Oxygen Evolution Reaction on Nanoporous Gold Modified with Ir and Pt: Synergistic Electrocatalysis between Structure and Composition

Active oxygen evolution reaction electrocatalysts for water splitting have received great attention because of their importance in the utilization of renewable energy sources. Here, the electrochemical oxygen evolution reaction activities of a nanoporous gold (NPG)‐based electrode in acidic media are investigated. The dependence of the oxygen evolution reaction activity on the NPG surface area shows that the large electrochemical surface areas of the NPG are effectively utilized to enhance electrocatalytic activity. The NPG surfaces are modified with Pt using atomic layer electrodeposition methods, and the resulting NPG@Pt exhibited enhanced electrocatalytic activities compared to those of the NPG and flat Pt electrodes. Ir‐modified NPG (NPG@Ir) electrodes are prepared by spontaneous exchange of Ir on NPG surfaces and exhibit enhanced electrocatalytic activity compared to that of flat Ir surfaces. The modification of NPG@Pt with Ir results in NPG@Pt/Ir electrodes, and their electrocatalytic activities exceed those of NPG@Ir. The enhanced oxygen evolution reaction activity on NPG@Pt/Ir over that on NPG@Ir surfaces is examined by X‐ray photoelectron spectroscopy. The oxygen evolution reaction activity on NPG@Pt/Ir surfaces demonstrates synergistic electrocatalysis between the nanoporous surface structure and active electrocatalytic components.     

Electrochemical Biosensor Based on Nanoporous Au/CoO Core–Shell Material with Synergistic Catalysis

An ultrathin CoO layer is deposited on the skeleton surfaces of a nanoporous gold (NPG) film by using atomic layer deposition, creating a flexible electrode. Detailed characterization demonstrates the superior performance of the flexible NPG/CoO hybrids for electrochemical catalysis. The NPG/CoO hybrid not only achieves high catalytic activity for glucose oxidation and H₂O₂ reduction, but also exhibits a linear dependence of the electrical signal on the concentration of glucose and H₂O₂ molecules in the electrolyte. Meanwhile, the sensitivity for H₂O₂ reduction can be as high as 62.5 μA mm ^?1 cm^?2 with linear dependence on the concentration in the range of 0.1–92.9 mm . The high sensitivity is proposed to result from the synergistic effect of Au and CoO at the interfaces, and the high conductivity of the gold skeleton with a large surface area. The superior electrochemical performance of this hybrid electrode is promising for future potential applications in various transitional‐metal‐oxide‐based electrochemical electrodes.     

Nanoporous Au Formation on Au Substrates via High Voltage Electrolysis**

Nanoporous Au (NPG) films have promising properties, making them suitable for various applications in (electro)catalysis or (bio)sensing. Tuning the structural properties, such as the pore size or the surface-to-volume ratio, often requires complex starting materials such as alloys, multiple synthesis steps, lengthy preparation procedures or a combination of these factors. Here we present an approach that circumvents these difficulties, enabling for a rapid and controlled preparation of NPG films starting from a bare Au electrode. In a first approach a Au oxide film is prepared by high voltage (HV) electrolysis in a KOH solution, which is then reduced either electrochemically or in the presence of H₂O₂. The resulting NPG structures and their electrochemically active surface areas strongly depend on the reduction procedure, the concentration and temperature of the H₂O₂-containing KOH solution, as well as the applied voltage and temperature during HV electrolysis. Secondly, the NPG film can be prepared directly by applying voltages that result in anodic contact glow discharge electrolysis (aCGDE). By carefully adjusting the corresponding parameters, the surface area of the final NPG film can be specifically controlled. The structural properties of the electrodes are investigated by means of XPS, SEM and electrochemical methods.     

Facet-Dependent Formation and Adhesion of Au Oxide and Nanoporous Au on Poly-Oriented Au Single Crystals**

Nanoporous Au (NPG) has different properties compared to bulk Au, making it an interesting material for numerous applications. To modify the structure of NPG films for specific applications, e. g., the porosity, thickness, and homogeneity of the films, a fundamental understanding of the structure formation is essential. Here, we focus on NPG prepared via electrochemical reduction from Au oxide formed during high voltage (HV) electrolysis on poly-oriented Au single crystal (Au POSC) electrodes. These POSCs consist of a metal bead, with faces with different crystallographic orientations and allow screening of the influence of crystallographic orientation on the structure formation for different facets in one experiment. The HV electrolysis is performed between 100 ms and 30 s at 300 V and 540 V. The amount of Au oxide formed is determined by electrochemical measurements and the structural properties are investigated by scanning electron and optical microscopy. We show that the formation of Au oxide is mostly independent of the crystallographic orientation, except for thick layers, while the macroscopic structure of the NPG films depends on experimental parameters such as the Au oxide precursor thickness and the crystallographic orientation of the substrate. Possible reasons for the frequently observed exfoliation of the NPG films are discussed.     

