Nanoporous Silver Research Articles

Interacting Metal-Insulator-Metal Resonator by Nanoporous Silver and Silk Protein Nanomembranes and Its Water-Sensing Application.

Planar and lithography-free metal–insulator–metal (MIM) resonators based on the Fabry–Pérot etalon are attractive for biochemical sensing applications because of their acceptable optical performance and cost-effectiveness. However, injecting analytes into the insulating layer where the optical field is localized (high light–matter interaction) is difficult. Here, planar and lithography-free MIM resonators interacting with their environment are reported. In the MIM, molecules of a liquid can infiltrate the inherent nanopores in the deposited silver nanomembrane and be absorbed into the silk protein hydrogel membrane. The silk layer is swollen when water molecules are absorbed, inducing a large shift in the resonance wavelength. Thus, in this study, the proposed MIM resonator was applied as a highly sensitive water sensor, and a water content as low as 0.008% in organic solvents could be determined by reading the shift in the transmission peak. This limit can be lowered further by using a high-resolution spectrometer and a thicker silk layer. In addition, the area of interaction can be artificially selected by applying an elastomer stamp and a patterned photoresist window.

Asynchronous Evolution of Nanoporous Silver on Dual-Phase Ag⁻Sn Alloys by Potentiostatic Dealloying in Hydrochloric Acid Solution.

Evolution behavior of the nanoporous architectures has been investigated via potentiostatic electrochemical dealloying of dual-phase AgxSn100−x (x = 20, 30, 40 at.%) alloys, which consist of β-Sn and ε-Ag3Sn phases with different volume fractions in 1.2 M HCl solution. The results show that the open-circuit potentials and corrosion potentials of dual-phase Ag–Sn alloys are determined by the less noble β-Sn phases rather than chemical compositions of the Ag–Sn precursor alloys. The potentiodynamic polarization curves show that the anodic dissolution of Ag–Sn alloys is divided into two stages including the first preferential dissolution of β-Sn phases and secondary dealloying of ε-Ag3Sn phases, which is associated with the order of the nanoporous evolution. Nanoporous silver (NPS) can be fabricated by potentiostatic dealloying of dual-phase Ag–Sn alloys in HCl solution. The dealloying of two phases is asynchronous: The less noble β-Sn phases are preferentially etched to generate the larger pores, and then the more noble ε-Ag3Sn phases are dealloyed to form the finer nanoporous structure. The significant surface diffusion of Ag adatoms at the applied potential higher than the pitting potential of ε-Ag3Sn phases during the dealloying results in the coarsening of nanoporous ligaments with a time dependence of d(t) t0.1. The fractions and the difference in electrochemical stabilities of the β-Sn and ε-Ag3Sn phases in dual-phase AgxSn100−x (x = 20, 30, 40 at.%) precursor alloys determines the final nanoporous structure.

Near-ideal compressive strength of nanoporous silver composed of nanowires

Nanoporous materials exhibit promising applications in energy storage, catalysis, and sensing. They are typically synthesized by dealloying, a costly and environmentally detrimental technology, valid only within a narrow compositional range of alloys. Surmounting these disadvantages, we assembled nanoporous silver materials via bottom-up nanoscale joining of nanowires, a technique also suitable for other metals. Furthermore, the resulting nanoporous materials exhibit an unprecedented, near-ideal compressive yield strength (∼2.6 GPa). Such an ultra-high strength, however, does not belong to the nanoporous materials composed of nanowires with the minimum length in our samples, challenging the smaller-is-stronger tenet. According to molecular dynamics simulations, such a strength degradation as nanowires shorten is attributed to the internal compressive stress arising from the five-fold twins within nanowires. Such internal stress maximizes at the center and diminishes near free surfaces, making the center part harder to compress than that adjacent to the free surfaces. The volume fraction of the latter increases as the nanowire shortens, diminishing the overall Young's modulus. For nanowires having aspect ratios smaller than six, a reduced Young's modulus lowers the yield strength since the yield strain is independent of aspect ratios. However, if the aspect ratio exceeds six, compression induces bending, reducing both yield strength and yield strain. Overall, this work not only provides new physical insights on the structure-mechanical property relationship for nanoporous silver, but also paves a new way for bottom-up synthesizing nanoporous metals with ultra-high compressive strength efficiently, economically, and environmental-friendly.

