Scalable synthesis of nanoporous boron for high efficiency ammonia electrosynthesis

Three-dimensional bicontinuous nanoporosity fabricated by dealloying can provide unique chemical properties in catalytic materials, which conventional nanoparticulate catalysts do not have. Although many solid elements in the periodic table have been fabricated as nanoporous materials by dealloying, technically important nanoporous boron has not been realized because of the poor diffusivity and high chemical stability of boron. Here we report a scalable top–down method to produce three-dimensional nanoporous boron by selectively leaching a less stable metal compound phase from rapidly solidified two-phase metal–boron alloys. The metalloid boron phase with relatively high chemical stability remains as the skeleton of a nanoporous structure. The resultant nanoporous boron with tunable pore sizes, and porosities, shows superior catalytic activities towards ammonia electrosynthesis. This work provides a new approach to fabricate nanoporous metalloids for a wide range of functional applications and brings boron, an important functional material, to the family of dealloyed nanoporous materials.     

Dealloying by metallic melt

Dealloying, which commonly involves corrosion processes in aqueous solutions, is a promising technique for preparing functional nanoporous metals. While this technique is ideal for preparing nanoporous noble metals such as of Au, it is not readily applicable to less-noble metals. Here, we propose a novel dealloying method employing a metallic melt, instead of an aqueous solution, as the dealloying liquid for a preparing of nanoporous metals. An atomic interaction among alloy components and metallic melt causes specific component to dissolve out from the alloy solid into the melt with self-organizing nanoporous structure by the remaining component. The dealloying method can be applied for preparation of nanoporous less-noble metal such as of Ti for the development of functional materials such as fluid filters, gas absorption media, and biomaterials.     

Hierarchical Nanoporous Copper Architectures via 3D Printing Technique for Highly Efficient Catalysts

Nanoporous metals represent a class of functional materials with unique bicontinuous open porous structural properties, making them ideal candidates for various catalyst applications. However, the pursuit of nanoporous properties, extremely small pores, and high surface area, results in the restriction of mass transport. Herein, a free‐standing hierarchical nanoporous Cu material, prepared by a selective laser melting 3D printing technique and a one‐step dealloying process, is presented as a highly efficient electrocatalyst for methanol oxidation. It is demonstrated that the digitally controlled hierarchical structure with macro‐ and nano‐scaled pores can be utilized for promoting and directing mass transport as well as for the enhancement of catalytic properties. This work highlights a facile, low‐cost, and alternative strategy for hierarchical nanoporous structure design that can be applied to binary, ternary, and quaternary metal alloys for various functional applications.     

General Synthesis of Nanoporous 2D Metal Compounds with 3D Bicontinous Structure

Although 2D layered metal compounds are widely exploited using various techniques such as exfoliation and vapor-phase-assisted growth, it is still challenging to construct the 2D materials in a 3D configuration with preservation of the unique physicochemical properties of the metal compounds. Herein, a general synthetic strategy is reported for a wide variety of 2D (atomic-scale thickness) metal compounds with 3D bicontinous nanoporous structure. 19 binary compounds including sulfides, selenides, tellurides, carbides, and nitrides, and five alloyed compounds, are successfully prepared via a surface alloy strategy, which are readily created by using a recyclable nanoporous gold assisted chemical vapor deposition process. These 3D nanoporous metal compounds with preserved 2D physicochemical properties, tunable pore sizes, and compositions for electrocatalytic applications, show excellent catalytic performance in the electrochemical N₂ reduction reaction. This work opens up a promising avenue for fundamental studies and potential applications of a wide variety of nanoporous metal compounds.     

Morphology,Oxidation, and Mechanical Behavior of Nanoporous Cu Foams

Nanoporous copper foams (65–80% porosity) are synthesized by dealloying different copper alloys, including Cu₂₀Zn₈₀, Cu₃₅Zn₆₅, and Cu₃₀Al₇₀. Ligament sizes, porosity, and oxide content are examined by scanning electron microscopy, Raman spectroscopy, and X‐ray powder diffraction to determine the nanoporous foam's structure and thermal stability. The pores and ligaments of the copper foams can range in size from 35 to 220 nm. Both parameters can be controlled by either the dealloying process or a subsequent heat treatment. The results show no copper oxide peaks for the as‐prepared samples; however, any heat treatments above 200 °C lead to the formation of Cu oxide (CuO and Cu₂O). These foams are shown to retain their structural integrity even after oxidation. A novel method is thus developed for synthesizing nanoporous Cu oxide foams by heat treating nanoporous copper.     

