Ligament Size Research Articles

Synthesis, microstructure, and mechanical properties of polycrystalline Cu nano-foam

AbstractA polycrystalline Cu foam with sub-micron ligament sizes was formed by creating a non-woven fabric via electrospinning with a homogeneous mixture of polyvinyl alcohol(PVA)-and copper acetate(Cu(Ac)2). Thermogravimetric measurement of the electrospun fabric of the precursor solution is reported. Oxidizing the precursor fabric at 773K formed an oxide nano-foam; subsequent heating at 573K with a reducing gas transformed the CuO nano-foam to Cu with a similar ligament and meso-scale pore size morphology. A cross-section prepared by focused ion beam lift-out shows the polycrystalline structure with multi-scale porosity. The mechanical property of the Cu nano-foam is measured by nano-indentation. The load-depth curves and deduced mechanical properties suggest that additional intra-ligament pores lead to unique structure-property relations in this non-conventional form of metal.

Nanoporous gold synthesized by plasma-assisted inert gas condensation: room temperature sintering, nanoscale mechanical properties and stability against high energy electron irradiation

With a plasma assisted gas condensation system it is possible to achieve high-purity nanoporous Au (np-Au) structures with minimal contaminations and impurities. The structures consist of single Au-nanoparticles, which partially sintered together due to their high surface to volume ratio. Through electron microscopy investigations a porosity >50% with ligament sizes between 20–30 nm was revealed. The elastic modulus of the np-Au was determined via peak force quantitative nanomechanical mapping and resulted in values of 7.5 ± 1.5 GPa. The presented structures partially sintered at room temperature, but proved to be stable to electron irradiation with energies of 7 MeV up to doses of 100 MGy. The electron irradiation stability opens the venue for electron assisted functionalization with biomolecules.

Ligament size-dependent electrocatalytic activity of nanoporous Ag network for CO2 reduction.

Electrochemical CO2 reduction (ECR) depends significantly on the nanostructures of electrocatalysts. Here we show a nanoporous Ag network catalyst (np-Ag) for efficient electrochemical reduction of CO2. The np-Ag samples with an average ligament size of 21 nm (denoted by np-Ag (21 nm)) and 87 nm (denoted by np-Ag (87 nm)) were fabricated by dealloying the rapidly solidified Mg80Ag20 (wt%) alloy ribbons in 1 wt% citric acid and 5 wt% phosphoric acid, respectively. The ligament size effect on the electrocatalytic activity and selectivity of CO2 conversion into CO is investigated. When catalysing CO2 reduction in 0.1 M KHCO3, the np-Ag (21 nm) catalyst exhibits a significantly enhanced selectivity with a faradaic efficiency for CO formation of 85.0% at -0.8 V versus RHE, about two times that (41.2%) over the np-Ag (87 nm). Additionally, a superior catalytic activity is also achieved over the np-Ag (21 nm), with a >2.5-fold increase in the CO partial current density relative to the np-Ag (87 nm). The improved selectivity and activity of np-Ag (21 nm) are attributed to the enhanced electrochemical surface area, higher local pH derived from ligament size effect, as well as more defect sites (i.e., grain boundaries) in ligaments.

Mechanical response of nanoporous metals: A story of size, surface stress, and severed struts

Abstract

Independent control over residual silver content of nanoporous gold by galvanodynamically controlled dealloying.

A new procedure was developed and characterized for the galvanodynamically controlled dealloying (GCD) of AuxAg100-x alloys to obtain nanoporous gold (npAu) mainly as an unsupported catalyst material for partial oxidation of alcohols. Such catalysts require residual Ag content of less than 1 at%. GCD was compared to the preparation of npAu by potentiostatically controlled deallyoing (PCD) and free corrosion (FC). The main advantage of GCD is the ability to obtain npAu with a predetermined residual Ag content including residual Ag contents below 1 at% while retarding the coarsening of the ligaments. For PCD and FC, there is a strong increase of ligament size with decreasing residual Ag content because the longer times required for dealloying unavoidably lead to coarsening of the npAu structure. On the other hand, GCD also prevented too high initial current density that leads to cracking of the samples and prevents formation of mechanically stable monoliths. GCD tolerates different compositions of the starting alloy for AuxAg100-x within the tested composition range (20 at% ≤ xAu ≤ 30 at%). The samples obtained by GCD were tested for methanol and ethanol oxidation and showed favorable characteristics for partial oxidation of methanol to methyl formate and of ethanol to ethyl acetate.

