Bicontinuous Nanoporous Gold Research Articles

Atomistic study of the impact response of bicontinuous nanoporous gold as a protection medium: Effect of porous-nonporous interface on failure evolution

Compared to their non-porous counterparts, metallic foams are known to exhibit improved functionality (e.g., damping capacity) when subjected to high-speed impact loading. Here, we report the results of a molecular dynamics study of bicontinuous nanoporous gold (NPG) subjected to impact loading. Additionally, we investigate the effects of a heterogeneous impact zone on the fate of a witness specimen initially protected by an NPG target. A cube-shaped flyer object (L0= 408.6 Å) composed of full-density f.c.c. gold (FDG) strikes with impact speed Uflyer= 1.0 km s−1 an initially stationary equal-sized NPG target, which subsequently transmits a dispersed shock wave into a protected FDG witness specimen located at the downstream end of the target. The NPG target is stochastic, with porosity ϕ= 0.5 and mean ligament diameter D¯L=64±6 Å. A corresponding simulation for an FDG target serves as a baseline case. As anticipated, the sharp, planar shock imparted by the FDG flyer rapidly becomes highly curved and broadened along the shock direction in the NPG target. Intense plastic and convective flows (ejecta and jetting) lead to spatially heterogeneous mass flow and energy localization, which persist across the entire length of the NPG target studied. Whereas the shock transmitted by the FDG target leaves the witness specimen intact and essentially undamaged, the heterogeneous stress and flow fields imparted by the NPG target results in destruction of the witness. Independent simulations for the same cube-shaped NPG target shocked on the three statistically equivalent {100} target faces reveal modest run-to-run variability in the target and witness responses. The difference in the effects of the NPG and FDG targets on the witness response motivates additional study of the failure evolution inside the witness. It appears from the preliminary results that the porous-nonporous interface might induce a failure pattern that, in some aspects, resembles failure waves observed in brittle solids.

Fast attenuation of high-frequency acoustic waves in bicontinuous nanoporous gold

We studied the formation and attenuation of GHz elastic waves in free-standing nanoporous gold films by MeV ultrafast electron diffraction and finite element simulations. By tracing the evolution of the high frequency acoustic waves in time domain, we found that the bicontinuous nanoporous structure in nanoporous gold films results in three-dimensionally acoustic response with low coherence, leading to fast attenuation of the elastic waves in comparison with solid gold films. The morphologically dominated dynamics indicates the nanoporosity plays an important role in the high-frequency acoustic energy relaxation, which shines a light on the applications of dealloyed nanoporous materials in nanodevices and sensors as GHz and THz acoustic filters and dampers.

Shock responses of nanoporous gold subjected to dynamic loadings: Energy absorption

Using the finite element analysis, we investigate the shock response, deformation behavior and energy–absorbing characteristics of stochastic bicontinuous nanoporous gold computationally, considering the effects of impact speed and relative density of materials. Three deformation modes during shocking, quasi–static compression, transition state and dynamic compression, are observed with the increasing impact speeds and the corresponding internal deformation mechanisms are demonstrated. The shock response under low–speed loading is similar to the R–LHP–L materials model, and it is more inclined to the R–S–H model as the impact speed rises. The peak stress, plateau stress, densification strain, energy, specific energy absorption (per unit volume) are assessed quantitatively. The peak stress shows a monotonous increase with impact speed, due to the enhanced inertial effect. The plateau stress and energy–absorbing efficiency are insensitive to low impact velocity, but significantly enhanced under high–speed loading. Remarkably, the results show that the energy absorption efficiency of nanoporous gold is better than conventional foam materials and nanofluidic energy absorption and dissipation system. The research not only further clarifies the shock responses and associated mechanisms of NPG, but also promotes its industrial application as an energy–absorbing material.

Flexible Co-Mo-N/Au Electrodes with a Hierarchical Nanoporous Architecture as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction.

Designing highly active and robust electrocatalysts for oxygen evolution reaction (OER) is crucial for many renewable energy storage and conversion devices. Here, self-supported monolithic hybrid electrodes that are composed of bimetallic cobalt-molybdenum nitride nanosheets vertically aligned on 3D and bicontinuous nanoporous gold (NP Au/CoMoNx ) are reported as highly efficient electrocatalysts to boost the sluggish reaction kinetics of water oxidation in alkaline media. By virtue of the constituent CoMoNx nanosheets having large accessible CoMoOx surface with remarkably enhanced electrocatalytic activity and the nanoporous Au skeleton facilitating electron transfer and mass transport, the NP Au/CoMoNx electrode exhibits superior OER electrocatalysis in 1 m KOH, with low onset overpotential (166 mV) and Tafel slope (46 mV dec-1 ). It only takes a low overpotential of 370 mV to reach ultrahigh current density of 1156 mA cm-2 , ≈140-fold higher than free CoMoNx nanosheets. The electrocatalytic performance makes it an attractive candidate as the OER catalyst in the water electrolysis.