Enhancing CO2 electroreduction on nanoporous silver electrode in the presence of halides

The effects of halide ions (F−, Cl−, Br−, and I−) on nanoporous Ag electrode for reduction of CO2 to CO were systematically assessed. The electrochemical measurements indicate that the catalytic performance of nanoporous Ag is closely related to the affinity of halide ions to Ag. The current density and Faradaic efficiency for CO production simultaneously increase with the increase of the radius of halide ions from F− to Cl− and then to Br−, and the disparity between catalytic performance will further widen with the negative shift of potential. While the nanoporous Ag in the presence of I− first exhibits the poorest electrocatalytic performance at low potential (<−0.69 V vs. RHE), then reverses the situation at high potential (>−0.89 V vs. RHE) in comparison to the other halide ions. Tafel plots indicate that the adsorption of halide ions on the surface of nanoporous Ag influences mechanistic pathways for CO2 reduction reaction. 8 h durability test demonstrates that the nanoporous Ag is catalytically stable in a mixed electrolyte containing bromide with only less than 5% deactivation. The density functional theory calculations enlighten that the weakened interaction between Ag atoms on the surface of electrocatalyst in the presence of halides can significantly enhance the binding energy of the intermediate COOH*, and thus effectively lowers the overpotential of CO2 reduction reaction.

Electrochemically Synthesized Porous Ag Double Layers for Surface-Enhanced Raman Spectroscopy Applications.

Double-layered nanoporous silver is fabricated by dealloying an electrodeposited AgCu double layer with different compositions in each layer. The pore/ligament size and porosity of each layer can be conveniently tailored by controlling the applied voltage profile when electrodepositing the AgCu double-layer precursors. Therefore, nanoporous Ag double layers with a tailor-made porous profile along the film thickness can be easily fabricated. The Ag structures thus obtained are particularly attractive as novel multifunctional enhancement substrates for surface-enhanced Raman spectroscopy (SERS) applications. When a higher porosity is created in the top layer, the double layer can trap more light because of the antireflection effect, enabling stronger SERS enhancement. On the other hand, with smaller pores formed in the top layer, the double layer readily works as a size-screening SERS substrate that can help distinguish SERS signals from a mixture of reagents with different sizes. The theoretical simulation shows good agreement with the experimental observation.

SDS‐modified Nanoporous Silver as an Efficient Electrocatalyst for Selectively Converting CO2 to CO in Aqueous Solution

Summary of main observation and conclusionSelectively electrochemical conversion of CO2 into organic fuel using renewable electricity is one of the most sought‐after processes. In this paper, we report the electrochemical reduction of CO2 (CO2RR) on the nanoporous Ag electrodes made of compacted Ag nanoparticles (AgNPs), which were prepared by one‐step reduction in the water phase with or without the surfactant sodium dodecyl sulfate (SDS). The scanning electron microscope (SEM) characterizations show that the compacted Ag electrodes have the nanoporous morphology formed by stacking AgNPs. Compared with the nanoporous Ag electrode without SDS modification (C‐AgNPs), the SDS‐modified AgNPs electrode (C‐AgNPs‐SDS) is highly effective in improving selective CO production in a wide range of potentials (–0.69 V — –1.19 V, vs. RHE), with a Faradaic efficiency of 92.2% and a current density of –8.23 mA·cm–2 for CO production at –0.79 V (vs. RHE). C‐AgNPs‐SDS is also catalytically stable with only less than 7% deactivation after 8 h of continuous electrolysis.

Nanoporous Gold Formation by Free Corrosion Dealloying of Gold-Silver Alloys in Nonoxidizing Acids Driven by Catalytic Oxygen Reduction Reaction

Highly concentrated nitric acid (molarity ~ 15 M), a strong oxidizing agent, has been extensively used to produce nanoporous gold by free corrosion dealloying of gold-silver alloys. This is because, kinetically, silver dissolution in dilute nitric acid with molarity 5 M (or lower) is not favorable at ambient temperature and pressure. We recently reported (Deng and Detsi in Nanoscale 9:11858, 2017; Fu et al. in ACS Appl Nano Mater 1(8):4164, 2018) on the use of catalytically driven reduction reactions to enhance the rate of nanoporous silver and nanoporous gold formation at ambient temperature and pressure by free corrosion dealloying in hydrochloric acid, a nonoxidizing acid. Here, we further elucidate these findings by combining our experimental studies with the mixed potential theory of corrosion to shed more light on the reaction mechanisms associated with free corrosion dealloying of silver-aluminum alloys, and gold-silver alloys in various corroding media and in the presence of a catalyst used to enhance the dealloying rate.

