Alloy Nanocrystals Research Articles

Intermetallic Nanocrystal Discovery through Modulation of Atom Stacking Hierarchy.

A library of compositionally and structurally well-defined Au-Cu alloy nanocrystals has been prepared via scanning probe block copolymer lithography. These libraries not only allow one to map compositional and structure space but also the conditions (e.g., cooling rate) required to access specific structures. This approach enabled the realization of a previously unobserved architecture, an intermetallic nanoprism, that is a consequence of hierarchical atom stacking. These structures exhibit distinctive diffraction patterns characterized by non-integer-index, forbidden spots, which serve as a diagnostic indicator of such structures. Inspection of the library's pseudospherical particles reveals a high-strain cubic-tetragonal interfacial configuration in the outer regions of the intermetallic nanocrystals. Since it is costly and time-consuming to explore the nanomaterials phase space via conventional wet-chemistry, this parallel kinetic-control approach, which relies on substrate- and positionally isolated particles, may lead to the rapid discovery of complex nanocrystals that may prove useful in applications spanning catalysis and plasmonic sensing.

(Digital Presentation) Design of Metal/Metal Oxide Nanomaterials for Highly Active, Selective, and Durable Electrocatalysts

Electrocatalysis is a key part of renewable energy conversion in the future energy system. Sustainable energy conversion and chemical production require catalyst structure with high activity, durability, and product selectivity. In general, nanoscale electrocatalysts suffer various degradation phenomena during electrocatalysis, which leads to critical performance loss. Recently, the various hybrid nanostructures (such as ordered structure, metal/carbon encapsulation, or metal/metal oxide) have been highly investigated to achieve promising catalytic performances and enhanced stabilities.In this presentation, we will cover three different types of nanomaterials as highly active and stable electrocatalysts for oxygen reduction reaction (ORR). First, the alloy nanoparticles with ordered structures exhibit novel catalytic properties from their unique electronic and geometric structures. In particular, Pt alloys with atomically ordered crystal structures have been found to largely improve both electrocatalytic activity and stability for ORR through increased electronic interaction between Pt and other transition metals. Similarly, we recently demonstrated that well-controlled Co-, Mn- and Fe-based ternary or binary oxide nanocatalysts have an exceptionally high ORR activity, in addition to the promising electrocatalytic stability. Therefore, it is very important to synthesize well-ordered alloy nanocrystals to obtain highly durable and active electrocatalysts with respect to their structural and compositional properties. Second, we will show the strategic employment of carbon shells on electrocatalyst surfaces to enhance stability in the electrochemical process. Carbon shells can beneficially shield catalyst surfaces from electrochemical degradation and physical agglomeration. Thus carbon shells can effectively preserve the initial active site structure during electrocatalysis. The carbon shell also provides a confined environment at interfaces, enabling unconventional electrochemical behaviors. Finally, we will suggest an effective strategy to construct metal/oxide interfaces, precisely modulating the metal/oxide interfacial interactions in the nanoscale. By controlling the interface and strain effect on catalytic activity, we can achieve high active and stable metal oxide systems for ORR. We would like to describe the details of the above results, for investigating structure-activity relationships in electrocatalytic processes. Only when we start to comprehend the fundamentals behind electrocatalysis on the structure and interface of metal/metal oxide nanocrystals, they can be further advanced to be sustainable in long-term device operation.

Electrodeposition of shaped PtIr alloy nanocrystals with high–index facets for the electro–catalytic oxidation of alcohols

Pt–group metal nanocatalysts enclosed with high–index facets have sufficient kink and step atoms on the surface, which could act as efficient sites for fuel cell reactions. Besides surface structure, alloying was another efficient strategy to obtain nanoparticles with excellent catalytic performance. So how about combining the surface structure of high–index facet with alloying to prepare Pt–based nanocatalysts? In this work, we have synthesized alloyed PtIr tetrahexahedral (THH) nanocrystals (NCs) with high–index facets (730) by electrodeposition. And PtIr trioctahedral (TOH) NCs with high–index facets (441) could be obtained by simply decreasing the upper limit potential (EU) of square–wave potential (SWP) with other condition keeping the same as that for the synthesis of PtIr THH NCs. The successful synthesis of alloyed PtIr NCs with high–index facets mainly lied in the repetitive adsorption/desorption of oxygenated species on the surface of nanoparticles. The prepared PtIr THH NCs exhibited excellent electrocatalytic activity and stability towards ethylene glycol oxidation reaction (EGOR) and glycerol oxidation reaction (GOR). Among the nano–catalysts, the peak in the forward scan of PtIr THH NCs for EGOR and GOR were 19.9 mA∙cm−2 and 17.6 mA∙cm−2, respectively, which were 9.5 and 6.8 times higher than those of commercial Pt/C. The excellent electro–catalytic performance of alloyed PtIr NCs could mainly be attributed to the surface structure effect of high–index facets and synergetic effect of the alloy structure. This work could provide a new idea for designing highly efficient nanocatalysts for fuel cells by combing surface structure effect and alloying effect together.

