Nanoporous Au Research Articles

Ethanol Electrooxidation on an Island-Like Nanoporous Gold/Palladium Electrocatalyst in Alkaline Media: Electrocatalytic Properties and an In Situ Surface-Enhanced Raman Spectroscopy Study.

The design of electrocatalysts with high activity and the understanding of the reaction mechanism for the ethanol oxidation reaction (EOR) are pivotal for commercializing direct ethanol fuel cells (DEFCs). Herein, island-like nanoporous gold/palladium (INPG/Pd) is designed as a highly efficient EOR electrocatalyst. For a better understanding of the reaction mechanism, in situ surface-enhanced Raman spectroscopy (SERS) in conjunction with H-D isotope replacement is used to investigate the dissociation and oxidation of CH3CH2OH on the INPG/Pd electrode, with a focus on identifying significant intermediate species in the reaction process. The results show that INPG/Pd has a higher electrocatalytic performance than INPG and indium-tin oxide (ITO) glass/palladium (Pd) due to the synergistic effect of NPG and Pd. INPG/Pd-10 shows the highest specific activity, the strongest charge-transfer ability, and relatively good stability. INPG/Pd presents better SERS sensitivity than ITO glass/Pd because of the plasma enhancement effect of nanoporous Au. The in situ Raman spectral results suggest that the oxidation of ethanol proceeds via a dual-pathway (C1 and C2) reaction mechanism. Dehydrogenation of ethanol can form acetaldehyde (CH3CHO) at -0.4 V. Meanwhile, the adsorbed acetaldehyde is oxidized to acetate from approximately -0.4 V, with the potential moving positively, which is the so-called C2 pathway. Alternatively, in the C1 pathway, CH3CHO and CH3CH2OH decomposed to intermediate species (adsorbed CO) on the INPG/Pd electrode due to C-C bond breaking at potentials of approximately -0.2 V. Subsequently, the CO species is oxidized to CO2 at more positive potentials.

(Electrodeposition Division Research Award) Impact of Electrodeposition on the Design and Synthesis of Nanoporous Functional Materials

The sustainable growth of efficient and durable functional nanomaterials applicable in a variety of practical fields is founded on the creative design, rational synthesis, extensive characterization, and realistic testing of products developed by countless hard-working research teams. Many of these nanomaterials must include high-cost and scarcely distributed elements and compounds that need to be either eliminated or minimally used as much as it is possible. Most commonly, such minimization has been approached by synthesizing the materials of interest in the form of nanoparticles. Nanoparticles are undoubtedly superior to other type of materials in a variety of fields and applications because of their large surface area to volume ratio and unique chemical and physical properties. Their implementation in practice also addresses well the objective for minimization of the use of expensive elements and compounds. At the same time, as class of nanomaterilas, the nanopartiles suffer some drawbacks like loss of material during synthesis, surface contamination/blockage by chemicals used in their synthesis, mechanical disconnection from carrier electrodes, and / or aggregation during exploitation. Such unwelcome developments often make it difficult to keep the cost low and/or lead to a reduction of the active surface area and thus, to loss of functionality, performance, and stability.A way of addressing some of the mentioned shortcomings is to employ electrochemical approaches for synthesizing alternative nanostructured materials directly on the carrier electrode. Such approach not only provides for better adhesion and contamination free surface, but also enables an efficient control of the amount of the deposited material along with flexibility in the structuring during the material synthesis, thus most-certainly reducing its overall cost of the final product. The electrochemical means inevitably include controlled electrodeposition of a binary / ternary alloy layer with a desired thickness and pre-selected elemental composition. A naturally following step in the material's synthesis is the selective oxidative dissolution of the less / least noble metal (a.k.a. de-alloying) to create a continuous nanoporous film with tunable pore and ligament size comprising a length-scale in the single-digit nanometer range. Finally, as-synthesized nanostructured films may either be employed directly for the purposes of the intended applications or be subjected to an additional surface functionalization by a further electrodeposition or electroless of a thin layer with specific properties that is aimed at boosting the material's functionality, performance and stability.This talk will introduce the use of electrochemical means in the design and synthesis of continuous nanoporous Au- and Cu-based functional alloy nanomaterials with applications in electrocatalysis, environmental protection, and electronic packaging. The discussed synthetic approaches will include bulk alloy electrodeposition, electrochemical de-alloying, and electrochemical atomic layer deposition for surface functionalization by films with a thickness in the range from a sub-monolayer to a few monolayers. The presentation of each class of nanostructured materials of interest to this talk will include a conceptual description of their synthetic routines, followed by an electrochemical and ultra-high vacuum-based characterization results, and concluded with a glimpse into the outcome of standard performance tests of the functionality, performance, and stability of said materials in intended applications. Finally, aspects of the materials performance associated with hypothesized mechanistic views will be critically discussed in comparison with other nanoparticulate and/or nanostructured counterpart materials in the literature.