Fabrication of nanoporous silver film by dealloying Ag/α-Fe2O3 nanocomposite for surface-enhanced Raman spectroscopy

A facile and reliable method for fabrication of nanoporous Ag film as surface-enhanced Raman spectroscopy (SERS) substrates was developed by dealloying the low-cost Ag/α-Fe2O3 nanocomposite in dilute acid solution. Influencing parameters such as different dealloying solutions, electrolyte concentration and dealloying time were optimized, and a uniform nanoporous Ag film of tunable and smaller ligament size was synthesized by etching the nanocomposite in sulfuric acid as a dealloying solution, while hydrochloric acid produced coarsened Ag nanoporous structures. Besides, the ligament size of the synthesized nanoporous Ag film increased with the electrolyte concentration and dealloying time. SERS activity of the as-synthesized nanoporous Ag film was evaluated using R6G as Raman probe, and the nanoporous Ag film with smaller ligament size exhibited strong SERS activity with a SERS enhancement factor of 7.47 × 106. The substrate exhibited good sensitivity and uniformity (RSD, 13.3%), and practical applicability for SERS detection of Sudan III, a synthetic organic dye banned as food additives, and achieved a detection limit of 0.5 μM. More importantly, the developed method ensures low-cost and reliable method of nanoporous materials fabrication from low-cost semiconductor based nanomaterials for applications in wide variety of fields.

In-situ synthesis, operation and regeneration of nanoporous silver with high performance toward oxygen reduction reaction

Oxygen reduction reaction (ORR) plays a major role in the efficient operation of many important electrochemical energy devices. Herein, a novel catalyst of nanoporous silver (NPS) is in-situ synthesized by a high voltage anodization method in 0.1 M KOH which also serves as the electrolyte for the catalysis of ORR. Favored by the continuous pore channels, the mass transfer is enhanced within the NPS. Theoretical calculations suggest that the strengthened interactions between OOH intermediates and the dominant Ag(110) surfaces on NPS as revealed by an adsorption test reduce the activation energy for the rate determining step and thus accelerate the ORR. Due to the enhanced mass transfer and facile kinetics, the NPS exhibits excellent ORR activity and even outperforms the commercial Pt/C. Fascinating more, the service life of NPS is significantly extended through an in-situ regeneration process. The integration concept we demonstrate in this work shed new lights on the preparation of long-life functional materials with high performance in applications including but not limited to ORR.

Formation mechanism of nanoporous silver during dealloying with ultrasonic irradiation.

Nanoporous silver (NPS) with an extreme coarsened 3-dimensional bi-continuous ligament and nanopore structure could be prepared by chemical dealloying with high-intensity ultrasonic irradiation (UI). The formation mechanism of NPS dealloying with UI was different from NPS obtained through free corrosion. It evolved into NPS with a new lump forming-disintegrating mechanism. Ultrasonic irradiation had strong effects on the dealloying process of NPS. The stirring effect produced by ultrasonic vibration could promote the corrosion of Cu and facilitate the diffusion of Ag atoms. Therefore, the coarsening rate of the ligament was increased significantly. Dealloying assisted by UI could generate an extremely coarsened microstructure of which ligament and pore sizes were much larger than those obtained from free corrosion dealloying.