Atomic Layer Electrodeposition of Pt on Nanoporous Au and its Application in pH Sensing

The preparation of nanoporous metal structures has received a substantial amount of attention because of the unique properties and various applications of these structures. In this work, the preparation of nanoporous Pt structures by modification of nanoporous gold (NPG) surfaces with Pt was achieved. An atomic layer electrodeposition (ALED) technique previously reported for the modification of flat Au surfaces with Pt was applied to the NPG surfaces to produce Pt‐modified NPG structures. The optimal ALED parameters, such as deposition potential, time, and number of cycles, for the preparation of Pt‐modified NPG structures were investigated. Scanning electron microscopy and energy‐dispersive X‐ray analysis confirmed the successful preparation of nanoporous Pt structures by ALED techniques. The Pt‐modified NPG performed well as a pH sensor with a Nernstian slope and negligible hysteresis. The method of preparing the nanoporous Pt structures reported in this work could be utilized in various applications such as electrocatalysis and electroanalysis.     

Electroless deposition of gold thin films on silicon for surface-enhanced infrared spectroelectrochemistry

Electroless (or chemical) deposition technique has been used in preparing Au island film electrodes on Si for in situ infrared spectroscopic studies of the electrochemical interface in attenuated total reflection mode. Owing to surface-enhanced infrared absorption (SEIRA) effect, absorption bands of molecules adsorbed on the chemically deposited films were one order of magnitude as large as those observed on smooth Au electrode surfaces. Although the enhancement factor was identical to that observed on vacuum evaporated Au island films, this simple method is superior to vacuum evaporation method with respect to the adhesion of the film, surface contamination, reproducibility, and cost.     

Al Electrode Modified by Au Atoms as a Novel Electrode for Electrocatalytic Oxidation of Thiosulfate

An aluminum electrode modified with gold atoms was introduced as a novel electrode. Gold atoms were deposited both chemically and electrochemically onto the aluminum electrode to make an aluminum/gold (Al/Au) modified electrode (ME). The experimental results showed that the Al/Au modified electrode prepared by chemical deposition, exhibits much more current than the electrochemical deposition method. The electrochemical behavior of the Al/Au modified electrode was studied by cyclic voltammometry. This modified electrode showed two pairs of peaks, a₁c₁ and a₂c₂, with surface‐confined characteristics in a 0.5 M phosphate buffer. The dependence of E_pa of the second peak (a₂c₂) on pH shows a Nernestian behavior with a slope of 55 mV per unit pH. The effect of different supporting electrolytes, solution's pH and different scan rates on electrochemical behavior of Al/Au modified electrode was studied. Au deposited electrochemically on a Pt electrode (Pt/Au) was also used as another modified electrode. A comparative study of electrochemical behavior of bare Al, Pt/Au and Al/Au modified electrodes showed that both Pt/Au and Al/Au electrodes have the ability of electrocatalytic oxidation of S₂O₃^2?, but the electrocatalytic oxidation on the latter was better than the former. The kinetics of the catalytic reaction was investigated by using cyclic voltammetry and chronoamperometry techniques. The average value of the rate constant for the catalytic reaction and the diffusion coefficient were evaluated by means of chronoamperometry technique.     

Electrooxidation of Glucose at Nanoporous Gold Surfaces: Structure Dependent Electrocatalysis and Its Application to Amperometric Detection

Electrooxidation of glucose is investigated at nanoporous gold (NPG) with controlled surface structures by applying different deposition charges during the formation of Ag? Au layers. As the deposition charge increases, the NPG surfaces exhibit smaller ligament/pore structures and the electrocatalytic oxidation of glucose becomes more effective. Voltammetric responses of NPG suggest that the electrocatalytic oxidation arises from the enrichment of (110) or (100) surface orientation of gold with higher deposition charges. The electrooxidation of glucose is retained at NPG surfaces with higher deposition charges in the presence of Cl^?, which suggests possible applications to the amperometric glucose detection in biological samples.     