Effects of the Parent Alloy Microstructure on the Thermal Stability of Nanoporous Au.

Nanoporous (NP) metals represent a unique class of materials with promising properties for a wide set of applications in advanced technology, from catalysis and sensing to lightweight structural materials. However, they typically suffer from low thermal stability, which results in a coarsening behavior not yet fully understood. In this work, we focused precisely on the coarsening process undergone by NP Au, starting from the analysis of data available in the literature and addressing specific issues with suitably designed experiments. We observe that annealing more easily induces densification in systems with short characteristic lengths. The NP Au structures obtained by dealloying of mechanically alloyed AuAg precursors exhibit lower thermal stability than several NP Au samples discussed in the literature. Similarly, NP Au samples prepared by annealing the precursor alloy before dealloying display enhanced resistance to coarsening. We suggest that the microstructure of the precursor alloy, and, in particular, the grain size of the metal phases, can significantly affect the thermal stability of the NP metal. Specifically, the smaller the grain size of the parent alloy, the lower the thermal stability.

Ultrahigh-energy and -power aqueous rechargeable zinc-ion microbatteries based on highly cation-compatible vanadium oxides

• Aqueous Zn-ion microbatteries are constructed by nanoporous metal/oxide electrodes. • Cation insertion/extraction kinetics depends on host/guest compatibility. • Zn x V 2 O 5 supported by nanoporous Au shows the highest capacity and rate capability. • Zn-ion microbatteries of nanoporous Au/Zn x V 2 O 5 outperform the present microdevices. Aqueous multivalent-metal-ion intercalation chemistries hold genuine promise to develop safe and powerful microbatteries for potential use in many miniaturized electronics. However, their development is beset by state-of-the-art electrode materials having practical capacities far below their theoretical values. Here we demonstrate that high compatibility between layered transition-metal oxide hosts and hydrated cation guests substantially boost their multi-electron-redox reactions to offer higher capacities and rate capability, based on typical bipolar vanadium oxides preintercalated with hydrated cations (M x V 2 O 5 ). When seamlessly integrated on Au current microcollectors with a three-dimensional bicontinuous nanoporous architecture that offers high pathways of electron transfer and ion transport, the constituent Zn x V 2 O 5 exhibits specific capacity of as high as ∼527 mAh g −1 at 5 mV s −1 and retains ∼300 mAh g −1 at 200 mV s −1 in 1 M ZnSO 4 aqueous electrolyte, outperforming the M x V 2 O 5 (M = Li, Na, K, Mg). This allows aqueous rechargeable zinc-ion microbatteries constructed with symmetric nanoporous Zn x V 2 O 5 /Au interdigital microelectrodes as anode and cathode to show high-density energy of ∼358 mWh cm −3 (a value that is forty-fold higher than that of 4 V/500 μAh Li thin film battery) at high levels of power delivery.