X-ray studies of nanoporous gold: Powder diffraction by large crystals with small holes

X-ray diffraction studies of nanoporous gold face the poorly understood diffraction scenario where large coherent crystals are riddled with nanoscale holes. Theoretical considerations derived in this study show that the ligament size of the porous network influences the scattering despite being quasi single crystalline. Virtual diffraction of artificially generated samples confirms the results but also shows a loss of long-range coherency and the appearance of microstrain due to thermal relaxation. Subsequently, a large set of laboratory X-ray investigations of nanoporous gold fabricated by different approaches and synthesis parameters reveal a clear correlation between ligament size and size of the coherent scattering domains as well as extremely high microstrains in samples with ligament sizes below 10 nm.

Relationship between dealloying conditions and coarsening behaviors of nanoporous copper fabricated by dealloying Cu-Ce metallic glasses

Monolithic nanoporous copper (NPC) with tunable ligament size (107–438nm) was synthesized by dealloying a new Cu-Ce binary glassy precursor in dilute H2SO4 aqueous solution. The effects of the dealloying conditions on the morphologies of NPC were evaluated comprehensively. The results show that the ligaments of NPC can significantly coarsen with the increase of acid concentration, elevation of reaction temperature or prolongation of immersion time. These coarsening behaviors can be well described by a diffusion based growth kinetic model. Moreover, the surface diffusivity and activation energy for diffusion of Cu atoms were also estimated to investigate the formation mechanism of NPC, which is mainly governed by dissolution of Ce element in the glassy precursor coupled with nucleation and growth of Cu clusters via the precursor/solution interface. In the experiment of the degradation of methyl orange (MO) dye, the NPC fabricated by Cu-Ce metallic glasses exhibits superior sono-catalytic activity.

Preparation and Characterization of Nanoporous Copper Films by Chemical Dealloying

Nanoporous Cu (np-Cu) materials can be easily prepared by chemical dealloying of Zn-Cu alloys. Tuning the surface area, the number of defects, the content of the less noble metal, and the mass transport properties of the nanoporous materials allows us to control its chemical, physical and catalytic properties as well as its mechanical properties for different applications. We present how the formation of the np-Cu films is controlled by the annealing temperature and kind of electrolyte. The Zn-Cu alloy films were prepared by electrodeposition of Zn onto a Cu surface. After the thermal annealing at three different temperatures, the Zn-Cu alloy films were dealloyed in acidic and alkaline media. Based on our results obtained from SEM, EDX, XPS and XRD, we correlate the structural changes (ligament size, pore size, composition) with the experimental dealloying conditions (annealing temperature, kind of electrolyte) to better understand the dealloying processes for the np-Cu films.

Nanoporous Au foams: Variation of effective Young's modulus with ligament size

This work focuses on the effects of ligament size on the mechanical behavior of NP Au foams. NP Au structures with ligament diameter between 5 and 200nm have been fabricated by selective Ag dissolution from Ag70Au30 alloys prepared by mechanical alloying and subsequent annealing. Nanoindentation experiments indicate that the effective Young's modulus increases as the ligament thickness decreases. The stiffening of NP Au foams with fine ligaments is ascribed to size effects connected with surface stress states. It is shown that a phenomenological model accounting for disorder and surface contributions to Young's modulus correctly predicts the observed behavior.