Strong yet Ductile High Entropy Alloy Derived Nanostructured Cermet.

Cermet is an important composite composed of ceramics and metals, which has widespread engineering applications but is notoriously known for its poor ductility. This work synthesizes freestanding cermet nanosheets through the reaction of high entropy alloy nanocrystals with the molecular structure of poly-vinyl alcohol, which results in a unique nanostructure consisting of metallic nanocrystals surrounded by complex amorphous ceramics. Atomic force microscopy indentation shows that such a high entropy alloy derived cermet is strong and tough at ambient temperature with a superb strength (∼3.2 GPa) and excellent ductility (∼50%), therefore overcoming the long-standing issue of brittleness facing conventional cermets.

Fast detection of ppm n-pentanol by PtAu alloy nanocrystals decorated flower-like WO3

Rapid detection of n-pentanol is of great significance. Herein, we presented an excellent n-pentanol gas sensor based on bimetallic PtAu alloy nanocrystals (NCs) modified flower-like WO3. PtAu NCs with relatively uniform size were synthesized through a reduction method in oleylamine solution. The sensitive materials of flower-like WO3 with different mass percentages of PtAu NCs were prepared by impregnation. WO3 with the loading amount of 0.5 wt% showed a response of 162 (Ra/Rg) for 20 ppm n-pentanol at 200 °C, which was 105 times higher than that for pure WO3. PtAu adding enhanced the response due to the sensitization and their synergistic effect. It is noteworthy that PtAu-modified WO3 exhibited ultrafast response/recovery speed (tres = 1 s, trec = 3 s) to n-pentanol, as far as we know it is the fastest n-pentanol sensor based on semiconductor until now. Flower-like WO3 3D nanostructures allow fast diffusion of gas molecules, resulting in the high rates of gas adsorption and desorption. At the same time, with the help of the bimetal catalysis, the reaction was further promoted. This work can provide a new solution strategy for fast detection and real-time monitoring of n-pentanol in chemical plants.

Facile one-pot preparation and luminescence mechanism of stable Bi3+-doped Cs2AgInCl6 double perovskite nanocrystals

Lead-free alloyed Cs2AgInCl6 nanocrystals (NCs) are currently considered as a new class of lead halide perovskite alternative due to their less toxicity and excellent intrinsic stability. However, their solution preparation process with oleylamine as ligands are easy to the formation of metallic Ag due to the reduction of Ag+, resulting in poor photoluminescence (PL) performance of NCs. Herein, a facile one-pot solution synthesis strategy was proposed to prepare stable Bi3+ doped Cs2AgInCl6 NCs via tri-n-octylphosphine (TOP) as co-ligands, and the emission mechanism was further investigated. The prepared Bi3+ doped Cs2AgInCl6 NCs show broad dual emission bands at around 410 nm and 565 nm, in which the violet emission band is ascribed to the surface defect states, while the yellow emission band of as-synthesized NCs is attributed to the 3P1→1S0 self-trapped excitons recombination of Bi3+ ions. Due to the introduction of BiCl6 states after doping with Bi3+ and well elimination of surface metallic silver by strongly coordinating of TOP, the PL quantum yield of as-synthesized NCs is obviously elevated from 1.25% to 5.10%. The prepared Bi3+ doped Cs2AgInCl6 NCs exhibit excellent storage stability and light stability, making it a promising single emitter fluorescence material for white light-emitting diode devices.