Elongated nanoporous Au networks improve somatic cell direct conversion into induced dopaminergic neurons for Parkinson's disease therapy

The efficient production of dopaminergic neurons via the direct conversion of other cell types is of interest as a potential therapeutic approach for Parkinson's disease. This study aimed to investigate the use of elongated porous gold nanorods (AuNpRs) as an enhancer of cell fate conversion. We observed that AuNpRs promoted the direct conversion of fibroblasts into dopaminergic neurons in vivo and in vitro. The extent of conversion of fibroblasts into dopaminergic neurons depended on the porosity of AuNpRs, as determined by their aspect ratio. The mechanism underlying these results involves specific AuNpR-induced transcriptional changes that altered the expression of antioxidant-related molecules. The generation of dopaminergic neurons via the direct conversion method will open a new avenue for developing a therapeutic platform for Parkinson's disease treatment. Statement of significanceIn this study, we applied modified gold nanoporous materials (AuNpRs) to the direct lineage reprogramming of dopaminergic neurons. The cell reprogramming process is energy-intensive, resulting in an excess of oxidative stress. AuNpRs facilitated the direct conversion of dopaminergic neurons by ameliorating oxidative stress during the reprogramming process. We have found this mechanistic clue from high throughput studies in this research work.

Stiffening of nanoporous gold: experiment, simulation and theory

By combining electron microscopy measurements, atomistic simulations and elastic homogenization theory, we theoretically investigate the Young’s modulus of nanoporous Au structures. Based on atomistic replicas generated starting from experimental tomographic evidence, atomistic simulations reveal that nanoporous Au stiffens as ligaments become finer, reproducing experimental findings obtained by nanoindentation of dealloyed samples. We argue that such a stiffening is neither due to surface stress nor to grain boundaries. Instead, we observe a direct quantitative correlation between the density of dislocations found in the material phase of the nanoporous structures and their Young’s modulus and we propose a microscopic explanation of the observed stiffening. In particular, we show that local stress and strain fields in the neighborhood of dislocation cores allow dislocations to work as reinforcing solutes.

Stretchable Non‐Enzymatic Fuel Cell‐Based Sensor Patch Integrated with Thread‐Embedded Microfluidics for Self‐Powered Wearable Glucose Monitoring

AbstractWearable sensor patches for continuous or intermittent monitoring of biomolecules from body fluids are highly desired as a stretchable integrated platform conformable and attachable to the human body. However, realizing such integrated sensor patches is very challenging because of the difficulty of achieving full stretchability, controlling transport of the body fluid, and minimizing power consumption. In this study, a stretchable and self‐powered microfluidic‐integrated sensor patch comprising a stretchable non‐enzymatic fuel cell‐based sweat glucose sensor and a stretchable cotton thread‐embedded microfluidic device is demonstrated. To endow the fuel cell‐based glucose sensor with stretchability, the anode and cathode electrodes with catalytic nanoporous Au (NPG) and NPG coated with Pt nanoparticles (PtNPs@NPG), respectively, are formed on an omnidirectionally stretchable substrate with a three‐dimensional micropattern that can effectively absorb the stress generated during deformation. In addition, a stretchable microfluidic device made by embedding cotton thread into a polydimethylsiloxane (PDMS) channel for powerless constant absorption and sweat flow is integrated into the fuel cell structure. The fully stretchable microfluidic‐integrated self‐powered sensor patch demonstrates excellent continuous monitoring of the sweat glucose concentration.

Sacrificial Silver Recovery during Nanoporous Gold Formation by Electrolytic Dealloying of Gold-Silver Alloy