Thermodynamic and kinetic investigation for peroxide hydrogen reduction reaction on nanoporous electrocatalysts

Owing to the unique three-dimensional bi-continuous (pore and ligament) structure of nanoporous metals, active sites and electron conduction are both promoted for electrocatalytic reactions. However, the narrowing pore sizes for increasing surface areas provide more active sites but hindered mass diffusion. Therefore, there are optimal values for the ligament and pore size of nanoporous metal electrocatalysts. As an outstanding electrocatalyst towards hydrogen peroxide reduction reaction (HPRR), the structure features of nanoporous gold (NPG) were investigated in detail by using electrochemical techniques, e.g., rotating disk electrode (RDE) in this paper. The experimental results demonstrated that residual silver in NPG did not afford a positive effect for HPRR although exhibiting good catalytic activity alone. Furthermore, by comparing the thermodynamic and kinetic properties of NPG samples with various ligament/pore sizes, NPG electrocatalyst with pore size of ~ 30 nm exhibited an optimal catalytic activity and stability for HPRR.

The Fabrication and Characterization of Bimodal Nanoporous Si with Retained Mg through Dealloying

The fabrication and characterization of bimodal nanoporous silicon films with retained magnesium, achieved through a novel approach utilizing free corrosion dealloying of precursor Si–Mg films in distilled water, is studied. Investigation of film structure and chemical composition using various techniques reveals important characteristics potentially relevant to lithium‐ion battery applications. Dealloying of precursor films results in a hierarchal structure, where larger ligaments have an average width of 83 nm and smaller ligaments an average width of 19 nm. A thin, porous surface layer is present on most dealloyed films and is largely composed of magnesium and silicon oxides, as verified by XPS surface analysis. TEM studies reveal that as‐dealloyed films are amorphous, but nanocrystalline silicon grains form after vacuum annealing at 500 °C. EDS mapping and XPS reveal three distinct chemical composition regions through the film thickness, where residual magnesium generally increases as a function of film thickness, with the highest amount of retained magnesium at the surface. The ligament size, composition, and structure, combined with the simple, non‐hazardous nature of the dealloying method, make this an attractive material and processing technique for efficient and scalable production of lithium‐ion battery anode material.

Effect of nodal mass on macroscopic mechanical properties of nanoporous metals

The current work investigates the effect of the nodal mass on the macroscopic mechanical behavior of nanoporous metals using the Finite Element Method. A nodal corrected beam modeling concept is introduced that allows local incorporation of the effective elastoplastic mechanical behavior of the nodal mass in the nodal area of a representative volume element (RVE). The calibration to the corresponding Finite Element solid model is achieved by integrating additional geometry and material parameters to the so-called nodal areas in the beam model. With this technique an excellent prediction can be achieved over a large range of deformation for different types of RVEs. From the results of the nodal corrected beam model, modified leading constants are determined in the scaling laws for Young's modulus and yield strength. The effect of the nodal correction is also studied with respect to various randomization levels. Finally, the ligament size dependent strength is analyzed by applying the proposed model to experimental data. It could be shown that the nodal correction improves the overall agreement with literature data, particularly for such data points that are related to samples with a high solid fraction.

Scaling laws and mechanical properties of nanoporous copper

AbstractThrough molecular dynamics simulations, the mechanical behavior of nanoporous copper under impact loading was investigated with relative densities ranging from 77.91% to 98.36%, focusing on deformation mechanism, the scaling laws and influence of ligament sizes. Results show that the classical Gibson-Ashby's scaling laws should be modified for prediction of both the Young's modulus and yield stress. A proportional relationship is established between cell wall thickness and yield stress, and new modified scaling equations are built for nanoporous copper with consideration on both relative mass density and size effects of ligaments. The size effect can be explained by larger surface area/volume ratio of samples with thinner ligament size and limited dislocation source activation due to narrow space between larger numbers of voids.

Simulation of the liquid break-up at an AdBlue injector with the volume-of-fluid method followed by off-line coupled Lagrangian particle tracking