Nanoporous silver cathode surface-treated by aerosol-assisted chemical vapor deposition of gadolinia-doped ceria for intermediate-temperature solid oxide fuel cells

Herein, a nanoporous silver surface treated with gadolinia-doped ceria (GDC) is evaluated as a cathode for intermediate-temperature solid oxide fuel cells operating below 500 °C. For uniform surface treatment on the porous silver, aerosol-assisted chemical vapor deposition (AACVD) is used; it is a non-vacuum process and is considered as an economical alternative to the expensive vacuum-environment thin-film fabrication methods. Consequently, a uniform coating of AACVD GDC on the Ag surface is successfully achieved, which is confirmed by high-resolution transmission electron microscopy. The optimized amount of AACVD GDC enhances fuel cell performance compared to cells with bare Ag, in terms of the power and long-term stability measured by current–voltage characteristics, electrochemical impedance spectroscopy, and potentiostatic amperometry. This performance is even more significant than that from the cell with a platinum cathode, which, to our best knowledge, is known as the best-performing catalyst for solid oxide fuel cells in the intermediate- and low-temperature regimes. The power enhancement is attributed to the improved kinetics with the GDC surface coating; moreover, this oxide decoration is proven effective in preventing the thermal agglomeration of Ag, as confirmed by the morphology comparison before and after the long-term test.

Cations promoting synthesis of self-supported nanoporous silver electrode and its catalytic activity for oxygen reduction reaction

Self-supported nanoporous metals are promising catalysts and battery materials due to their continuous channels, excellent conductivity and high surface area. In this work, we found that Cu2+ cations could promote the spontaneous formation of AgCl on Ag surface via microcell reactions. Applying a potential to the silver electrode promotes the surface enrichment of Cu2+ cations, and further speeds the formation of AgCl. After electro-reduced, AgCl was transformed to self-supported silver electrode with nanoporous structures, which shows good catalytic activity for the oxygen reduction reaction and electrochemical stability together with superior methanol tolerance property, compared to the pristine silver electrode. The approach used in this work provides an alternative way to prepare self-supported nanoporous metals.

Microfluidic approach for in situ synthesis of nanoporous silver microstructures as on-chip SERS substrates

Nanoporous silver microstructures (np-AgMSs) were first in situ synthesized using a microfluidic system. Chloride induced precipitation with NH4OH was used to prepare AgCl as a sacrificial template. Exploiting the microfluidic platform, effects of reactant concentrations on the morphology of AgCl templates were studied by simply adjusting the input flow rates. The AgCl templates were subsequently reduced into Ag0 or np-AgMSs using chemical reduction. The synthesized np-AgMSs were tested as on-chip SERS substrates using p-aminothiophenol as an analyte. It was found that the np-AgMSs with multipod structures (tripod and tetrapod) provided the highest SERS signal compared to other structures. Finally, the multipod np-AgMSs were successfully used as on-chip SERS substrates for determination of thiocyanate in human saliva. Linearity of the proposed approach was found to be in the range of 1–20 μM with R2 = 0.9887. The LOD and LOQ were 0.1 and 1.5 μM, respectively. Accuracy of the system was investigated from % recovery of spiked thiocyanate standard solutions and found to be in the range of 90–110%. In addition, reproducible Raman signals of thiocyanate were achieved with relative standard deviations of less than 9% for both intraday and interday SERS measurements. These indicated high accuracy and precision of the whole system.

Electrochemical behavior and application of a silver electrode in a 1 M LiPF6 solution

The electrochemical behavior of a silver electrode was studied with lithium sheets as the counter and reference electrodes in a 1 M LiPF6 (EC/DMC = 1:1, v/v) solution. Cyclic voltammetry results showed that there were some cathodic reductions of the silver electrode. The phases of the electrodes were studied using X-ray diffraction. New compound phases (labeled as phase ⅠⅠ) appeared after the silver electrode was polarized under 0 mV (vs. Li/Li+) for 25 h. Phase ⅠⅠ was a silver lithium alloy phase (AgLi). A new metallic silver material (phase ⅠⅠⅠ) was formed after the lithium was removed from phase ⅠⅠ by reacting with water. Scanning electron microscopy experiments showed that the new metallic silver had a nanoporous morphology, with particle sizes ranging from 100 to 800 nm and pore sizes ranging from 100 to 500 nm. The thickness of the formed nanoporous silver was ∼17 μm.

Self-Generated Nanoporous Silver Framework for High-Performance Iron Oxide Pseudocapacitor Anodes.