Tuning Surface Structure of 3D Nanoporous Gold by Surfactant‐Free Electrochemical Potential Cycling

3D dealloyed nanoporous metals have emerged as a new class of catalysts for various chemical and electrochemical reactions. Similar to other heterogeneous catalysts, the surface atomic structure of the nanoporous metal catalysts plays a crucial role in catalytic activity and selectivity. Through surfactant‐assisted bottom‐up synthesis, the surface‐structure modification has been successfully realized in low‐dimensional particulate catalysts. However, the surface modification by top‐down dealloying has not been well explored for nanoporous metal catalysts. Here, a surfactant‐free approach to tailor the surface structure of nanoporous gold by surface relaxation via electrochemical redox cycling is reported. By controlling the scan rates, nanoporous gold with abundant {111} facets or {100} facets can be designed and fabricated with dramatically improved electrocatalysis toward the ethanol oxidation reaction.     

去合金化制备纳米多孔金属材料的研究进展

用去合金化制备的孔隙尺寸小于100nm的纳米多孔金属材料,开拓了多孔金属材料一个新的应用领域.目前的研究主要集中于通过不同的合金体系制备出不同的纳米多孔金属,分别介绍了纳米多孔金、铂、铜、钯、钛的制备工艺,并对孔洞形成的溶解-再沉积机制、体扩散机制、表面扩散机制、渗流机制及相分离模型进行了简述.对纳米多孔金的现有研究表明,纳米多孔金具有良好的化学稳定性、高的比表面积以及高的屈服强度,目前应用研究包括作为热交换器、传感器及催化材料等方面.     

Facile fabrication of ultrathin freestanding nanoporous Cu and Cu-Ag films with high SERS sensitivity by dealloying Mg-Cu(Ag)-Gd metallic glasses

Nanoporous metals prepared by dealloying have attracted increasing attention due to their interesting size-dependent physical,chemical,and biological properties.However,facile fabrication of metallic ultra-thin freestanding nanoporous films(UF-NPFs)by dealloying is still challenging.Herein,we report a novel strategy of facile preparation of flexible Cu,Cu3Ag,and CuAg UF-NPFs by dealloying thick Mg-Cu(Ag)-Gd metallic glass ribbons.During dealloying,the local reaction latent heat-induced glass transition of the precursor ribbons leads to the formation of a solid/liquid interface between the initially dealloyed nanoporous layer and the underlying supercooled liquid layer.Due to the bulging effect of in situ gen-erated H2 on the solid/liquid interface,Cu,Cu3Ag,and CuAg UF-NPFs with thicknesses of～200 nm can self-peel off from the outer surface of the dealloying ribbons.Moreover,it was found that the surface-enhanced Raman scattering(SERS)detection limit of Rhodamine 6G(R6G)on the Cu and CuAg UF-NPF substrates are 10-6 M and 10-11 M,respectively,which are lower than most of the Cu and Cu-Ag sub-strates prepared by other methods.This work presents a reliable simple strategy to synthesize a variety of cost effective and flexible metallic UF-NPFs for functional applications.     

General synthesis of sponge-like ultrafine nanoporous metals by dealloying in citric acid

A general method is proposed to synthesize ultrafine nanoporous Cu,Ag,and Ni with novel sponge-like morphologies,high porosities,and large surface areas.The materials are produced by dealloying Mg65M25Y10 (M =Cu,Ag,and Ni) metallic glasses in citric acid.Citric acid played a key role due to its capping effect,which reduced the surface diffusion of metals.A structural model consistent with the sponge-like morphology was constructed to calculate the porosity and the surface area.The mechanism of the dealloying process in dtric add,involving ligament formation and coarsening,was illustrated.The mechanism was capable of explaining the experimental trends of dealloying,especially the morphology.A glucose sensor,which can be further developed into a high-precision real-time glucose monitor for medical use,was constructed using sponge-like nanoporous copper.Our findings are not only relevant to understanding the dealloying mechanism of metallic glasses,but also provide promising materials for multiple applications.     

Nanoporous copper sonocatalysts prepared by dealloying Cu–Al–Ti amorphous ribbons

ABSTRACT

Nanoporous copper (NPC) with a controllable ligament width was prepared by chemically dealloying Cu₄₅Al₄₅Ti₁₀ amorphous ribbons in dilute HCl solution. X-ray diffraction and scanning electron microscopy analysis confirmed the 3D, bicontinuous, nanoporous structure constituting fcc-Cu ligaments of 39–79?nm thickness. The coarsening of NPC ligaments increased significantly with an increase of HCl concentration, reaction temperature or time. The surface diffusivity and activation energy of NPC were also calculated. Finally, the catalytic activity of NPC was validated with the ultrasound and H₂O₂-assisted degradation of methyl orange (MO), where 99% MO was degraded within 15?min.     

