Nanoporous Ag Research Articles

(Invited) Electrochemical Tuning Sensing and Catalytic Performance of Catalytic Materials -from Nanoscale to Atomic Scale

In this talk, we will introduce two facile methods for the preparation of a nanoporous Ag foam and a promising electrocatalyst for the oxygen reduction reaction (ORR). Ag has a high electrical and thermal conductivity, low cost, and is relatively abundant in the earth compared to other Pt-group metals. Unlike Pt-related noble metals, the poor hydrogen evolution reaction (HER) activity of Ag in acidic environments makes it rarely considered as a potential HER electrocatalyst. We demonstrate a spongy-like highly nanoporous Ag foam obtained using a simple Ag surface treatment through the repetitively electrochemical anodic formation of AgBr/cathodic reduction back to Ag exhibited Pt-like high HER activities. Bimetallic Pt-Ni with Pt on the outermost layer and an innermost layer enriched in Ni, referred to as (Pt3Ni(Pt-skin), is a promising configuration of electrocatalyst for the ORR in fuel cells. We prepare a core (Pd)/shell (Pt3Ni(Pt-skin)) catalyst (Pt3Ni(Pt-skin)/Pd/C) from Zn underpotential deposition (UPD) on a Ni UPD modified Pd/C catalyst, facilitating Pt atomic layer-by-layer growth on the Ni surface through the galvanic replacement process. The Pt3Ni(Pt-skin) thin layer with ultra-low Pt loading presents not only a breakthrough in Pt specific activity and Pt mass activity, both of which were superior to state-of-the-art Pt-Ni catalysts, but also in high durability owing to the perfect Pt3Ni(Pt-skin) construction

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.

Formation of ultrafine spongy nanoporous metals (Ni, Cu, Pd, Ag and Au) by dealloying metallic glasses in acids with capping effect

The formation of ultrafine spongy nanoporous Ni, Cu, Pd, Ag and Au obtained by dealloying Mg-based metallic glasses in citric acid, tartaric acid, malic acid and amber acid is systematically investigated. The nanoporosity can generally be tuned by the capping efficiency of the acids, the precursor composition and dealloying conditions. Capping effect dominated dealloying is responsible for the novel nanoporosity via significantly increasing the activation energy of the surface diffusion process. This work demonstrates that capping agents involved dealloying is efficient to obtain ultrafine spongy nanoporosity and is important to further understand the dealloying mechanisms.

Facile preparation of nanoporous Ag decorated with CeO2 nanoparticles for surface-enhanced Raman scattering

Abstract

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.

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.

Design and preparation of nanoporous Ag–Cu alloys by dealloying Mg–(Ag,Cu)–Y metallic glasses for antibacterial applications

Highly antibacterial and long-term oxidation-resistant nanoporous Ag–Cu alloys were fabricated by dealloying Mg–(Ag,Cu)–Y metallic glasses.

Nanostructured amalgams with tuneable silver–mercury bonding sites for selective electroreduction of carbon dioxide into formate and carbon monoxide

Nanoporous Ag–Hg amalgams with tuneable Ag–Hg bonding sites exhibit switchable product selectivity of CO2electroreduction towards formate or CO.

Ag surface segregation in nanoporous Au catalysts during CO oxidation

The present study focuses on the modification of surface compositional profiles induced in nanoporous (NP) Au catalysts by the catalytic oxidation of carbon monoxide to carbon dioxide in the presence of oxygen. The phenomenon has deep implications concerning the catalytic behavior of NP Au foams in particular, and more in general for the design of more efficient catalysts. Aimed at gaining deeper insight into the mechanisms governing surface segregation, we exposed NP Au foams containing residual Ag to a mixture of gaseous carbon monoxide and oxygen at different temperature. Structural and surface composition analyses pointed out the concomitant occurrence of both NP Au coarsening and Ag surface segregation processes. Experimental findings suggest for Ag surface segregation a two-stage kinetics. During the initial, rapid coarsening of the NP Au structure, Ag surface segregation is mediated by surface rearrangements, which allow the Ag atoms to reach the surface at anomalously fast rate. As coarsening decelerates, the slower diffusion of buried Ag atoms towards the surface predominates, due to favorable chemical interactions with adsorbed oxygen. This novel mechanism’s understanding can benefit strategic areas of science and technology.