Numerical simulation of the liquid break-up and prediction of the droplet size distribution of injection devices is a challenging task. For predicting the disintegration process, interface capturing techniques, e.g. the volume-of-fluid method, are mandatory. The extensive computational effort caused by the high resolution requirements in space and time determine the limitation for many applications. Validations of such simulations with experimental data can be only rarely found in the literature. This study presents a comprehensive numerical and experimental investigation of the break-up process at an industrial low-pressure three-hole injector. The numerical simulations for three different injection pressures are performed with an efficient volume-of-fluid approach. At certain time instants the volume fraction field is scanned for coherent liquid structures, and the ligament size distribution is evaluated. The simulated mean droplet diameters and the droplet size distribution functions show a good agreement with the performed measurements. The high computational effort, caused by the fine mesh resolution, limits the axial extension of the simulated domain, here fifty times the nozzle diameter. In order to extend the simulated spray domain, an off-line coupling approach with a Lagrangian particle method is proposed. Therefore, the detached droplets from the volume-of-fluid simulation are used as initial conditions for the subsequent Lagrangian spray simulation on a coarser computational grid. This presented second step determines the link between liquid disintegration on a micro-scale, and spray simulation on a macro-scale. The aim of this study was to develop a predictive simulation tool for determination of the injector characteristics.

Indentation size effect for spherical nanoindentation on nanoporous gold

We find that the indentation size effect (ISE) occurs during spherical indentation on nanoporous gold (np-Au). The hardness increases as the indenter radius decreases at a fixed representative strain. We prepare np-Au samples with a ligament size of 26nm by free-corrosion dealloying and perform multiple spherical nanoindentations with nominal indenter radii of 4, 12, and 50μm. A nanomechanics model for the ISE during spherical indentation is developed, and the model accurately describes the hardness depending on the indenter radius. We also correlate the ISE trends of spherical and Berkovich indentations along with their differences in terms of the representative strain and the indentation work.

Preparation and Characterization of Nanoporous Copper Films By (electro)Chemical Dealloying

Nanostructured materials are of great interest in different fields such as solar cells, batteries, fuel cells and sensors.1-3 The chemical and physical properties of nanostructured materials strongly differ from those of the corresponding bulk materials. For instance, with decreasing size, quantum-physical effects get more relevant.2 Nanoporous materials offer not only new chemical and physical properties by nano-sizing but also very high surface area and good mass transport properties through the pores.3 A promising synthetic approach to prepare nanoporous copper (np-Cu) materials is the (electro)chemical dealloying process.4 Dealloying is the dissolution of the less noble metal (Zn) from a Cu-based alloy. The Cu-based alloys are prepared by electrodeposition of Zn on a Cu surface followed by thermal annealing step in an inert atmosphere. We showed that by modifying the dealloying conditions, np-Cu films with tuneable structure (e.g. ligament size and shape) and chemical composition (e.g. the residual content of the foreign atom and surface oxides) can be produced. For the characterization, a broad combination of microscopic and spectroscopic techniques was used: XPS, XRD, EDX, SEM and XANES/EXAFS. Our study shows the relationship between the dealloying parameters such as temperature, electrolyte, concentration etc. and the material properties of the np-Cu such as ligament size and shape, pore distribution and content of the foreign atom. First, the annealing temperature controls the crystal phase and the degree of alloying of the Zn-Cu film. The crystalline phase and chemical surface composition of the master alloy have a strong influence on the resulting structure after the dealloying step. In particular, with higher annealing temperature, the dealloying proceeds slower and the ligament-pore size ratio increases. Furthermore, the variation of the dealloying conditions (electrolyte concentration and time) leads to different structures of the np-Cu films, indicating that the mechanism and kinetics of the dealloying significantly change. One of the critical parameters is the effect of the supporting electrolyte. For instance, in HCl, a homogeneous ligament structure is formed, whereas in NaOH solution the dealloying mainly proceeds at the grain boundaries of the Zn-Cu alloy and forms cracks. Large ligament sizes and strong oxidized surfaces with very low content of Zn was observed. Moreover, the temperature during the dealloying can be used to tune the ligament and pore size. For example, using a HCl solution with the same concentration, the ligament size can be adjusted from 30 to 300 nm in a temperature range between 10 and 70 °C. Another observation is that by increasing dealloying time, the Zn content further decreases. In basic solutions, the absence of Zn leads to an increased formation of copper oxide at the surface. The aim of our work is to better understand the formation mechanism and kinetics of the np-Cu. The next step is the rational design of various np-Cu materials as electrocatalysts for the electrochemical carbon dioxide reduction reaction (CO2RR). 1. A. S. Aricò, P. Bruce, B. Scrosati, J. Tarascon, and W. van Schalkwijk, Nature Materials, 366-377, 4 (2005) 2. W. A. Badawy, Journal of Advanced Research, 123-132, 6(2) (2015) 3. M. Oezaslan, M. Heggen, and P. Strasser, J. Am. Chem. Soc., 514-524, 134(1) (2012) 4. Z. Zhang, Y. Wang, Z. Qi, W. Zhang, J. Qin, and J. Frenzel, J. Phys. Chem. C, 12629-12636, 113 (2009)

