Electrochemical Synthesis of Nanostructured Ordered Intermetallic Materials under Ambient Conditions.

ConspectusThe enhanced catalytic properties of alloy nanostructures have made them a focus of extensive research in the field of catalysis. Alloy nanostructures can be classified into two types: disordered alloys (also known as solid solutions) and ordered intermetallics. The latter are of particular interest as they possess long-range atomic scale ordering, which leads to well-defined active sites that can be used to accurately assess structure-property relationships and their impact on (electro)catalytic performance.While many ordered intermetallics (OICs) have been synthesized and evaluated as electrocatalysts, there is still a lack of understanding on how the local structure of atoms controls their catalytic performance. Ordered intermetallics are difficult to synthesize and often require high-temperature annealing for the atoms to equilibrate into ordered structures. High temperature processing results in aggregated structures (usually >30 nm) and/or contamination from the support, which can decrease their performance and preclude these materials from being used as model systems for elucidating insight into structure and electrochemical properties. Therefore, alternative methods are required to enable more efficient atomic ordering while maintaining some level of morphological control.This Account delves into the potential of electrochemical methods as a practical alternative for synthesizing ordered intermetallics at lower temperatures. Specifically, it explores the viability of electrochemical dealloying and electrochemical deposition to synthesize Pd-Bi and Cu-Zn intermetallics at room temperature and atmospheric pressure. These methods have proven useful in synthesizing phases that are typically inaccessible under ambient conditions. The high homologous temperatures at which these materials are synthesized provide the necessary atomic mobility required for equilibration and formation of ordered phases, thus making the direct synthesis of ordered intermetallic materials at room temperature by electrochemical means a reality.Beyond synthesis, the electrocatalytic performance of these intermetallics was assessed for the oxygen reduction reaction (ORR), which is an important process employed in fuel cells. The OICs displayed increased performance with respect to commercial Pd/C and Pt/C benchmarks because of lower coverages of spectator species. Furthermore, these materials exhibited improved methanol tolerance.This Account provides valuable insights into the electrochemical synthesis of ordered intermetallics and their potential use as highly effective catalysts for electrocatalytic reactions. By using electrochemical methods, it is possible to obtain ordered intermetallics with unique atomic arrangements and tailored properties, which can be optimized for specific catalytic applications. With further research, electrochemical synthesis methods may enable the development of new and improved ordered intermetallics with even higher catalytic activity and selectivity, making them ideal candidates for use in a wide range of industrial processes. Furthermore, the ability to access intermetallics under milder conditions may accelerate the ability to use these materials as model systems for revealing fundamental insight into electrocatalyst structure and function.

Sensitive organic electrochemical transistor biosensors: Comparing single and dual gate functionalization and different COOH-functionalized bioreceptor layers.

We developed new measurement configurations based on organic electrochemical transistors (OECTs). Three types of COOH-functionalized bioreceptor layers were deposited on indium tin oxide (ITO) electrodes on poly(ethylene terephthalate) (PET) substrates and their performance was tested using single gate functionalization organic electrochemical transistor (S-OECT) and dual gate functionalization organic electrochemical transistor (D-OECT) configurations. The three layers included one p-type semiconductor, one insulator, and one self-assembled layer, and the dual gates were connected in series through buffer solutions, so the solution-electrode interfaces had the opposite polarities. We investigated the sensitivities of these systems using the human IgG antigen-human IgG antibody receptor pair for main experiments, and drifts of antibody-functionalized gates without analytes as control experiments. Drifts without analyte can obscure the real sensitivity. We show that the D-OECT has the capability to cancel the drifts, and is also beneficial for showing the sensitivity more exactly. This configuration has the ability to increase the accuracy of antibody-antigen interaction detection, and further decrease or eliminate the effect of ions in the buffer solution. We also prove that the D-OECT can work well with different bioreceptor materials, which indicates that the system can be further applied to different conditions.

Promoting Bifunctional Water Splitting by Modification of the Electronic Structure at the Interface of NiFe Layered Double Hydroxide and Ag.

Electrochemical water splitting is a promising method for the renewable production of high-purity hydrogen via the hydrogen evolution reaction (HER). Ni-Fe layered double hydroxides (Ni-Fe LDHs) are highly efficient materials for mediating the oxygen evolution reaction (OER), a half-reaction for water splitting at the anode, but LDHs typically display poor HER performance. Here, we report the preparation of self-organized Ag@NiFe layered double hydroxide core-shell electrodes on Ni foam (Ag@NiFe/NF) prepared by galvanic etching for mediating both the HER and OER (bifunctional water-splitting electrocatalysis). This synthetic strategy allowed for the preparation of organized hierarchical architectures which displayed improved the electrochemical performance by tuning the electronic structure of the catalyst and increasing the surface area utilization. X-ray photoelectron spectroscopy (XPS) and theoretical calculations revealed that electron transfer from the Ni-Fe LDH to Ag influenced the adsorption of the reaction intermediates leading to enhanced catalytic activity. The Ag@NiFe/NF electrode displayed overpotentials as low as 180 and 80 mV for oxygen and hydrogen evolution, respectively, at a current density of 10 mA cm-2, and improvements in the specific activity by ∼5× and ∼1.5× for the oxygen and hydrogen evolution reaction, respectively, compared to benchmark NiFe hydroxide materials. Additionally, an integrated water-splitting electrolyzer electrode can be driven by an AA battery.