Study of the growth mechanisms of nanoporous Ag flowers for non-enzymatic glucose detection

Highly sensitive and selective non-enzymatic glucose detection was developed using nanoporous Ag flowers on a Ni substrate. The cyclic scanning electrodeposition (CSE) method was used to fabricate Ag flowers on a Ni substrate in an alkaline electrolyte. The nanoporous Ag flowers were then formed by repeated CSE in NaOH. The growth mechanisms of the nanoporous Ag flowers were systematically studied, and these mechanisms can be extended to the formation of other metal, bimetal or metal oxide. The synthesized three-dimensional nanoporous Ag flowers on the Ni substrate were used in the electro-oxidation of glucose, demonstrating a wide linear range (0.1 μM to 1 mM), fast response time (<2 s), low detection limit of 0.1 μM (S/N = 3) and a high sensitivity to detect glucose in the presence of uric acid (UA) and ascorbic acid (AA) at the level of their physiological concentrations. Apart from the nanoporous Ag flowers, the formation of a NiO thin layer on the Ni substrate during CSE also contributed to the high selectivity. This work indicates the potential for developing a fast, sensitive, selective and stable electrochemical sensor for diabetes diagnosis.

Nanoporous Ag Films Prepared by Cluster‐Source Sputtering as Substrates for Surface‐Enhanced Raman Scattering

By mounting a cluster source on a conventional sputtering system, a new sputtering system is devised for the deposition of nanoporous metal thin films at room temperature. First, the diameter of the nozzle is fixed while the pressure inside the cluster source is kept at several hundreds of mTorr. By adjusting the length of the cluster source, nanoporous films of different thickness can be formed at room temperature. The Raman response characteristics according to the process conditions are then analyzed, and the applicability of the sputtered thin film as a substrate for surface‐enhanced Raman scattering (SERS) is examined. The results show that the Raman intensity increases with increasing film thickness and reaches a saturated value at a thickness of 2 µm. Therefore, the proposed system takes advantage of an existing sputtering process, and it is possible to form a nanoporous metal film with thickness of several micrometers or more that can be used as a Raman substrate.

Evolution of the nanoporous microstructure of sintered Ag at high temperature using in-situ X-ray nanotomography

Silver pastes sintering is a potential candidate for die bonding in power electronic modules. The joints, obtained by sintering, exhibit a significant pore fraction, thus reducing the density of the material compared to bulk silver. The mechanical properties (Young's modulus, yield strength and ultimate tensile stress) are then drastically altered. However, while careful analysis of the nanoporous structure has been reported in 2D, little is known about its quantitative spatial evolution during thermal aging and more specifically during temperature jumps endured by the assembly during operation. Here, high temperature evolutions of the 3D nanoporous structure have been observed in-situ using a heater fitted into the beamline 6-2c at SLAC-SSRL. Segmentation of the porosity and subsequent statistical analysis of the tomographic dataset reveal a complex evolution of the porous nanostructure including growth, separation and coalescence at a constant density. Such an analysis provides insight into the microstructural evolution of sintered nanoporous Ag joints in-service.

Tunable magnetic properties of the nanoporous hybrid multilayer arrays

Hybrid multilayer films system permits a lot of realization methods of tailoring of the magnetic properties. In this work, nanoporous hybrid multilayer system Ag was fabricated from anodic aluminum oxide (AAO) by a magnetron sputtering methods of room temperature. It is demonstrated that the magnetic properties can be flexibly manipulated by the substrate, noble metal (i.e. silver), the thickness of the ferromagnetic magnetic material (CoFeB) and the indium tin oxide (ITO).

Reduction-Induced Decomposition: Spontaneous Formation of Monolithic Nanoporous Metals of Tunable Structural Hierarchy and Porosity

The beauty of dealloying, i.e., the selective dissolution of an alloy, lies in the spontaneous formation of bicontinuous nanoporous metal, whose robustness, conductivity, and high specific area are otherwise difficult to achieve in a monolithic form. However, a uniform structure of nanoporous metal requires a homogeneous alloy precursor, whose laborious fabrication has been limiting the application of dealloying and dealloyed materials. Here, we replace the alloy precursor with a compound, and design another type of selective dissolution, reduction-induced decomposition (RID). Using the RID of AgCl as an example, we chemically reduced bulk AgCl samples to create bicontinuous nanoporous Ag that resembles dealloyed structures. The monolithic material possesses a uniform ligament width of 72 nm and a specific area of 7.57 m2/g. The ligament width can be tuned in a range from 30 nm to 1 μm through coarsening, and the porosity from 57% to 87% by replacing silver cations in the compound with sodium cations. The...