Elucidating growth mechanism and shape evolution of highly branched PtNi alloy nanocrystals and their electrocatalytic performance

Due to the difficulty in controlling thermodynamics and kinetics of the growth for branched bimetallic metal alloy nanocrystals in a solution system, understanding the growth mechanisms of this class of nanostructures is critical for their shape-controlled synthesis. Herein, we report an approach to control over shape of branched PtNi alloy nanocrystals by tuning the molar ratio of Pt to Ni in an oleyamine solution. The detailed analyses confirmed that the morphologies of PtNi alloy nanocrystals evolve from dendrites (less than 5 at% Ni) to multipods (5–10 at% Ni) to star shapes (10–20 at% Ni) and finally to polyhedrons (100 at% Ni) as the nickel content in PtNi alloy nanocrystals increase. During the morphology evolution, the growth mechanism changes from oriented attachment growth to a new mechanism combining oriented attachment and anisotropic overgrowth, then to kinetically controlled overgrowth and finally to thermodynamic control growth. More importantly, we find that the growth of branched PtNi alloy nanocrystals is always along the [220] direction, and the growth direction determines the number of branches in nanocrystals. Electrochemical results show that highly branched PtNi alloy nanocrystals exhibit significantly enhanced electrocatalytic activity and durability compared with commercial Pt catalysts. These in-depth findings will provide new hints to precisely control synthesis of highly branched PtNi nanocrystals.

Stably Immobilizing Sub‐3 nm High‐Entropy Pt Alloy Nanocrystals in Porous Carbon as Durable Oxygen Reduction Electrocatalyst

AbstractImmobilizing ultrafine high‐entropy Pt alloy nanocrystals (HENs) in porous carbon (PC) with strong interface interaction, which is crucial to efficient and durable oxygen reduction reaction (ORR), remains a challenge. In this study, an anchoring‐carbonization strategy for strongly bonding sub‐3 nm HENs in ordered mesoporous carbon is reported. When heating the orderly assembled composites containing hydrophobic organometallic precursors, structure directing agent F127, and hydrophilic resol, F127 is first removed at moderate temperature region, leading to reduced metallic species directly anchored on partially carbonized resol framework. Then along further temperature increase, in situ carbonization of resol around the HENs, enables more effective plane bonding rather than a single point contact of HENs with carbon. It not only enhances the anchor strength, but also inhibits migration and size growth of HENs along the subsequent high‐temperature carbonization. As a result, senary, septenary, octonary, and denary Pt‐based HENs with average nanocrystal sizes of 2.2, 2.6, 2.9, and 2.8 nm, respectively, are successfully imbedded into porous carbon. In electrocatalytic ORR, porous carbon‐supported senary Pt‐based HENs (6‐HENs/PC) exhibit superior activity and durability, representing a promising electrocatalyst. This strategy allows for the preparation of a range of ultrafine HENs strongly loaded on porous carbon for wide applications.

Electrodeposition of alloy PtPd nanocrystals with shape transforming from low–index facet to high–index facet

High–efficient Pt–based electrocatalyst played important role in fuel cell reaction. Shape–controlled synthesis and alloying are two widely used method to optimize electro–catalytic performance of nanoparticles. In this work, we have successfully synthesized shaped PtPd alloy nanocrystals with high–index facet as well as low–index facet via square wave potential (SWP) procedure. The shape transformation from low–index facet to high–index facet could be easily achieved by simply adjusting the parameter of SWP procedure. The periodic adsorption and desorption of hydrogenated or oxygenated species played a key role in the shape–controlled synthesis of PtPd alloy nanocrystals during the process of synthesis. Compared with commercial Pt/C, the prepared PtPd nanocrystals showed great electrocatalytic performance for ethylene glycol oxidation reaction (EGOR). The peak current densities of PtPd convex tetrahexahedral (THH) nanocrystals and PtPd concave nanocubes (NCs) were 21.5 mA ∙ cm−2 and 16.5 mA ∙ cm−2, which were 11.9 and 9.2 times higher than that of commercial Pt/C, respectively. The PtPd alloy nanocrystals also showed much better stability compared with commercial Pt/C. The excellent electro–catalytic performance could attribute to the surface structure and effect of alloying.