Free-standing nanoporous gold for direct plasmon enhanced electro-oxidation of alcohol molecules

Localized surface plasmon resonance (LSPR) excitation enhanced chemical and electrochemical reactions represent a promising pathway for solar-to-chemical energy conversion. However, plasmonic catalysts usually suffer from low collection efficiency of hot charge carries generated by LSPR excitation due to short lifetime of hot carriers and fast electron-hole recombination. Schottky barriers between plasmonic catalysts and semiconducting supports have been utilized to prevent the electron-hole recombination. However, the interfacial barriers also hinder low-energy hot charge collection and thus affect solar-to-chemical energy conversion efficiency. Here we report that bicontinuous nanoporous gold as a Schottky barriers-free direct plasmonic catalyst can significantly enhance electro-oxidation of alcohol molecules. A high energetic hole yield of 0.486% is achieved, which is 4 times higher than that of discrete plasmonic AuAg nanoparticles. The direct plasmonic catalyst offers the highest methanol oxidation current density of 531 µA cm−2 among all known Au catalysts. This work provides compelling evidence that higher hot carrier collection efficiency can be achieved from direct plasmonic electrocatalysis without the assistance of Schottky junctions and has important implications in developing high efficiency plasmonic catalysts for photo-enhanced electrochemical reactions.

Dynamics of Electron–Phonon Coupling in Bicontinuous Nanoporous Gold

The nanosize effect on electron–phonon interactions in free-electron noble metals has been a subject of intense discussion because of their important applications in physics, chemistry, and biomedicine. However, the interference from supports of dispersive nanoparticulate metals has led to controversial observations. We utilize freestanding, bicontinuous nanoporous gold (NPG) films to investigate electron–phonon interaction dynamics using ultrafast MeV electron diffraction. Compared to solid gold films, NPG shows faster electron–phonon interaction and thus noticeably higher electron–phonon coupling constant. The results demonstrate that surface states of electrons and phonons play an important role in electron–phonon coupling of nanostructured materials.

Three-Dimensional Bi-Continuous Nanoporous Gold/Nickel Foam Supported MnO2 for High Performance Supercapacitors

A three-dimensional bi-continuous nanoporous gold (NPG)/nickel foam is developed though the electrodeposition of a gold–tin alloy on Ni foam and subsequent chemical dealloying of tin. The newly-designed 3D metal structure is used to anchor MnO2 nanosheets for high-performance supercapacitors. The formed ternary composite electrodes exhibit significantly-enhanced capacitance performance, rate capability, and excellent cycling stability. A specific capacitance of 442 Fg−1 is achieved at a scan rate of 5 mV s−1 and a relatively high mass loading of 865 μg cm−2. After 2500 cycles, only a 1% decay is found at a scan rate of 50 mV s−1. A high power density of 3513 W kg−1 and an energy density of 25.73 Wh kg−1 are realized for potential energy storage devices. The results demonstrate that the NPG/nickel foam hybrid structure significantly improves the dispersibility of MnO2 and makes it promising for practical energy storage applications.

Hierarchical nanoporosity enhanced reversible capacity of bicontinuous nanoporous metal based Li-O2 battery.

High-energy-density rechargeable Li-O2 batteries are one of few candidates that can meet the demands of electric drive vehicles and other high-energy applications because of the ultra-high theoretical specific energy. However, the practical realization of the high rechargeable capacity is usually limited by the conflicted requirements for porous cathodes in high porosity to store the solid reaction products Li2O2 and large accessible surface area for easy formation and decomposition of Li2O2. Here we designed a hierarchical and bicontinuous nanoporous structure by introducing secondary nanopores into the ligaments of coarsened nanoporous gold by two-step dealloying. The hierarchical and bicontinuous nanoporous gold cathode provides high porosity, large accessible surface area and sufficient mass transport path for high capacity and long cycling lifetime of Li-O2 batteries.

