygen evolution reaction (OER). In this work, a two-step strategy, including alloying and phosphating processes, is reported to synthesize trimetallic amorphous PdCuNiP phosphide nanoparticles for efficient OER under alkaline conditions. The synergistic effect between Pd, Cu, Ni, and P elements, as well as the amorphous structure of the obtained PdCuNiP phosphide nanoparticles, would boost the intrinsic catalytic activity of Pd nanoparticles towards a wide range of reactions. These obtained trimetallic amorphous PdCuNiP phosphide nanoparticles exhibit long-term stability, nearly a 20-fold increase in mass activity toward OER compared with the initial Pd nanoparticles, and 223 mV lower in overpotential at 10 mA cm-2. This work not only provides a reliable synthetic strategy for multi-metallic phosphide nanoparticles, but also expands the potential applications of this promising class of multi-metallic amorphous phosphides.

Tailoring Amorphous PdCu Nanostructures for Efficient C-C Cleavage in Ethanol Electrooxidation.

The large-scale application of direct ethanol fuel cells has long been obstructed by the sluggish ethanol oxidation reaction at the anode. Current wisdom for designing and fabricating EOR electrocatalysts has been focused on crystalline materials, which result in only limited improvement in catalytic efficiency. Here, we report the amorphous PdCu (a-PdCu) nanomaterials as superior EOR electrocatalysts. The amorphization of PdCu catalysts can significantly facilitate the C-C bond cleavage, which thereby affords a C1 path faradic efficiency as high as 69.6%. Further tailoring the size and shape of a-PdCu nanocatalysts through the delicate kinetic control can result in a maximized mass activity up to 15.25 A/mgPd, outperforming most reported catalysts. Notably, accelerated durability tests indicate that both the isotropic structure and one-dimensional shape can dramatically enhance the catalytic durability of the catalysts. This work provides valuable guidance for the rational design and fabrication of amorphous noble metal-based electrocatalysts for fuel cells.

Mastering the surface strain of platinum catalysts for efficient electrocatalysis.

Platinum (Pt) has found wide use as an electrocatalyst for sustainable energy conversion systems1-3. The activity of Pt is controlled by its electronic structure (typically, the d-band centre), which depends sensitively on lattice strain4,5. This dependence can be exploited for catalyst design4,6-8, and the use of core-shell structures and elastic substrates has resulted in strain-engineered Pt catalysts with drastically improved electrocatalytic performances7,9-13. However, it is challenging to map in detail the strain-activity correlations in Pt-catalysed conversions, which can involve a number of distinct processes, and to identify the optimal strain modification for specific reactions. Here we show that when ultrathin Pt shells are deposited on palladium-based nanocubes, expansion and shrinkage of the nanocubes through phosphorization and dephosphorization induces strain in the Pt(100) lattice that can be adjusted from -5.1 per cent to 5.9 per cent. We use this strain control to tune the electrocatalytic activity of the Pt shells over a wide range, finding that the strain-activity correlation for the methanol oxidation reaction and hydrogen evolution reaction follows an M-shaped curve and a volcano-shaped curve, respectively. We anticipate that our approach can be used to screen out lattice strain that will optimize the performance of Pt catalysts-and potentially other metal catalysts-for a wide range of reactions.

General Synthesis of Amorphous PdM (M = Cu, Fe, Co, Ni) Alloy Nanowires for Boosting HCOOH Dehydrogenation.

Noble metal-based nanomaterials with amorphous structures are promising candidates for developing efficient electrocatalysts. However, their synthesis remains a significant challenge, especially under mild conditions. In this paper, we report a general strategy for preparing amorphous PdM nanowires (a-PdM NWs, M = Fe, Co, Ni, and Cu) at low temperatures by exploiting glassy non-noble metal (M) nuclei generated by special ligand adsorption as the amorphization dictator. When evaluated as electrocatalysts toward formic acid oxidation, a-PdCu NWs can deliver the mass and specific activities as high as 2.93 A/mgPd and 5.33 mA/cm2, respectively; these are the highest values for PdCu-based catalysts reported thus far, far surpassing the crystalline-dominant counterparts and commercial Pd/C. Theoretical calculations suggest that the outstanding catalytic performance of a-PdCu NWs arises from the amorphization-induced high surface reactivity, which can efficiently activate the chemically stable C-H bond and thereby significantly facilitate the dissociation of HCOOH.

Deposition of Atomically Thin Pt Shells on Amorphous Palladium Phosphide Cores for Enhancing the Electrocatalytic Durability.