One‐Pot Synthesis of Hat‐like PdAu Alloy Open Nanostructures with Improved Oxidase‐like Activities

AbstractControlled assembly of two‐dimensional (2D) noble metal nanocrystals into three‐dimensional (3D) open architecture with reasonable interparticle distance is of great value to solve the re‐stacking issue for practical application, which yet achieves limited progress for bimetallic nanocrystals. Herein, we report the synthesis of hat‐like palladium‐gold (PdAu) alloy nanocrystals with the building block of nanosheet and explored their catalytic properties when serving as oxidase‐like nanoenzyme. The success of current synthesis lies on the co‐reduction of Au and Pd precursors under kinetics‐controlled condition, together with the use of glutathione molecules as the shape‐directing agent. The use of OTAC as capping agent is crucial for the unique morphology, where wire‐like and quasi‐spherical products are obtained, respectively, when the capping agent is replaced by CTAB and CTAC, respectively. Systematic investigations reveal that the oxidase‐like activities of these PdAu nanocrystals are highly morphology‐dependent. In particular, the PdAu alloy nanohats displayed the highest oxidase‐like activity, as well as a wide linear range (100–5000 μM) and low detection limit (25 μM). The present study drives the development of synthetic strategies that allows the construction of 2D bimetallic nanocrystals with a 3D open structure, and validates their improved performance as nanoenzyme towards practical sensing applications, which may find important use in the rational design of biocatalysis, bioassays, and nano‐biomedicine.

A sequential hydrogen-adsorption-assisted bond-weakening strategy for preparing sub-2-nm ordered Pt alloy nanocrystals

<h2>Summary</h2> Synthesis of highly active sub-2 nm Pt-based alloy nanocrystals for oxygen reduction reaction (ORR), especially by a confinement-free route, remains a challenge. Herein, we report a sequential hydrogen-adsorption-assisted bond-weakening strategy. To Pt₃ clusters deposited on Fe–N₅–C surface, due to stronger Pt–H than N–H interaction, H would firstly adsorb on Pt and then on N, which then weakens Pt–Pt and Fe–N bonds, improves atomic mobility, and thus significantly decreases the kinetic barriers for alloying Pt clusters with single Fe species. As a result, Pt alloys can be prepared at a lower temperature, which largely inhibits the size growth and successfully leads to 1.55 nm Pt₃Fe and 1.25 nm Pt₃Ni intermetallic nanocrystals. Under an ultralow cathodic loading of 0.03 mg_Pt cm^-2, the H₂–O₂ fuel cell assembled with Pt₃Ni cathode delivers extraordinary activity (1.60 A cm^-2@0.67 V; 13.7 W mg_Pt^-1 for the whole cell).

Yolk‐Shell AuAgPt Alloy Nanostructures with Tunable Morphologies: Plasmon‐Enhanced Photothermal and Catalytic Properties

Herein, the preparation of AuAgPt alloy yolk‐shell nanostructures with tunable elemental compositions and morphologies by conducting the mild, successive galvanic replacement reaction (GRR) between Au@Ag core‐shell nanocubes and H2PtCl6 is reported. In particular, the resulting products can be featured with well‐defined cubic frames and the distribution of all the three metals over the whole particle, which is enabled by the unique “etching and regrowth” mechanism. Due to the strong absorbance in visible region, the current products exhibit superb performances in two localized surface plasmon resonance (LSPR)‐based applications, photothermal heat conversion, and plasmon‐enhanced catalytic reduction of 4‐nitrophenol (4‐NP). In particular, the optimized photothermal heat conversion efficiency and the apparent rate constant of photocatalytic 4‐NP reduction reach 12.9% and 0.155 min−1, respectively, for the sample AuAgPt‐6 and AuAgPt‐12, respectively. Herein the present study, a feasible strategy for fabrication of plasmonic‐active multimetallic alloy nanocrystals with yolk‐shell structure and cubic frame‐like edges is offered and their promising use in photothermal heat conversion and photocatalysis is validated, which could be extended to other alloys or material combinations for rational design of nanoscale materials with plasmon‐enhanced properties.