Polyvinylpyrrolidone macromolecules function as a diffusion barrier during dealloying

The inhibition effect of polyvinylprrolidone (PVP) during dealloying on the formation of nanoporous Cu from a Ti₆₀Cu₄₀ amorphous alloy in hydrofluoric acids (HF) was investigated. A bicontinuous nanoporous Cu structure formed on Ti₆₀Cu₄₀ after dealloying. The pore size of nanoporous Cu formed in HF solution was 71 nm, but this decreased to 12 nm and 11 nm after dealloying for the same period of dealloying time of 43.2 ks when 0.01 M and 0.03 M PVP, respectively, was added into the 0.03 M HF base solution. The surface diffusivity was estimated to decrease from 2.5 × 10⁻¹⁸ m² s⁻¹ in 0.03 M HF solution to 1.84 × 10⁻²¹ m² s⁻¹ when 0.01 M PVP was added, and to 1.42 × 10⁻²¹ m² s⁻¹ when 0.03 M PVP was added. More PVP macromolecules were adsorbed onto the nanoporous Cu surface in the 0.03 M HF solution with the addition of 0.03 M PVP than when 0.01 M PVP was added to the solution, which resulted in the formation of smaller nanopores. The suppressed diffusion of Cu adatoms due to the PVP macromolecule resulted in the formation of finer Cu ligaments than that formed in 0.03 M HF solution. This long chain organic molecule was shown to act as a diffusion barrier for the diffusion of metal adatoms during dealloying and to elaborate the nanoporous structure.     

Preparation and characterization of nanoporous Cu6Sn5/Cu composite by chemical dealloying of Al–Cu–Sn ternary alloy

In this article, a new ternary Al–Cu–Sn alloy system has been exploited to fabricate nanoporous Cu₆Sn₅/Cu composite slices through chemical dealloying in a 20 wt% NaOH solution at an elevated temperature. The microstructure of the sliced nanoporous Cu₆Sn₅/Cu composite was characterized using x-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and transmission electron microscopy. The experimental results show that multi-phase precursor alloy comprises α-Al, Sn, and θ-Al₂Cu phases. The new phase Cu₆Sn₅ emerges through dealloying, and the as-dealloyed samples have three-dimensional (3D) structure composed of large-sized channels (hundreds of nanometers) and small-sized channels (tens of nanometers). Both the large- and small-sized pores are 3D, open and bicontinuous. The synergetic dealloying of α-Al and θ-Al₂Cu in the three-phase Al–Cu–Sn alloy and fast surface diffusion of Cu atoms and Sn atoms result in the formation of Cu₆Sn₅/Cu composite with bimodal channel size distributions. In addition, the dealloying duration plays a significant role in the formation of Cu₆Sn₅ and the length scales of the small-sized ligament/channels at a settled temperature.     

纳米多孔金属力学性能的若干研究进展

纳米多孔金属是近年来发展起来的一类具有纳米级双连续孔洞和高表面积的新型功能材料,具备如化学性能、力学性能、表面拉曼散射性能等多方面的优异特性,在催化、传感、新能源、生物医学等诸多领域拥有广阔的应用前景。围绕纳米多孔金属的制备、力学性能和尺度特性等,展开细述了相关的研究工作,并重点针对力学性能方面的研究进展,如尺度方程、破坏机理、表面效应和表面应力,以及脱合金制备方法和制备过程中的力学问题进行了讨论,并对将来的研究方向进行了展望。     

Progress in the Physisorption Characterization of Nanoporous Gas Storage Materials

Assessing the adsorption properties of nanoporous materials and determining their structural characterization is critical for progressing the use of such materials for many applications, including gas storage. Gas adsorption can be used for this characterization because it assesses a broad range of pore sizes, from micropore to mesopore. In the past 20 years, key developments have been achieved both in the knowledge of the adsorption and phase behavior of fluids in ordered nanoporous materials and in the creation and advancement of state-of-the-art approaches based on statistical mechanics, such as molecular simulation and density functional theory. Together with high-resolution experimental procedures for the adsorption of subcritical and supercritical fluids, this has led to significant advances in physical adsorption textural characterization. In this short, selective review paper, we discuss a few important and central features of the underlying adsorption mechanisms of fluids in a variety of nanoporous materials with well-defined pore structure. The significance of these features for advancing physical adsorption characterization and gas storage applications is also discussed.     

A two-step method for synthesis of micron sized nanoporous silver powder and ZnO nanoparticles

Micron-sized nanoporous silver powder with pore size of ～100–160 nm and specific surface area of ～4.7–5.5 m²/g was synthesized from three mechanically alloyed Ag-Zn powders (composition: 25, 50 and 75 at.% Zn). Dealloying was carried out at free corrosion conditions in NaOH, HCl and AgNO₃ solutions. Both partial and complete dealloying were obtained by suitable choice of electrolyte and time of exposure. Zn in the solution after dealloying was recovered in the form of ZnO nanoparticles with particle size of 55.7 ± 18 nm. The effect of composition and electrolyte on the degree of dealloying was also studied.     