As an excellent electrocatalyst, platinum (Pt) is often deposited as a thin layer on a nanoscale substrate to achieve high utilization efficiency. However, the practical application of the as-designed catalysts has been substantially restricted by the poor durability arising from the leaching of cores. Herein, by employing amorphous palladium phosphide (a-Pd-P) as substrates, we develop a class of leaching-free, ultrastable core-shell Pt catalysts with well-controlled shell thicknesses and surface structures for fuel cell electrocatalysis. When a submonolayer of Pt is deposited on the 6 nm nanocubes, the resulting Pd@a-Pd-P@PtSML core-shell catalyst can deliver a mass activity as high as 4.08 A/mgPt and 1.37 A/mgPd+Pt toward the oxygen reduction reaction at 0.9 V vs the reversible hydrogen electrode and undergoes 50 000 potential cycles with only ∼9% activity loss and negligible structural deformation. As elucidated by the DFT calculations, the superior durability of the catalysts originates from the high corrosion resistance of the disordered a-Pd-P substrates and the strong interfacial Pt-P interactions between the Pt shell and amorphous Pd-P layer.

H2-Induced coalescence of Pt nanoparticles for the preparation of ultrathin Pt nanowires with high-density planar defects.

Construction of planar defects within a metallic catalyst can significantly improve its catalytic performance. However, it remains a huge challenge to introduce planar defects during the synthesis of metallic catalysts. In this work, we have reported an effective approach for the preparation of Pt nanowires with high-density planar defects. The success of the approach mainly relies on the attaching and merging of small Pt nanoparticles at low temperatures with the assistance of H2. By comparing the catalytic activities of Pt nanowires with high-density planar defects and commercial Pt/C catalysts toward methanol oxidation reactions, we show that the existence of planar defects can markedly enhance the electrocatalytic performance of the Pt nanowires. The Pt nanowires of 2.0 nm in diameter show a factor of 6.1 enhancement in specific activity and a factor of 5.4 enhancement in mass activity, respectively, for this reaction, compared to the commercial Pt/C catalyst. The method developed in this work could be an effective route to introduce planar defects within Pt catalysts, endowing them with much enhanced catalytic properties.

Etching-Assisted Route to Heterophase Au Nanowires with Multiple Types of Active Surface Sites for Silane Oxidation.

The construction of multiple types of active sites on the surface of a metallic catalyst can markedly enhance its catalytic activity toward specific reactions. Here, we show that heterophase gold nanowires (Au NWs) with multiple types of active surface sites can be synthesized using an etching-assisted process, yielding the highest reported turnover frequency (TOF) for Au catalysts toward the silane oxidation reaction by far. We use synchrotron powder X-ray diffraction (PXRD) and aberration-corrected (scanning) transmission electron microscopy (TEM) to show that the Au NWs contain heterophase structures, planar defects, and surface steps. Moreover, the contribution to the catalytic performance from each type of active sites was clarified. Surface steps on the Au NW catalysts, which were identified using aberration-corrected (scanning) TEM, were shown to play the most important role in enhancing the catalytic performance. By using synchrotron PXRD, it was shown that a small ratio of metastable phases within Au NWs can enhance catalytic activity by a factor of 1.35, providing a further route to improve catalytic activity. Of the three types of surface active sites, surface terminations of planar defects such as twin boundaries (TB) and stacking faults (SF) are less active than metastable phases and surface steps for Au catalysts toward the silane oxidation reaction. Such an etching-assisted synthesis of heterophase Au NWs promises to open new possibilities for catalysis, plasmonic, optics, and electrical applications.

Engineering Surface Structure of Pt Nanoshells on Pd Nanocubes to Preferentially Expose Active Surfaces for ORR by Manipulating the Growth Kinetics.

Synthesis of Pt nanoshells on substrates can increase the utilization efficiency of Pt atoms and reduce the amount of Pt used in the applications. However, it is still an enormous challenge in tailoring the required crystal facets of Pt nanoshells on a given substrate. In this work, we demonstrate a facile and convenient approach capable for generating Pt octahedral islands with tunable sizes and densities on Pd nanocubes by manipulating the deposition rate. The key to this synthesis is the fine control over the deposition rate of Pt on Pd seeds. Because of the different reactivities at the surface sites, the deposition of Pt can be controlled at a certain site by carefully tuning the deposition rate. With a low concentration of reductant (8.33 mg/mL of glucose), surface diffusion dominates the process, and thus the Pt cubic shells form on Pd cubic seeds. In contrast, when a higher amount of the reductant (16.67 mg/mL of glucose) is added, the deposition starts to dominate the growth of Pt shells. In this case, the deposition would be controlled at the corners, forming eight large Pt octahedra on a cubic Pd seed. Further increasing the deposition rate can induce much higher deposition rates, in which case, the deposition of Pt would likely take place not only at the corners, but also the edge and surface sites of the seeds. Not surprisingly, this growth habit can result in the formation of high-density octahedral islands on Pd cubic seeds. With the same amount of precursor supply, the higher the densities of Pt islands, the smaller the size of the octahedral islands on Pd nanocubes. Unlike other synthetic methods, the size of the octahedral islands on Pd seeds can be even controlled to be smaller than 3 nm by controlling the amount of the Pt precursor. Considering the excellent performance of {111} facets of Pt catalysts toward ORR, the Pt nanocages with small octahedral islands on the surfaces can exhibit a high activity with a mass activity 0.68 A/mg, as high as 5.2 times of that of commercial Pt/C.