Constructing hotspots through star-shaped gold-copper alloy nanocrystals for laser initiation of explosives

Laser initiation technology is an effective way to detonate high energy materials safely and reliably, which has broad application prospect in military blasting. However, there are many obstacles in high-power laser propagating through optical fiber, which directly affect the initiation of high-energy explosives under low-power conditions. Photosensitizer is a necessary substance for initiating low energy and low sensitivity explosives. Herein we developed gold-copper alloys nanocrystals as free-standing laser energy converter material to reduce the laser initiation threshold and delay time. By controlling the nanocrystals morphology, more hotspots were created in the nanocrystals and the ultraviolet–visible-near infrared (UV–VIS-NIR) absorption peak of nanocrystals was regulated to around 808 nm. The laser irradiation result showed that the temperature of the gold-copper alloys nanocrystals was close to 280 ℃ within 10 s (808 nm, 1 W·cm−2). In the practical applications, the cyclotrimethy lenetrinitramine (RDX) initiation threshold can be reduced by 83.47% and B/KNO3 delay time by 43.95% by using gold-copper alloy nanocrystals as laser energy absorber. This result indicates that the gold-copper alloy as an independent laser absorbing material can effectively initiate low-sensitive explosives under low-power conditions.

Promoting the Water-Reduction Kinetics and Alkali Tolerance of MoNi4 Nanocrystals via a Mo2 TiC2 Tx Induced Built-In Electric Field.

Mo-Ni alloy-based electrocatalysts are regarded as promising candidates for the hydrogen evolution reaction (HER), despite their vulnerable stability in alkaline solution that hampers further application. Herein, Mo2 TiC2 Tx MXene, is employed as a support for MoNi4 alloy nanocrystals (NCs) to fabricate a unique nanoflower-like MoNi4 -MXn electrocatalyst. A remarkably strong built-in electric field is established at the interface of two components, which facilitates the electron transfer from Mo2 TiC2 Tx to MoNi4 . Due to the accumulation of electrons at the MoNi4 sites, the adsorption of the catalytic intermediates and ionic species on MoNi4 is affected consequently. As a result, the MoNi4 -MX10 nanohybrid exhibits the lowest overpotential, even lower than 10% Pt/C catalyst at the current density of 10mAcm-2 in 1 m KOH solution (122.19vs 129.07mV, respectively). Furthermore, a lower Tafel slope of 55.88mVdec-1 is reported as compared to that of the 10% Pt/C (65.64mVdec-1 ). Additionally, the MoNi4 -MX10 catalyst also displays extraordinary chemical stability in alkaline solution, with an activity loss of only 0.15% per hour over 300h of operation. This reflects the great potential of using MXene-based interfacial engineering for the synthesis of a highly efficient and stable electrocatalyst.

Tuning the Shape of Gold‐Silver Nanocrystals by Separately Controlling the Metal‐Atom Concentration in a One‐Pot Synthesis

AbstractWe report a niche synthetic strategy to manipulate the shape of gold‐silver (AuAg) nanocrystals based on kinetics control. The success of current work relies on the use of two syringe pumps to add Au and Ag precursor solutions independently at distinct injection rates, which allows the separate control over the feeding rate of Au and Ag atoms, respectively. Such strategy is validated effective in both one‐pot reduction and seeded growth involving spherical and rod‐likes Au seeds. By conducting orthogonal experiments setting two injection rates at three levels, a set of morphologies are obtained. Interestingly, AuAg alloy nanocrystals in the form of multipod, hollow protruded particles, and branched hollow nanoplate, that are rare to obtain via conventional routes, are successfully screened out, exhibiting the unique advantage of this strategy. The current work provides a feasible approach to applying kinetics control over AuAg bimetallic nanocrystals, which could be extended to other noble metals or material combinations.

Synthesis of PtCuFe alloy nanoframes with high-index facets for efficient electro-oxidation of liquid fuels

By controlling the structure, surface and composition of Pt-based ternary alloy nanocrystals, the electro-catalytic performances of Pt-based catalysts can be improved effectively. In this paper, a one-pot method for synthesis of PtCuFe nanoframes (PtCuFe NFs) with tunable structure and defect density is reported. By controlling the heating mode and the type of ligand, then changing the nucleation/growth kinetics, two types of PtCuFe NFs were obtained. These types of catalysts exhibited excellent electro-catalytic performance for methanol electrooxidation (MOR) and formic acid electrooxidation (FAOR). Specifically, the dendritic nanoframes (d-NFs) showed high electro-catalytic activity for MOR and FAOR, which were 8.34 times and 3.68 times higher than commercial Pt/C, respectively. The stability was also greatly improved. The excellent electrocatalytic activity of PtCuFe NFs is attributed to the defect sites of high-index facets and open framework structures. This strategy provides a new idea for improving the electrocatalytic performance of Pt-based catalysts.