Electrolytic dealloying has proven to be a practical technique for fabricating nanoporous metals such as nanoporous Au (NP-Au). This widely studied material has novel properties that are useful in applications ranging from chemical sensing to energy storage. Building off the success of this field, issues concerning the sustainability of fabricating NP-Au through dealloying need to be further explored, such as the large amount of valuable sacrificial Ag material dissolved in solution after dealloying is complete. In this work, an electrolytic cell is created consisting of a Au–Ag alloy foil working electrode and a Ag foil counter and reference electrode immersed in a non-aqueous Ag+ ion electrolyte. When a small overpotential is applied, the working electrode is dealloyed to form NP-Au. Unique to this configuration, Ag removed from the working electrode is simultaneously plated on the counter electrode, resulting in high yield recovery of Ag sacrificial material, which can be used to create new Au–Ag parent alloys. This integrated process of simultaneous dealloying and recovery of the sacrificial material significantly improves the sustainability and economic viability of dealloying. From these results, a generalized electrolytic dealloying fabrication procedure is derived that can be extended to a wide range of nanoporous metals.

The effects of Al2Au phase and oxide film on dealloying performances of Al50Au50 ribbon

The dealloying performances of the free side and roller side of the Al50Au50 ribbons are investigated under different dealloying conditions. The free side consisting of Al2Au, AlAu, and AlAu2 phases undergoes more severe corrosion than the monophasic AlAu roller side. The Al2Au phase not only acts as an anodic electrode in galvanic corrosion of free side but also leaves nanoporous Au behind and induces further dealloying of AlAu by the capillary action. The oxide film is confirmed as Al–O film with solubilized Au atoms. During corrosion, the Au atoms in oxide film tend to segregate into Au-rich and Au-lacked regions. The Au-rich region remained can enhance the Au-content on surface and impede the further corrosion.

Estimation of Nanoporous Au Young’s Modulus from Serial Block Face-SEM 3D-Characterisation

Nanoporous Au has been subjected to serial block face-scanning electron microscopy (SBF-SEM) 3D-characterisation. Corresponding sections have been digitalized and used to evaluate the associated mechanical properties. Our investigation demonstrates that the sample is homogeneous and isotropic. The effective Young’s modulus estimated by an analytical multiscale approach agrees remarkably well with the values stated in the literature.

Ultrasonic irradiation of nanoporous Au

We investigated the effects of ultrasonic irradiation (UI) on nanoporous (NP) Au structures fabricated by dealloying. To this aim, we leaned an ultrasonic horn against the sample surface and carried out experiments at different temperature. We found that UI induces coarsening, analogously to the process driven by annealing.

Hierarchically Structured Black Gold Film with Ultrahigh Porosity for Solar Steam Generation.

Plasmonic metals demonstrate significant potential for solar steam generation (SSG) because of their localized surface plasmon resonance effect. However, the inherently narrow absorption spectra of plasmonic metals significantly limit their applications. The fabrication of nanostructures is essential to achieve broadband solar absorption for high-efficiency vapor generation. Herein, a self-supporting black gold (Au) film with an ultrahigh porosity and a hierarchically porous structure is fabricated by formulating an extremely dilute Cu99 Au1 precursor and controlling the dealloying process. In situ and ex situ characterizations reveal the dealloying mechanism of Cu99 Au1 in a 1 m HNO3 solution as that involving the phase transformation of Cu(Au) → Au(Cu) → Au, giant volume shrinkage (≈87%), structural evolution/coarsening of ligaments, and development of ultrahigh porosity (86.2%). The multiscale structure, consisting of ultrafine nanoporous nanowires, aligned nanogaps, and various microgaps, provide efficient broadband absorption over 300-2500nm, excellent hydrophilicity, and continuous water transport. In particular, the nanoporous black Au film shows high SSG performance with an evaporation rate of 1.51kg m-2 h-1 and a photothermal conversion efficiency of 94.5% under a light intensity of 1kW m-2 . These findings demonstrate that the nanoporous Au film has great potential for clean water production and seawater desalination.

Hardening of Nanoporous Au Induced by Exposure to Different Gaseous Environments.