Ordered Intermetallic Pd3Bi Prepared by an Electrochemically Induced Phase Transformation for Oxygen Reduction Electrocatalysis.

The synthesis of alloys with long-range atomic-scale ordering (ordered intermetallics) is an emerging field of nanochemistry. Ordered intermetallic nanoparticles are useful for a wide variety of applications such as catalysis, superconductors, and magnetic devices. However, the preparation of nanostructured ordered intermetallics is challenging in comparison to disordered alloys, hindering progress in material development. Herein, we report a process for converting colloidally synthesized ordered intermetallic PdBi2 to ordered intermetallic Pd3Bi nanoparticles under ambient conditions by electrochemical dealloying. The low melting point of PdBi2 corresponds to low vacancy formation energies, which enables the facile removal of the Bi from the surface while simultaneously enabling interdiffusion of the constituent atoms via a vacancy diffusion mechanism under ambient conditions. The resulting phase-converted ordered intermetallic Pd3Bi exhibits 11 times and 3.5 times higher mass activity and high methanol tolerance for the oxygen reduction reaction compared with Pt/C and Pd/C, respectively, which is the highest reported for a Pd-based catalyst, to the best of our knowledge. These results establish a key development in the synthesis of noble-metal-rich ordered intermetallic phases with high catalytic activity and set forth guidelines for the design of ordered intermetallic compounds under ambient conditions.

Rapid Room-Temperature Synthesis of a Metastable Ordered Intermetallic Electrocatalyst.

Metal alloys with atomic scale ordering (ordered intermetallics) have emerged as a new class of high performance materials for mediating electrochemical reactions. However, ordered intermetallic nanostructures often require long synthesis times and/or high temperature annealing to form because a high-activation energy barrier for interdiffusion must be overcome for the constituent metals to equilibrate into ordered structures. Here we report the direct synthesis of metastable ordered intermetallic Pd31Bi12 at room-temperature in minutes via electrochemical deposition. Pd31Bi12 is highly active for the reduction of O2 to H2O, delivering specific activities over 35× higher than those of commercial Pt and Pd nanocatalysts, placing it as the most active Pd-based catalyst, to the best of our knowledge, reported under similar testing conditions. Stability tests demonstrate minimal loss of activity after 10,000 cycles, and a retention of intermetallic crystallinity. This study demonstrates a new method of preparing ordered intermetallics with extraordinary catalytic activity at room temperature, providing a new direction in catalyst discovery and synthesis.

Hierarchically Ordered Two-Dimensional Coordination Polymers Assembled from Redox-Active Dimolybdenum Clusters.

Coordination polymers (CPs) supporting tunable through-framework conduction and responsive properties are of significant interest for enabling a new generation of active devices. However, such architectures are rare. We report a redox-active CP composed of two-dimensional (2D) lattices of coordinatively bonded Mo2(INA)4 clusters (INA = isonicotinate). The 2D lattices are commensurately stacked and their ordering topology can be synthetically tuned. The material has a hierarchical pore structure (pore sizes distributed between 7 and 33 Å) and exhibits unique CO2 adsorption (nominally Type VI) for an isotherm collected at 195 K. Furthermore, cyclic voltammetry and electrokinetic analyses identify a quasi-reversible feature at E1/2 = -1.275 V versus ferrocene/ferrocenium that can be ascribed to the [Mo2(INA)4]0/-1 redox couple, with an associated standard heterogeneous electron transfer rate constant ks = 1.49 s-1. The tunable structure, porosity, and redox activity of our material may render it a promising platform for CPs with responsive properties.

Tuning of Silver Catalyst Mesostructure Promotes Selective Carbon Dioxide Conversion into Fuels.

An electrode's performance for catalytic CO2 conversion to fuels is a complex convolution of surface structure and transport effects. Using well-defined mesostructured silver inverse opal (Ag-IO) electrodes, it is demonstrated that mesostructure-induced transport limitations alone serve to increase the turnover frequency for CO2 activation per unit area, while simultaneously improving reaction selectivity. The specific activity for catalyzed CO evolution systematically rises by three-fold and the specific activity for catalyzed H2 evolution systematically declines by ten-fold with increasing mesostructural roughness of Ag-IOs. By exploiting the compounding influence of both of these effects, we demonstrate that mesostructure, rather than surface structure, can be used to tune CO evolution selectivity from less than 5 % to more than 80 %. These results establish electrode mesostructuring as a powerful complementary tool for tuning both catalyst selectivity and efficiency for CO2 conversion into fuels.