Nanoporous gold: 3D structural analyses of representative volumes and their implications on scaling relations of mechanical behaviour

We present a quantitative study of the salient structural parameters identified from so-called ‘representative volumes’ of the bicontinuous nanoporous gold (NPG) network, and examine the validity of self-similarity in describing its evolution. The approach is based on 3D-focused ion beam tomography applied to as-dealloyed and isothermally annealed NPG samples. After identifying sufficiently large representative volumes, we show that the ligament width distributions coarsen in a sufficiently self-similar, time-invariant manner, while the scaled connectivity density shows a self-similar ligament network topology. Using these critical parameters, namely mean ligament diameter and connectivity density, the Gibson–Ashby scaling laws for the mechanical response of cellular materials are revisited. The inappropriateness of directly applying the Gibson–Ashby model to NPG is demonstrated by comparing finite element method compression simulations of both the NPG reconstruction and that of the Gibson–Ashby solid model; rather than the solid volume fraction, we show that an effective load-bearing ring structure governs mechanical behaviour.

Manipulating the d-Band Electronic Structure of Platinum-Functionalized Nanoporous Gold Bowls: Synergistic Intermetallic Interactions Enhance Catalysis

Bicontinuous nanoporous gold (NPG) is a high-performance catalyst characterized by its excellent electrochemical stability and immense active surface area with high electrolyte accessibility. However, the intrinsic catalytic activity of NPG is still lower compared to other metals (such as Pt), thus impeding its applicability in a commercial catalytic system. Herein, we incorporate secondary Pt metal with inherently strong catalytic activities into a zero-dimensional (0D) nanoporous gold bowl (NPGB) to develop Pt-NPGB bimetallic catalyst. Our strategy effectively exploits the highly accessible surface area of NPGB and the manipulative d-band electronic structure brought about by the synergistic intermetallic interaction for enhanced catalytic performance and durability. Deposition of Pt on the NPGB catalyst directly modulates its d-band electronic structure, with the electronic energy of Pt-NPGBs tunable between −3.93 to −4.24, approximating that of chemically resistant gold (−4.35 eV). This is vital to we...

Hardening under compression in Au foams

We report the results of compressive tests for a model bi-continuous nanoporous gold structure using atomistic simulations focusing on the densification regime and the plasticity mechanisms taking place. The stress – strain response follows a logarithmic evolution proportional to the inverse of strain and we provide a simple scaling law for this behavior. Hardening is correlated to the dislocation density and an exponential dependence is revealed. The plasticity mechanisms were inspected revealing the presence of Hirth and Frank partials contributing to the hardening of the sample. Lomer–Cottrel locks, perfect dislocations, and twinning were also found.

On-Chip Micro-Pseudocapacitors for Ultrahigh Energy and Power Delivery.

Microscale supercapapcitors based on hierarchical nanoporous hybrid electrodes consisting of 3D bicontinuous nanoporous gold and pseudocapacitive manganese oxide deliver an excellent stack capacitance of 99.1 F cm-3 and a high energy density of 12.7 mW h cm-3 with a retained high power density of 46.6 W cm-3.

Mechanical response of nanoporous gold

We report the results of computational tensile and compressive tests for model bi-continuous nanoporous gold structures using atomistic simulations with empirical many-body potentials and molecular dynamics. The results are compared with the predictions of scaling laws for coarser-scale foams and with available experimental data. We find a surprising substantial tension/compression asymmetry in yield due to the surface stress that sets the filament under compression, providing a bias favoring yielding in compression. We provide a model for our results based on a ligament strength value close to the theoretical strength of Au, and the surface stress. The model predicts a significant tension/compression asymmetry for ligament sizes below ∼10nm and pore collapse for ligament sizes below 1nm. We also observe an unexpected compaction tendency under tension characterized by a decrease in the total volume of the sample of 15% for samples deformed under tension by 30%. The mechanism of the compaction involves a decrease in the average pore size and pore collapse resulting from plasticity within the ligaments.

Three-dimensional nanoporous gold for electrochemical supercapacitors

We report the capacitive performances of an electrochemical supercapacitor with three-dimensional nanoporous gold (NPG) as both current collectors and electrodes. The bicontinuous NPG with a large internal surface and high conductivity shows promising capacitive properties in both a conventional aqueous electrolyte and a green ionic liquid electrolyte. In comparison with the performance in the aqueous electrolyte, the NPG-based supercapacitor in the ionic liquid electrolyte shows higher energy density because of the high operation voltage arising from the low ionic mobility of the room-temperature ionic liquid.