Tuning the Ligament Size and the Content of the Foreign Atom of the Nanoporous Copper for the Electrochemical Carbon Dioxide Reduction

During the electrochemical reduction reaction of carbon dioxide (CO2RR) on metallic copper surfaces several C1 and C2 products can be formed. The reaction mechanisms and kinetics, yet, are unclear to date. Different reaction mechanisms for CO2RR are postulated.1-3 For instance, the adsorption of negatively charged intermediates at the catalytically active surface might play a very important role in the formation of C2 products.3 Further critical issues for the CO2RR are addressed: (i) high overpotentials of up to 1 V, (ii) broad product distribution, (iii) fast degradation by catalyst poisoning and (iv) competition reaction at high cathodic potentials, referred to as hydrogen evolution reaction. Thus, tailoring the structure, morphology and chemical composition of Cu-based materials by adding other metals is one of the promising strategies to control the catalytic activity and product distribution during the CO2RR.4-5 In our study, we investigated the catalytic properties of a novel nanoporous copper film (np-Cu) for the CO2RR. The np-Cu films were prepared by de-alloying process of the less noble metal (Zn) from a Zn-Cu alloy. This technique allows us to tailor the ligament and pore size as well as the content of the Zn in the np-Cu. For the characterization of the np-Cu we used SEM, TEM, EDX, XRD and XPS. The catalytic performance of different np-Cu materials for the CO2RR was evaluated by linear sweep voltammetry and chronoamperometry. To correlate the measured current during the CO2RR polarization curves, the formed products were quantified by using gas chromatography and ion chromatography. Our results show how the ligament structure and the Zn content of the np-Cu have an effect on the activity and selectivity during the CO2RR. Depending on the experimental conditions, the dealloying in alkaline media largely forms copper oxide with very low content of Zn. In contrast, after dealloying in acidic media the ligament size and pore distribution of np-Cu are more uniform and contains a higher amount of Zn. The polarization curves for both np-Cu materials showed higher activity for the CO2RR compared to that for a flat Cu surface. We showed that a high content of Zn at the surface promotes the formation of hydrogen, while the copper oxide leads to higher selectivity to C2 products. Based on our results, we provide a deeper insight into the mechanism and kinetics for the CO2RR on np-Cu materials. 1. K. P. Kuhl, E. R. Cave, D. N. Abram, and T. F. Jaramillo, Energy Environ. Sci., 7050-7059, 5 (2012) 2. A. A. Peterson, F. Abild-Pedersen, F. Studt, J. Rossmeisl, and J. K. Nørskov, Energy Environ. Sci., 1311-1315, 3 (2010) 3. R. Kortlever, J. Shen, K. J. P. Schouten, F. Calle-Vallejo, and M. F. M. Koper, J. Phys. Chem. Lett., 4073-4082, 6 (2015) 4. R. Reske, M. Duca, M. Oezaslan, K. J. P. Schouten, M. T. M. Koper, and P. Strasser, J. Phys. Chem. Lett., 2410-2413, 4 (2013) 5. A. Dutta, M. Rahaman, N. C. Luedi, M. Mohos, and P. Broekmann, ACS Catal., 2804-2814, 6 (2016)