This work focuses on the mechanical behaviour of nanoporous Au samples alternately exposed to ozone and carbon dioxide. Nanoporous Au was fabricated by freely corroding the Ag70Au30 parent alloys prepared by mechanical alloying in the form of powder and subsequently compacted by cold pressing. Dealloying was performed in acidic solution, and conditions were suitably adjusted to obtain fine nanoporous Au structures with ligaments about 15 nm thick. Nanoporous Au samples with increasingly thicker ligaments, up to about 40 nm, were fabricated by annealing the pristine nanoporous Au structure for different time intervals at 473 K. For all of the samples, the cyclic variation of gaseous atmosphere results in a macroscopic strain variation due to the occurrence of surface oxidation and reduction processes. We show that the reiterated cyclic exposure to the different gases also induces the progressive hardening of nanoporous Au, which can be ascribed to irreversible strain contributions. For nanoporous Au samples with ligaments that are 15 nm thick, after 50 exposure cycles, the yield strength increases approximately from 49 MPa to 57 MPa. A systematic investigation on coarser nanoporous Au structures indicates that, with the same exposure cycles, the degree of hardening decreases with the ligament thickness.

In Situ Study of Nanoporosity Evolution during Dealloying AgAu and CoPd by Grazing-Incidence Small-Angle X-ray Scattering.

Electrochemical dealloying has become a standard technique to produce nanoporous network structures of various noble metals, exploiting the selective dissolution of one component from an alloy. While achieving nanoporosity during dealloying has been intensively studied for the prime example of nanoporous Au from a AgAu alloy, dealloying from other noble-metal alloys has been rarely investigated in the scientific literature. Here, we study the evolution of nanoporosity in the electrochemical dealloying process for both CoPd and AgAu alloys using a combination of in situ grazing-incidence small-angle X-ray scattering (GISAXS), kinetic Monte Carlo (KMC) simulations, and scanning transmission electron microscopy (STEM). When comparing dealloying kinetics, we find a more rapid progression of the dealloying front for CoPd and also a considerably slower coarsening of the nanoporous structure for Pd in relation to Au. We argue that our findings are natural consequences of the effectively higher dealloying potential and the higher interatomic binding energy for the CoPd alloy. Our results corroborate the understanding of electrochemical dealloying on the basis of two rate equations for dissolution and surface diffusion and suggest the general applicability of this dealloying mechanism to binary alloys. The present study contributes to the future tailoring of structural size in nanoporous metals for improved chemical surface activity.

Influence of surface stress on the mechanical response of nanoporous metals studied by an atomistically informed continuum model

The influence of surface stresses on the deformation behavior of nanoporous metals is considered to be the main reason for the tension-compression asymmetry observed in atomistic simulation studies of nanoporous metals. While it is difficult to differentiate between the contributions of linear and non-linear elasticity, dislocation activity, topology and surface stresses from atomistic simulations, continuum mechanics allow to disentangle these effects by systematically varying the applied constitutive relations. In this study, we investigate the elastic response of a nanoporous Au and Ag structure, both by molecular statics simulations and an elastic continuum model, which includes surface stresses and elastic anisotropy. The 3-dimensional periodic atomistic nanoporous structure is triangulated and transferred to the continuum model, where periodicity, elastic anisotropy, and surface stresses are considered. The results show that surface stress acts as a mechanical pre-load and that the elastic response is sensitive to the loading direction. An asymmetric behavior is observed in the elastic regime. Moreover, we show that sites, where dislocation activity is starting, show high von Mises stresses.

Self-catalytic growth of one-dimensional materials within dislocations in gold

Dislocations in metals affect their properties on the macro- and the microscales. For example, they increase a metal's hardness and strength. Dislocation outcrops exist on the surfaces of such metals, and atoms in the proximity of these outcrops are more loosely bonded, facilitating local chemical corrosion and reactivity. In this study, we present a unique autocatalytic mechanism by which a system of inorganic semiconducting gold(I) cyanide nanowires forms within preexisting dislocation lines in a plastically deformed Au-Ag alloy. The formation occurs during the classical selective dealloying process that forms nanoporous Au. Nucleation of the nanowire originates at the surfaces of the catalytic dislocation outcrops. The nanowires are single crystals that spontaneously undergo layer-by-layer one-dimensional growth. The continuous growth of nanowires is achieved when the dislocation density exceeds a critical value evaluated on the basis of a kinetic model that we developed.