Electrochemical Studies of Chemically Modified Nanometer‐Sized Electrodes

Self‐assembled monolayers (SAMs) of 4‐aminothiophenol (4‐ATP) has been successfully deposited onto nanometer‐sized gold (Au) electrodes. The cyclic voltammograms obtained on a 4‐ATP SAMs modified electrode show peaks and the peak height is proportional to the scan rate, which is similar to that on an electroactive SAMs modified macro electrode. The electrochemical behavior and mechanism of outer‐sphere electron transfer reaction on the 4‐ATP SAMs modified nanometer‐sized electrode has also been studied. The 4‐ATP SAMs modified electrode is further modified with platinum (Pt) nanoparticles. Electrochemical behaviors show that there is electrical communication between Pt nanoparticles and Au metal on the Pt nanoparticles/4‐ATP SAMs/Au electrode. However, scanning electron microscopic image shows that the Pt nanoparticles are not evenly covered the electrode.     

Voltammetric behavior of gold nanotrench electrodes

We report the fabrication and electrochemical response of a gold nanoband electrode located at the bottom of a glass/epoxy nanotrench, hereafter referred to as a gold nanotrench electrode. Gold nanotrench electrodes of 12.5 and 40 nm in width with various depths from a few tens of nanometers to approximately 4 μm are fabricated and further characterized by cyclic voltammetry. The fabrication of a Au nanotrench electrode follows a simple electrochemical etching process in which a small AC signal is applied to an inlaid Au nanoband electrode submersed in a NaCl solution. The voltammetric behavior of a Au nanotrench electrode is characterized by a quasi-steady-state response at lower scan rates (e.g., <1 V/s for a 12.5-nm-wide electrode). We present an analytical expression for the quasi-steady-state diffusion-limited current of the nanotrench electrode based upon the analysis of the mass-transport resistance. Finite-element simulation of steady-state and transient voltammetric responses of the nanotrench electrodes provides additional insights for the analytical model. Peak-shaped transient voltammetric responses were observed at scan rates as low as 5 V/s for both inlaid and nanotrench electrodes. This result may suggest that the exposed area of the nanoband electrode is much greater than that expected from the fabrication of the inlaid bands. However, the extent to which this is seen is greatly decreased in the nanotrench electrode by a smoothing effect during etching. Our results confirm previous reports of excess overhanging metal and delamination crack contributing significantly to the shape and magnitude of the voltammetric response.     

Size-tuneable and micro-patterned iron nanoparticles derived from biomolecules via microcontact printing SAM-modified substrates and controlled-potential electrolyses

Site-selected and size-controlled iron nanoparticles were prepared on coplanar surfaces via microcontact printing of SAM-modified Au/mica electrodes and controlled-potential electrolytic reactions using ferritin biomolecules. Ferritin molecules packed like a full monolayer on 6-amino-1-hexanethiol (AHT)- and 11-amino-1-undecanethiol (AUT)-modified Au/mica surface via electrostatic interactions, which did not depend on the chain length of the amino terminal alkane thiols. After heat-treatment at 400 degrees C for 60 min, iron oxide nanoparticles (ca. 5 nm in diameter) derived from ferritin cores were observed at the Au/mica surface by atomic force microscopy (AFM). On the study on the electrochemistry of ferritin immobilized onto AHT- and AUT-modified Au/mica electrodes, the redox response of the ferritin immobilized AHT-modified electrode was clearly observed. On the other hand, no redox peak for ferritin was obtained at the AUT-modified electrode. The electron transfer between ferritin and the electrode through the AUT membrane could not take place. The difference in the electrochemical response of ferritin immobilized onto AHT- and AUT-modified Au/mica was caused by the chain length of the amino terminal alkane thiols. Uniform patterns of AHT and AUT on the Au/mica electrode surface were performed by use of a poly(dimethylsiloxane) (PDMS) stamp. After the immobilization of ferritin onto both AHT- and AUT-modified electrode surfaces, the modified electrode was applied to a -0.5 V potential for 30 min in a phosphate buffer solution. After this procedure, the PDMS stamp patterning image appeared by scanning electron microscopy (SEM) image. The SEM results induced by the size change of the ferritin core consisting of iron(III) by electrolysis.     

Ethanol oxidation at metal–zeolite-modified electrodes in alkaline medium. Part-1: gold–zeolite-modified graphite electrode

Gold–zeolite-modified graphite (AuZG) electrode shows higher catalytic activity for ethanol oxidation in alkaline medium compared with massive gold or gold-modified graphite (Au/G) electrodes. The activity of this electrode depends on the amount of zeolite loaded on the graphite surface and on the soaking time in Au³⁺ solution. The effects of both scan rate and ethanol concentration on the anodic peak height are indicatives of a diffusion-controlled process. Current decay measurements indicate that the activity of studied electrodes towards poisoning tolerance decreases in the following order: AuZG > Au/G > Au. This paper is dedicated to Prof. Dr. T. Iwasita for her 65th birthday.     