The rapid development of electric vehicles is increasing the demand for next-generation fast-charging energy storage devices with a high capacity and long-term stability. Metal oxide/hydroxide pseudocapacitors are the most promising technology because they show a theoretical capacitance that is 10-100 times higher than that of conventional supercapacitors and rate capability and long-term stability that are much higher than those of Li-ion batteries. However, the poor electrical conductivity of metal oxides/hydroxides is a serious obstacle for achieving the theoretical pseudocapacitor performance. Here, a nanoporous silver (np-Ag) structure with a tunable pore size and ligament is developed using a new silver halide electroreduction process. The structural characteristics of np-Ag (e.g., large specific surface area, electric conductivity, and porosity) are desirable for metal oxide-based pseudocapacitors. This work demonstrates an ultra-high-capacity, fast-charging, and long-term cycling pseudocapacitor anode via the development of an np-Ag framework and deposition of a thin layer of Fe2O3 on its surface (np-Ag@Fe2O3). The np-Ag@Fe2O3 anode shows a capacitance of ∼608 F g-1 at 10 A g-1, and ∼84.9% of the capacitance is retained after 6000 charge-discharge cycles. This stable and high-capacity anode, which can be charged within a few tens of seconds, is a promising candidate for next-generation energy storage devices.

A novel dealloying strategy for fabricating nanoporous silver as an electrocatalyst for hydrogen peroxide detection

A mild and low-cost strategy for fabricating nanoporous silver (NPS) thin films loaded on Ni wire were reported via co-electrodeposition of Ag-Sn alloy and subsequent dealloying in alkaline solution. Scanning electron microscope (SEM) and energy dispersive spectrum (EDS) were performed to investigate the morphology and composition of the electrodes. The crystal structures of the precursor alloy and NPS were examined by X-ray diffraction (XRD). After selectively etching the Sn out of the Ag-Sn alloy, an unordered three-dimensional bicontinuous nanoporous structure was observed, with a pore size of ∼50–120 nm and a ligament size of ∼40–200 nm. The NPS electrodes presented linear current response to hydrogen peroxide under the concentration ranging from 0.010 to 8.000 mM, with an excellent correlation coefficient (R2) of 0.99998. The high sensitivity, reliability, anti-interference ability, stability and short response time indicate its potential in real-time monitoring H2O2.

Effect of nanoindentation depth and ligament size on mechanical properties of nanoporous silver

Mechanical properties of nanoporous silver have been investigated by nanoindentation. Experiment shows that the hardness and elastic modulus are both involved with strong surface size effect. Under the same loading rate, hardness decreases from 56.7 to 26 MPa along with indentation depth increasing from 200 to 2000 nm, while the corresponding elastic modulus increases from 2.68 to 3.73 GPa. In the case of the same indentation depth, the hardness decreases from 53 to 7.4 MPa with ligament size increasing from 100 to 198 nm, and the corresponding elastic modulus reduces from 4.7 to 0.17 GPa. The calculated indentation size effect index m is 0.34, revealing that nanoporous silver has a good ductility in micro-state, but brittle in the macro-state.

Structure analysis of precursor alloy and diffusion during dealloying of Ag-Al alloy.

Nanoporous silver (NPS) was fabricated by dealloying Ag–Al alloy ribbons with nominal compositions of 30, 35 and 40 at% Ag (corresponding to hypoeutectic composition, eutectic composition and hypereutectic composition, respectively). The microstructures of the Ag–Al precursor and as-dealloyed samples were observed using a scanning electron microscope (SEM) and a transmission electron microscope (TEM) as well as via focused ion beam (FIB) technique. We concluded that with the increase in Ag content from 30 to 40 at%, the diameter of ligament increased from 70 ± 15 nm to 115 ± 35 nm. Due to the method of crystalline solidification and the distribution of α-Al(Ag) and γ-Ag2Al phases, the as-dealloyed Ag35Al65 alloy exhibited a homogeneous ligament/pore structure, whereas the microstructures of Ag30Al70 and Ag40Al60 showed thinner and coarser ligament structures, respectively.

Formation of hierarchically-ordered nanoporous silver foam and its electrocatalytic properties in reductive dehalogenation of organic compounds

Nanostructuring of silver notably improved its electrocatalytic activity in reductive dehalogenation of a variety of aryl and alkyl bromides.

A high-performance SDC-infiltrated nanoporous silver cathode with superior thermal stability for low temperature solid oxide fuel cells

A simple and effective infiltration method is demonstrated to improve the thermal stability and electrochemical performance of a nanoporous silver cathode.