Surface-enhanced Raman scattering of trace Hg2+ on a 4-mercaptobenzoic acid-modified nanoporous Au substrate

A Au-Ga alloy layer is synthesized on a Au sheet substrate at low temperature and a new nanoporous Au material is then prepared based on Ga removal from the Au-Ga alloy by a electrochemical method. X-ray diffraction analysis confirms that the grains of the newly formed Au layer are refined and scanning electron microscopy suggests that the newly formed Au layer has a nanoporous structure with a pore size of∼16 nm. The cyclic voltammetry curve of the prepared nanoporous Au in a 1 M KCl solution presents a square structure of a supercapacitor with a specific capacitance of mF·cm-2, which is 86 times that of the smooth Au electrode. The 4-mercaptobenzoic acid (4-MBA) modified nanoporous Au material is used as a Raman substrate to detect Hg2+ with surface-enhanced Raman scattering. from 1.0×10−10 mol·L-1 to 1.0×10−5 mol·L-1), and the limit of detection is 1.0×10−10 mol·L-1.

Porosity effects on nanoporous Au Young’s modulus

Nanoporous Au is a model system extensively studied to investigate how porosity affects the mechanical behaviour. To fabricate nanoporous Au structures with similar ligament diameter, but different solid volume fractions, we applied tailored dealloying procedures to Ag-Au alloys with Au atomic fraction ranging from 0.20 to 0.41. Their Young’s moduli, measured by nanoindentation, correlate with the Au atomic fraction. The scaling law we observe suggests a crucial role of percolation thresholds in the definition of mechanical properties.

High-efficient CO2 electrocatalysis over nanoporous Au film enabled by a combined pore engineering and ionic liquid-mediated approach

Electrochemical reduction of CO2 in ionic liquid-based electrolytes has received increasing interest due to its high selectivity and low overpotential. However, it has been plagued with a low yield rate due to the typically lower conductivity and higher viscosity of the electrolytes compared to aqueous electrolytes. Here, we report a rational electrode-electrolyte design by using nanoporous Au film electrode in combination with a 1-ethyl-3methylimidazolium tetrafluoroborate (EMIMBF4) ionic liquid-based binary electrolyte for electrochemical CO2 conversion. The CO2 reduction activity increases with the increase of the pore size of Au film, which is attributed to the increase in electrocatalytic surface area and enhancement in mass transport. Experimental and density functional theory studies show that EMIMBF4 ionic liquid increases the CO2 reduction activity and selectivity. Electrocatalytic reduction of CO2 with Faradaic efficiency up to 92.5% and mass activity of 3.0 A/g for CO is achieved at −0.55 V (vs RHE) in ionic liquid/water electrolyte.

Reflection color tuning of a metal–insulator–metal cavity structure using arc plasma deposition of gold nanoparticles

Structural colors have diverse applications in optical filters, surface decoration, photovoltaics, and digital displays. In this study, an arc plasma deposition (APD) method was employed for fabrication of the top Au layer in the asymmetric Fabry–Perot cavity structure of Au/Si3N4/Al. With an increasing number of APD pulses (nP), the morphology of the Au layer gradually changed from a dispersion of nanoparticles (NPs) to a nanoporous film and then to a dense film, which in turn led to a change in the refractive index of the Au layer. The structural colors of the Au/Si3N4/Al structure were mainly governed by the interference effect. The optical constants of the NP-dispersed and nanoporous Au layers could be reasonably estimated based on Bruggeman's effective medium approximation. When the Au layers deposited with different nP were combined with dielectric Si3N4 layers of various thicknesses (hd), the full range of color hue with a wide range of color chroma could be realized using the Au/Si3N4/Al trilayer structure. The color hue was predominantly governed by hd, and the color chroma gradually increased with nP for a given value of hd.