电化学氧化增强金属钴卟啉的自组装研究

应用电化学方法将钴(Ⅱ)卟啉氧化成钴(Ⅲ)卟啉以增强它与4-巯基吡啶(MP)自组装膜的轴向配位作用,从而快速制备了有序钴卟啉自组装修饰电极CoTMPP/MP/Au(E).电化学石英晶体微天平(EQCM)测试证实电化学氧化对钴卟啉膜生长过程的增强作用.Ram an光谱及修饰电极电催化还原氧研究显示,该修饰电极与经过长时间浸渍法得到的CoTMPP/MP/Au(I)修饰电极具有完全相似的有序结构和性质.与直接将钴卟啉吸附在电极表面的CoTMPP/Au修饰电极相比,以巯基吡啶为桥键得到的钴卟啉修饰电极具有更好的电催化活性和稳定性.     

Synthesis and electrocatalytic activity of Au/Pt bimetallic nanodendrites for ethanol oxidation in alkaline medium

Gold/Platinum (Au/Pt) bimetallic nanodendrites were successfully synthesized through seeded growth method using preformed Au nanodendrites as seeds and ascorbic acid as reductant. Cyclic voltammograms (CVs) of a series of Au/Pt nanodendrites modified electrodes in 1M KOH solution containing 1M ethanol showed that the electrocatalyst with a molar ratio (Au:Pt) of 3 exhibited the highest peak current density and the lowest onset potential. The peak current density of ethanol electro-oxidation on the Au(3)Pt(1) nanodendrites modified glassy carbon electrode (Au(3)Pt(1) electrode) is about 16, 12.5, and 4.5 times higher than those on the polycrystalline Pt electrode, polycrystalline Au electrode, and Au nanodendrites modified glassy carbon electrode (Au dendrites electrode), respectively. The oxidation peak potential of ethanol electro-oxidation on the Au(3)Pt(1) electrode is about 299 and 276 mV lower than those on the polycrystalline Au electrode and Au dendrites electrode, respectively. These results demonstrated that the Au/Pt bimetallic nanodendrites may find potential application in alkaline direct ethanol fuel cells (ADEFCs).     

Potential-dependant SERS interaction of ortho-substituted N-benzylamino(boronphenyl)methylphosphonic acid with Ag,Au, and Cu electrode surfaces

In this study we present surface-enhanced Raman spectroscopy (SERS) investigations of ortho-substituted N-benzylamino(boronphenyl)methylphosphonic acid (N-benzylamino-(2-boronphenyl)-R-methylphosphonic acid; o-PhR) adsorbed onto a roughened in oxidation–reduction cycles (ORC) Ag, Au, and Cu electrode surfaces at different applied electrode potentials. Based on the spectral changes in bandwidth and wavenumber of selected bands upon the alternation of the applied electrodes potential the manner and differences in the interaction of o-PhR with the Ag, Au, and Cu electrode surfaces were determined. Briefly, the spectral patterns on Ag and Au suggest that, generally, the both aromatic rings are involved in the o-PhR/electrode interaction, whereas the boronophenyl ring only interacts with Cu. Also, the boronic acid and phosphonic acid groups participate in the o-PhR interaction with all the types of electrodes. However, the type of the used electrode material and the applied electrode potential have influence onto the mode of adsorption.     

Preparation of a Novel NH2 + Ion Implantation Modified Electrode and Its Study of the Electrochemical Behavior of Hemoglobin

Abstract

A novel functional electrode was obtained by implanting NH₂ ⁺ into ITO film (NH₂/ITO) for the first time. The NH₂/ITO surface showed a better affinity to gold nanoparticles than bare ITO. Gold nanoparticles were deposited on the surface of NH₂/ITO electrode (Au/NH₂/ITO). The Au/NH₂/ITO and NH₂/ITO electrodes were used to observe the electrochemical behavior of Hemoglobin (Hb) immobilized on the electrodes surfaces. The peak current value of Hb immobilized on NH₂/ITO increased compared with on bare ITO while peak current value of Hb immobilized on Au/NH₂/ITO increased compared with on Au/ITO. Linkage between the ‐NH₂ implanted into the ITO film and the ‐COOH of Hb was thought to be the reason for the increase of active Hb coverage on NH₂/ITO compared with bare ITO. Increase of active Hb coverage on Au/NH₂/ITO compare with Au/ITO was attributed to the different amount of gold nanoparticles deposited. Results showed the novel NH₂/ITO and Au/NH₂/ITO electrodes exhibited good stability, reproducibility besides selectivity and sensitivity. The electrode process of Hb immobilized on Au/NH₂/ITO was quasi‐reversible with adsorption. The electrode reaction rate constant k_s and other related constants were determined. X‐ray photoelectron spectroscopy (XPS), field‐emission scanning electron microscopy (FE‐SEM), and impedance spectra were used to characterize the different surfaces.     