An etching-assisted route for fast and large-scale fabrication of non-layered palladium nanosheets.

To date, great progress has been made in the shape-controlled synthesis of noble-metal nanocrystals. However, there still exists a major gap between academic studies and industrial applications due to the inability to produce nanocrystals in large quantities while retaining their uniformity. To help fill this gap, herein, we provide a new route to scale up and accelerate the production of non-layered palladium nanosheets (Pd NSs) by incorporating etching while retaining effective capping during the synthesis. The key to this rapid synthesis is the etching induced by selected etchants (e.g., Fe3+/Fe2+, Cl-/O2, Br-/O2, and I-/O2). Specifically, this synthesis can be accomplished within 3 min, reaching a yield as high as 7.2 g L-1 h-1. The thickness of Pd NSs can be tuned to 1.6, 2.0, 2.3, and 3.5 nm by controlling the etching and reducing rates via choosing different type of etchants. Moreover, these non-layered Pd NSs are fabricated in an aqueous solution without the addition of any organic compounds; therefore, the surface of these NSs is extremely clean. When used as a catalyst for the formic acid oxidation reaction, the as-prepared non-layered Pd NSs exhibit a mass activity as high as 1350 mA mg-1, which is 3.7 times higher than that of commercial Pd/C, due to their much larger electrochemical surface area (66.2 m2 g-1, which is 2.7 times higher than that of commercial Pd/C).

Inflating hollow nanocrystals through a repeated Kirkendall cavitation process.

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

Synthesis of Pd Nanoframes by Excavating Solid Nanocrystals for Enhanced Catalytic Properties.

Synthesis of metal nanoframes has been of great interest for their open structures and high fractions of active surface sites, which gives rise to outstanding performance in catalysis. In this work, Pd nanoframes with well-defined structures have been successfully prepared by directly excavating solid nanocrystals. The success of this synthesis mainly relies on the fine control over the oxidative etching and regrowth rates. Due to the different regrowth rates at three typical types of surface sites (e.g., corners, edges, and faces), the removal of Pd atoms can be controlled at a certain site by carefully tuning the rates of the oxidative etching and regrowth. Without the presence of the reducing agent, etching dominates the process, resulting in the shape transformation of nanocrystals with well-defined shapes (e.g., octahedra) to cuboctahedra. In contrast, when a certain amount of the reducing agent (e.g., HCHO) is added, the regrowth rate at the corner and edge sites can be controlled to be equivalent to the etching rate, while the regrowth rate at the face sites is still smaller than the etching rate. In this case, the etching can only take place at the faces; thus, Pd nanoframes could be obtained. On the basis of this approach, solid Pd nanocrystals with different shapes, including cubes, cuboctahedra, octahedra, and concave cubes, have been successfully excavated to the corresponding nanoframes. These nanoframes can unambiguously exhibit much enhanced catalytic activity and improved durability toward formic acid oxidation reaction due to their three-dimensional (3D) open frameworks compared with solid Pd octahedra catalysts.

Creation of Controllable High-Density Defects in Silver Nanowires for Enhanced Catalytic Property.

Structural defects have been proven to determine many of the materials' properties. Here, we demonstrate a unique approach to the creation of Ag nanowires with high-density defects through controllable nanoparticles coalescence in one-dimensional pores of mesoporous silica. The density of defects can be easily adjusted by tuning the annealing temperature during synthetic process. The high-density defects promote the adsorption and activation of more reactants on the surface of Ag nanowires during catalytic reactions. As a result, the as-prepared Ag nanowires exhibit enhanced activities in catalyzing dehydrogenative coupling reaction of silane in terms of apparent activation energy and turnover frequency (TOF). We show further that the silane conversion rate can be enhanced by maximizing the defect density and thus the number of active sites on the Ag nanowires, reaching a remarkable TOF of 8288 h(-1), which represents the highest TOF that has been achieved by far on Ag catalysts. This work not only proves the important role of structural defects in catalysis but also provides a new and general strategy for constructing high-density defects in metal catalysts.