Electrodeposition of Metals and Metal Oxides into Nanoporous Gold

Nanoporous gold (NPG) is usually fabricated by the selective electrolytic dissolution (dealloying) of Ag from a binary Ag-Au alloy, or sometimes from a ternary alloy such as Ag-Au-Pt. The resulting ligament-pore structure is bicontinuous and has an extraordinarily high surface area-to-volume ratio, posing it as a promising candidate for catalyst applications [1-2]. The ligament size can be as small as 3-5 nm. While a lot of recent research has been focusing on the structure/property relationships of NPG as a functional material in its own right [2-5], the further use of NPG as a sophisticated template for fabricating complex nanostructured materials is worthy of attention. Figure 1 - SEM image of FIB (focused ion beam) cross-section showing the dealloyed layer after deposition of Cu. [5] Recent work showing atomic-scale characterization of NPG by electrodeposition of a fully compact Cu support opens the door to implementing that newly developed method to the creation of finely tuned nanostructures using NPG as a template [5]. Fabrication of nanostructures/nanosurfaces pertaining to the interests of the catalysis community will be demonstrated through the electrodeposition of metals, such as Cu and Co, and metal oxides, such as Cr2O3 and MoO2, on NPG. Furthermore, notable characteristics of NPG and the electrodeposition methodology that enables such compact deposits have been investigated experimentally and theoretically through reaction-transport modelling. A key aspect is the use of thin nanoporous layers, such that the characteristic diffusion time through the layer is short, in the ms range, but there are other subtleties. Electron diffraction and X-ray diffraction reveal the crystalline structure and orientation of the deposits and the associated growth mechanism. For metal deposition within a nanoporous material, not only ordinary thermodynamics have to be considered, but also the “subpotential” curvature-driven deposition described by Lee et al. [6].

Nanoporous Cu Thin Films for Electrochemical CO2 Reduction

Copper catalysts have been extensively researched as catalysts for electrochemical reduction of CO2 (CO2RR) due to their capability to catalyze production of hydrocarbons. The selectivity and activity of Cu catalysts for CO2RR are greatly dependent on the surface structure of the catalysts, and due to this dependence, many forms of Cu (ranging from high quality single crystals to densely populated arrays of supported nanostructures) have been investigated as CO2RR catalysts. Nanoporous (np) metals, which are produced through selective dissolution of an element from a parent alloy or compound, have inherent properties not typically present in other nanostructured materials that can strongly affect their catalytic properties, including concave surfaces with unique surface structures and open porosity that can cause mass transport effects during catalysis. Nanoporous Au and Ag, have been demonstrated as efficient and selective catalysts for oxidation of CO and reduction of CO2 to CO relative to their bulk metals. However, np-Cu has yet to be comprehensively studied as a CO2RR catalyst. To date, np-Cu has been primarily produced from homogeneous bulk materials which are not well suited for practical use as a potential CO2RR catalyst due to long dealloying treatments required to produce np-Cu that remains stable, without continued dealloying of the non-noble metal, at low CO2RR overpotentials. In this work, we demonstrate new electrochemical methods for producing fully dealloyed thin films of high-quality, crack-free np-Cu on inert substrates in significantly shorter processing times compared to bulk samples. We examine the activity and selectivity of np-Cu film catalysts produced by several dealloying methods for CO2RR. These results are presented in relation to other Cu catalysts with focus on the catalytic effects of the unique structural properties of np-Cu and the effects of post dealloying treatments to tune pore and ligament sizes of the np-Cu films. In addition to np-Cu films, we discuss how the presented techniques offer a path for production of stable thin films of other nanoporous metals on complex geometries. This work is supported by the Department of Energy award number DE-SC0008686.

Indentation size effect in nanoporous gold

We find that hardness of nanoporous gold (np-Au) measured by nanoindentation tends to increase with decreasing indentation depth, similar to the indentation size effect (ISE) in solid materials. While ISE in solid materials is attributed to a local increase in density of geometrically necessary dislocations (GNDs), the origin of ISE in np-Au has not been studied systematically. We prepare four np-Au samples with ligament sizes of 26, 73, 126, and 630 nm by free corrosion dealloying and post heat treatments. For the normalized hardness (hardness/macroscopic hardness) vs normalized indentation depth (indentation depth/three times ligament size), we find that the ISE trends for three np-Au samples of ligament sizes 26, 73, and 127 nm are almost identical, while an enhanced ISE is shown for np-Au with greatest ligament size, 630 nm. We investigate ISE in np-Au based on nanomechanics model for nanoindentation on np-Au with a sharp indenter, uniaxial compression and pure shear testing for np-Au.