Mesostructure-Induced Selectivity in CO2 Reduction Catalysis.

Gold inverse opal (Au-IO) thin films are active for CO2 reduction to CO with high efficiency at modest overpotentials and high selectivity relative to hydrogen evolution. The specific activity for hydrogen evolution diminishes by 10-fold with increasing porous film thickness, while CO evolution activity is largely unchanged. We demonstrate that the origin of hydrogen suppression in Au-IO films stems from the generation of diffusional gradients within the pores of the mesostructured electrode rather than changes in surface faceting or Au grain size. For electrodes with optimal mesoporosity, 99% selectivity for CO evolution can be obtained at overpotentials as low as 0.4 V. These results establish electrode mesostructuring as a complementary method for tuning selectivity in CO2-to-fuels catalysis.

A New Synthetic Route to Microporous Silica with Well-Defined Pores by Replication of a Metal-Organic Framework.

Microporous amorphous hydrophobic silica materials with well-defined pores were synthesized by replication of the metal-organic framework (MOF) [Cu3 (1,3,5-benzenetricarboxylate)2 ] (HKUST-1). The silica replicas were obtained by using tetramethoxysilane or tetraethoxysilane as silica precursors and have a micro-meso binary pore system. The BET surface area, the micropore volume, and the mesopore volume of the silica replica, obtained by means of hydrothermal treatment at 423 K with tetraethoxysilane, are 620 m(2) g(-1) , 0.18 mL g(-1) , and 0.55 mL g(-1) , respectively. Interestingly, the silica has micropores with a pore size of 0.55 nm that corresponds to the pore-wall thickness of the template MOF. The silica replica is hydrophobic, as confirmed by adsorption analyses, although the replica has a certain amount of silanol groups. This hydrophobicity is due to the unique condensation environment of the silica precursors in the template MOF.

Microporous brookite-phase titania made by replication of a metal-organic framework.

Metal-organic frameworks (MOFs) provide access to structures with nanoscale pores, the size and connectivity of which can be controlled by combining the appropriate metals and linkers. To date, there have been no reports of using MOFs as templates to make porous, crystalline metal oxides. Microporous titania replicas were made from the MOF template HKUST-1 by dehydration, infiltration with titanium isopropoxide, and subsequent hydrothermal treatment at 200 °C. Etching of the MOF with 1 M aqueous HCl followed by 5% H2O2 yielded a titania replica that retained the morphology of the parent HKUST-1 crystals and contained partially ordered micropores as well as disordered mesopores. Interestingly, the synthesis of porous titania from the HKUST-1 template stabilized the formation of brookite, a rare titania polymorph.

Broadband light absorption with multiple surface plasmon polariton waves excited at the interface of a metallic grating and photonic crystal.

Light incident upon a periodically corrugated metal/dielectric interface can generate surface plasmon polariton (SPP) waves. This effect is used in many sensing applications. Similar metallodielectric nanostructures are used for light trapping in solar cells, but the gains are modest because SPP waves can be excited only at specific angles and with one linear polarization state of incident light. Here we report the optical absorptance of a metallic grating coupled to silicon oxide/oxynitride layers with a periodically varying refractive index, i.e., a 1D photonic crystal. These structures show a dramatic enhancement relative to those employing a homogeneous dielectric material. Multiple SPP waves can be activated, and both s- and p-polarized incident light can be efficiently trapped. Many SPP modes are weakly bound and display field enhancements that extend throughout the dielectric layers. These modes have significantly longer propagation lengths than the single SPP modes excited at the interface of a metallic grating and a uniform dielectric. These results suggest that metallic gratings coupled to photonic crystals could have utility for light trapping in photovoltaics, sensing, and other applications.

Wafer-scale fabrication of plasmonic crystals from patterned silicon templates prepared by nanosphere lithography.

By combining nanosphere lithography with template stripping, silicon wafers were patterned with hexagonal arrays of nanowells or pillars. These silicon masters were then replicated in gold by metal evaporation, resulting in wafer-scale hexagonal gratings for plasmonic applications. In the nanosphere lithography step, two-dimensional colloidal crystals of 510 nm diameter polystyrene spheres were assembled at the air-water interface and transferred to silicon wafers. The spheres were etched in oxygen plasma in order to define their size for masking of the silicon wafer. For fabrication of metallic nanopillar arrays, an alumina film was grown over the nanosphere layer and the spheres were then removed by bath sonication. The well pattern was defined in the silicon wafer by reactive ion etching in a chlorine plasma. For fabrication of metal nanowell arrays, the nanosphere monolayer was used directly as a mask and exposed areas of the silicon wafer were plasma-etched anisotropically in SF6/Ar. Both techniques could be used to produce subwavelength metal replica structures with controlled pillar or well diameter, depth, and profile, on the wafer scale, without the use of direct writing techniques to fabricate masks or masters.