Selectivity on Etching: Creation of High-Energy Facets on Copper Nanocrystals for CO2 Electrochemical Reduction.

Creating high-energy facets on the surface of catalyst nanocrystals represents a promising method for enhancing their catalytic activity. Herein we show that crystal etching as the reverse process of crystal growth can directly endow nanocrystal surfaces with high-energy facets. The key is to avoid significant modification of the surface energies of the nanocrystal facets by capping effects from solvents, ions, and ligands. Using Cu nanocubes as the starting material, we have successfully demonstrated the creation of high-energy facets in metal nanocrystals by controlled chemical etching. The etched Cu nanocrystals with enriched high-energy {110} facets showed significantly enhanced activity toward CO2 reduction. We believe the etching-based strategy could be extended to the synthesis of nanocrystals of many other catalysts with more active high-energy facets.

Lattice-mismatch-induced twinning for seeded growth of anisotropic nanostructures.

Synthesis of anisotropic nanostructures from materials with isotropic crystal structures often requires the use of seeds containing twin planes to break the crystalline symmetry and promote the preferential anisotropic growth. Controlling twinning in seeds is therefore critically important for high-yield synthesis of many anisotropic nanostructures. Here, we demonstrate a unique strategy to induce twinning in metal nanostructures for anisotropic growth by taking advantage of the large lattice mismatch between two metals. By using Au-Cu as an example, we show, both theoretically and experimentally, that deposition of Cu to the surface of single-crystalline Au seeds can build up strain energy, which effectively induces the formation of twin planes. Subsequent seeded growth allows the production of Cu nanorods with high shape anisotropy that is unachievable without the use of Au seeds. This work provides an effective strategy for the preparation of anisotropic metal nanostructures.

Redox reaction induced Ostwald ripening for size- and shape-focusing of palladium nanocrystals.

We report here that size- and shape-focusing can be achieved through the well-known Ostwald ripening process to produce high-quality metal nanocrystals (NCs). Using Pd as an example, we show that the addition of small NCs of appropriate sizes could help in modulating the growth of larger NCs and enable excellent control over both the size and shape uniformity of the products. A detailed mechanistic study showed that the self-focusing of Pd NCs relied on a dissolution and regrowth process induced by redox reaction of HCHO. With the assistance of HCHO, injection of small sacrificial nanocrystals (SNCs), with sizes below a critical value, into larger seeds results in the dissolution of the SNCs and subsequent deposition onto the larger ones, thus allowing the formation of monodisperse Pd NCs. We have identified the critical radius of the SNCs to be ∼5.7 nm for Pd, and verified that SNCs with sizes larger than that could not effectively support the growth of larger seeds. More interestingly, since Ostwald ripening involves matter relocation, this synthetic approach could even break the self-termination growth habits of metal NCs and produce nanocrystals with sizes that are not conveniently accessible by direct growth.

Monitoring the shape evolution of Pd nanocubes to octahedra by PdS frame markers.

We report the use of compounds formed on the surface of nanocrystals as markers to effectively monitor the shape evolution during nanocrystal growth. By labelling the corners and edges of cubic Pd seeds with PdS through sulfuration, we obtain clear insight into their shape transition to octahedra. This work significantly expands the scope of the marker technique for studying nanoparticle shape evolution.

Synthesis of rhodium concave tetrahedrons by collectively manipulating the reduction kinetics, facet-selective capping, and surface diffusion.

This paper describes a facile synthesis of Rh tetrahedrons with concave side faces by collectively manipulating the reaction kinetics, facet-selective capping, and surface diffusion of atoms. Specifically, a combination of Na3RhCl6, triethylene glycol, l-ascorbic acid, and citric acid provides the right conditions for generating the concave tetrahedrons. After the formation of small Rh tetrahedral seeds through self-nucleation, the subsequently generated Rh atoms were selectively deposited onto the corner sites to generate Rh tetrapods. At the same time, the deposited atoms could diffuse from the corners to edges to generate concave side faces because the diffusion to face sites was restrained by the citric acid adsorbed on the {111} facets. This study offers deep insight into the growth mechanism involved the formation of noble-metal nanocrystals with concave surfaces. The Rh concave tetrahedrons were encased by a mix of {111} and {110} facets, showing great potential for catalytic applications.

A dispersive scattering centers-based strategy for dramatically enhancing the photocatalytic efficiency of photocatalysts in liquid-phase photochemical processes: a case of Ag nanosheets.

A dispersive scattering centers-based strategy was proposed to enhance the photocatalytic efficiency of photocatalysts in liquid-phase photochemical processes. Photocatalytic efficiencies of the photocatalyst, Degussa P25, in water splitting and photodegradation were markedly enhanced by using Ag nanosheets as dispersive scattering centers.