Making Ag Present Pt-like Activity for Hydrogen Evolution Reaction

Silver (Ag) has a high electrical and thermal conductivity, low cost, and is relatively abundant in the earth compared to other Pt-group metals. Unlike Pt-related noble metals, the poor hydrogen evolution reaction (HER) activity of Ag in acidic environments makes it rarely considered as a potential HER electrocatalyst. We demonstrate a spongy-like highly nanoporous Ag foam obtained using a simple Ag surface treatment through the repetitively electrochemical anodic formation of AgBr/cathodic reduction back to Ag exhibited Pt-like high HER activities. The spongy-like nanoporous Ag foam effectively increases the number of surface active sites and lowers the Gibbs free energy of adsorbed atomic hydrogen (Had) on catalysts (ΔGM-H) from multiple Ag grains constructing nanoarchitecture branches.

Conversion Reaction Synthesis: A Versatile Route to Nanoporous Transition Metals

Nanoporous Metals (NPMs) are nanostructured pure metals with pores ranging from approximately 1-100 nm in diameter. They have been sought-after for their unique combinations of metallic characteristics and nanostructured size-effect properties, ranging from catalysis, plasmonics, battery electrodes to actuators. Many synthesis routes for nanoporous metals have been developed in the past 30 years, the oldest and most popular being the dealloying method. This method involves chemically or electrochemically removing the less-noble element from an alloy, leaving behind a nanoporous metal. Other techniques include combustion synthesis, block-copolymer templating, or directed assembly of metal nanoparticles. Here, we demonstrate a scalable, room temperature synthesis method capable of producing a variety of nanoporous metal structures. Nanoporous Fe, Co, Au, Cu, Ag and Ni were formed by reacting metal-halide salts with organolithium reducing agents to form a metal/lithium salt nanocomposite. During this conversion reaction, the metal phase coalesces into an interconnected network of fine metal filaments surrounded by a lithium salt matrix. When the lithium salt is removed via dissolution in a polar organic solvent, the metal filament network remains and is an effective nanoporous pure metal (See Figure 1 for representative images). Nitrogen adsorption analysis and microscopy studies showed that these materials have high specific surface area (up to ~160 m2/g) with pore diameters ranging from 2 nm to 50 nm. This technique can also synthesize hybrid nanoporous structures of two or more metals by performing the conversion reaction on mixed precursors. Metals (e.g. Co and Cu) were found to stabilize each other when converted into a hybrid nanoporous structure, resulting in a mixed nanoporous metal with smaller pore sizes and higher surface areas than each respective element in its pure nanoporous metal form. This synthesis pathway greatly expands our access to new compositions and microstructures of nanoporous metals. Figure 1

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.

Flexible Bimetallic Nanoporous Cu-Ag Synthesized by Electrochemical Dealloying for Battery-Type Electrodes with High Electrochemical Performance

In this work, a flexible three-dimensional nanoporous bimetallic Cu-Ag (NPCuAg) has been fabricated by a simple electrochemically dealloying Zr45Cu35Ag20 glassy ribbon in HF aqueous solution. The influence of dealloying treatment time on the thickness of nanoporous layer and the flexibility of the dealloyed ribbon was investigated. It is found that the thickness of NPCuAg is approximately proportional to dealloying treatment time, and the NPCuAg shows a maximum nanoporous layer ∼9 μm (one side) while possessing good flexibility. Furthermore, nanoporous CuOAg (NPCuOAg) was obtained through a simple oxidation treatment. The NPCuOAg shows a hierarchical structure where CuO nanosheets are uniformly embedded in 3D nanoporous Ag skeleton. The NPCuOAg battery-type electrode exhibits superior electrochemical performance, which has great potential application in flexible high-performance energy storage devices.

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.