纳米多孔Pd-Cu/Pd-Ag催化剂的制备及其电催化性能EI北大核心CSCD

针对直接甲醇燃料电池(direct methanol fuel cell,DMFE)对高效阳极催化剂的需求,设计研发Ca-Mg-Pd-M(M=Cu,Ag)非晶合金前躯体体系,并采用去合金化制备系带-孔道双连续结构的纳米多孔Pd-Cu/Pd-Ag合金。通过设计前驱体合金比例可调节多孔结构的元素比例和尺寸,Pd元素可与Cu,Ag元素形成连续固溶体,在去合金化过程中可以降低Cu,Ag元素的扩散,进而细化纳米多孔的系带尺寸(由100 nm减小到10 nm)。相较于纳米多孔Pd,纳米多孔Pd-Cu/Pd-Ag合金表现出更优异的甲醇催化活性(催化电流强度:45 mA/mg)和抗毒化能力(J f/J b值为1.56),还具有低成本的优点,在直接甲醇燃料电池阳极催化剂方面有着良好的应用前景。     

Fabrication of nanoporous silver and microstructural change during dealloying of melt-spun Al–20 at.%Ag in hydrochloric acid

Ag nanoporous structure has been fabricated by dealloying of Al–20 at.%Ag in 2 M HCl at room temperature. We describe the changes of phase constitution and microstructural morphology during the dealloying. The bi-phase structure of α-Al and γ-Ag₂Al was obtained using melt-spinning, and considerable amount of γ-Ag₂Al was suppressed. According to the phase quantitative analysis, 5.6 % γ-Ag₂Al was precipitated at the grain boundary and inside the grain. At the early stage of dealloying process, the deep channel occurred at the interface of α-Al and γ-Ag₂Al due to concentration gradient of Ag; and then, α-Al was preferentially dealloyed with comparison to γ-Ag₂Al and formed 3-dimensional nanoporous structure which showed the open-pore/ligament bi-continuous structure. After 720 min, γ-Ag₂Al was dealloyed and formed 2-dimensional pored-wall structure. At the end of dealloying reaction, the initial grain/intermetallic wall structure was maintained. The Ag nanoporous structure was gradually coarsened with the increase of dealloying time, from 25 ± 5 nm at 20 min to 113 ± 22 nm at 720 min. The calculated coefficient of surface diffusion of Ag was 2.34 × 10^?17 m²/s. The surface diffusivity of Ag obtained in this study showed two orders of magnitude higher than the Au dealloyed from Au–Ag precursor at the similar temperature.     

Nanoporous Cu wide ribbons with good mechanical integrity

In this work we report the fabrication of nanoporous Cu wide ribbon samples by electrochemical dealloying the minor Si alloyed γ-Cu₃₀Mn₇₀ solid solution alloys. The γ-Cu₃₀Mn₇₀ structure was found to be capable of trapping a minor amount of Si as solute by liquid quenching. Ribbon samples of 10 mm wide with tunable thickness were made at the Cu₂₈Mn₇₀Si₂ composition. The γ-Cu₂₈Mn₇₀Si₂ alloy can be turned into wide ribbon nanoporous Cu with the pore size of ～30–50 nm and with good mechanical integrity. A thin layer of Si-enriched glue structure with ～50 nm thickness was formed continuously along the grain boundaries to play the role of reinforcement for the dealloyed structure.     

Dealloying of Au-Ag thin films with a composition gradient: Influence on morphology of nanoporous Au

Nanoporous Au can be formed by dealloying Au-Ag alloys and, depending on the initial alloy composition, produce a variety of microstructural features. We investigated a wide compositional range and found three regimes, based on initial Au content, that yield varying degrees of dealloying in thin films. Between 22 and 26 at.% Au, dealloying produces nearly pure Au with an open nanoporous structure. However, above 36 at.% Au, only the grain boundaries dealloy, leaving islands of retained Au-Ag alloy. For intermediate compositions, a transitional microstructure results, with final Ag concentration ranging from 50 at.% down to 4 at.%. Film cracking was observed after dealloying, and lower initial Au content correlated with a higher degree of cracking and a higher pore fraction.     

Large pore size nanoporous materials from the self-assembly of asymmetric bottlebrush block copolymers

Asymmetric polystyrene-polylactide (PS-PLA) bottlebrush block copolymers have been shown to self-assemble into a cylindrical morphology with large domain spacings. PLA cylinders can be selectively etched out of the shear-aligned polymer monoliths to generate nanoporous materials with an average cylindrical pore diameter of 55 nm. The remaining bottlebrush backbone provides a functional, hydrophilic coating inside the nanopores. This methodology significantly expands the range of pore sizes attainable in block copolymer based nanoporous materials.