Epitaxial Growth of Diamond-Shaped Au1/2Ag1/2CN Nanocrystals on Graphene.

Epitaxial synthesis of inorganic nanomaterials on pristine 2D materials is of interest in the development of nanostructured devices and nanocomposite materials, but is quite difficult because pristine surfaces of 2D materials are chemically inert. Previous studies found a few exceptions including AuCN, AgCN, CuCN, and Cu0.5Au0.5CN, which can be preferentially synthesized and epitaxially aligned onto various 2D materials. Here, we discover that Au1/2Ag1/2CN forms diamond-shaped nanocrystals epitaxially grown on pristine graphene surfaces. The nanocrystals synthesized by a simple drop-casting method are crystallographically aligned to lattice structures of the underlying graphene. Our experimental investigations on 3D structures and the synthesis conditions of the nanocrystals imply that the rhombic 2D geometries originate from different growth rates depending on orientations along and perpendicular to 1D molecular chains of Au1/2Ag1/2CN. We also perform in situ TEM observations showing that Au1/2Ag1/2CN nanocrystals are decomposed to Au and Ag alloy nanocrystals under electron beam irradiation. Our experimental results provide an additional example of 1D cyanide chain families that form ordered nanocrystals epitaxially aligned on 2D materials, and reveal basic physical characteristics of this rarely investigated nanomaterial.

Atmosphere-Induced Transient Structural Transformations of Pd-Cu and Pt-Cu Alloy Nanocrystals.

We have investigated the transformations of colloidal Pd–Cu and Pt–Cu bimetallic alloy nanocrystals (NCs) supported on γ-Al2O3 when exposed to a sequence of oxidizing and then reducing atmospheres, in both cases at high temperature (350 °C). A combination of in situ diffuse reflectance infrared Fourier transform spectroscopy and X-ray absorption spectroscopy was employed to probe the NC surface chemistry and structural/compositional variations in response to the different test conditions. Depending on the type of noble metal in the bimetallic NCs (whether Pd or Pt), different outcomes were observed. The oxidizing treatment on Pd–Cu NCs led to the formation of a PdCuO mixed oxide and PdO along with a minor fraction of CuOx species on the support. The same treatment on Pt–Cu NCs caused a complete dealloying between Pt and Cu, forming separate Pt NCs with a minor fraction of PtO NCs and CuOx species, the latter finely dispersed on the support. The reducing treatment that followed the oxidizing treatment largely restored the Pd–Cu alloy NCs, although with a residual fraction of CuOx species remaining. Similarly, Pt–Cu NCs were partially restored but with a large fraction of CuOx species still located on the support. Our results indicate that the noble metal present in the bimetallic Cu-based alloy NCs has a strong influence on the dealloying/migrations/realloying processes occurring under typical heterogeneous catalytic reactions, elucidating the structural/compositional variations of these NCs depending on the atmospheres to which they are exposed.

The best of both worlds in material synthesis to understand metal-support interactions

The best of both worlds in material synthesis to understand metal-support interactions

Composition adjustment of PtCoCu alloy assemblies towards improved hydrogen evolution and methanol oxidation reaction

Binary PtCo, PtCu and ternary PtCoCu alloys with different surface element distributions were prepared to study the effect of composition on electrocatalytic hydrogen evolution reaction (HER) and methanol oxidation reaction (MOR). A one-pot method was used to synthesize self-assembled alloy nanocrystals in one step. By adjusting the composition of synthesized alloys, their properties were optimized. The changes of morphology and property of the alloys after adding Cu were discussed. The HER and MOR properties of alloys were affected by the composition. Especially, the ternary alloy revealed better properties than that of the binary alloy. The starting potential of PtCoCu alloy in HER reaction under visible light is 14 mV and the overpotential under 10 mA cm−1 is 44 mV, which revealed excellent cycle stability. The current density at 0.7 V for MOR reaction can reach 4.2 mA cm−1. This is ascribed to the composition of alloy. Cu in alloys resulted in plasma-induced effective electron transfer. This result provides a feasible strategy for the design of effective multifunctional electrocatalysts.