Electrochemical detection of hydrazine using a highly sensitive nanoporous gold electrode

A facile alloy–dealloy technique performed in aqueous media was employed to prepare a nanoporous gold (NPG) electrode that demonstrated extremely high sensitivity toward hydrazine oxidation. An Ag_∼60Au_∼40 alloy was electrodeposited at a constant potential on sequentially Cr- and Au-deposited indium tin oxide (Au/Cr/ITO) from a bath that contained sulfuric acid, thiourea, HAuCl₄·3H₂O, and AgNO₃. The dealloying step was performed in concentrated HNO₃, where Ag in the alloy was selectively oxidized to leave the NPG structure. The NPG electrode was employed to study the hydrazine oxidation in basic phosphate buffer solution (PBS), and the results were compared with those obtained using the gold nanoparticle (AuNP)-modified ITO (AuNP/ITO) electrode. The NPG electrode demonstrated an unusual surface-confined behavior, which probably resulted from the thin-layer characteristics of the nano-pores. Hydrazine was detected by hydrodynamic chronoamperometry (HCA) at +0.2 V (vs. Ag/AgCl). The steady-state oxidative current exhibited a linear dependence on the hydrazine concentration in the concentration range of 5.00 nM–2.05 mM, and the detection limit was 4.37 nM (σ = 3). This detection limit is the lower than the detection limits reported in the current literature concerning the electrochemical detection of hydrazine. The NPG electrode indeed demonstrates greater stability after hydrazine detection than the AuNP/ITO electrode.     

Multivariate optimisation of electrochemically pre-treated electrodes used in a voltammetric electronic tongue

The use of experimental design as a tool to optimise electrochemically cleaned electrodes applied in a voltammetric electronic tongue is described. A simple and quick activation of electrode surfaces is essential for this type of device, especially for on-line applications in industrial processes. The electronic tongue consisted of four metal electrodes, e.g. Au, Ir, Pt, and Rh in a three-electrode configuration. Current was measured as a function of large potential pulses of decreasing amplitude applied to each electrode. Preliminary results showed that electrochemical cleaning activated the electrode surfaces to similar extent as polishing. Settings of potential and time for each electrode was determined with experimental design in a solution containing 1.0 mM K₄[Fe(CN)₆] in 0.1 M phosphate buffer (pH 6.8). Electrode surfaces were deactivated in-between measurements in a complex liquid, like tea. Optimal settings for potential and time in the electrochemical cleaning procedure at each electrode were chosen at recoveries of 100% (compared to polished electrodes). The recoveries were larger than 100% when too large potentials and times were applied. This could be explained by the fact that the electrode areas increased and therefore also the current responses. Principal component analysis (PCA) was used to investigate the stability of the electrode settings at 100% recoveries. No obvious trends of drift in the signals were found.     

Electorchemical Studies of Cytochrome c on Electodes Modified by Single—Wall Carbon Naotubes

Single-wall carbon nanotubes(SWNTs) modified gold electrodes were prepared by using two different methods.The electrochemical behavior of cytochrome c on the modified gold electrodes was investigated.The first kind of SWNT-modified electrode (noted as SWNT/Au electrode)was prepared by the adsorption of carboxylterminated SWNTs from DMF dispersion on the gold electrode.The oxidatively processed SWNT tips were covalently modified by coupling with amines (AET) to form amide linkage.Via Au-S chemical bonding,the self-assembled monolayer of thiol-unctionalized nanotubes on gold surface was fabricated so as to prepare the others SWNT-modified electrode (noted as SWNT/AET/Au electrode).It was shown from cyclic voltammetry cxperiments that cytochrome c exhibited direct electrochemical responses on the both electrodes, but only the current of controlled diffusion existed on the SWNT/Au electrode while both the currents of controlled diffusion and adsorption of cytochrome c occurred on the SWNT/AET/Au electrode.Photoelastic Modulation Infared Reflection Absorpthion Spectroscopy (PEM-IRRAS) and Quartz Crystal Microbalance (QCM) were employed to verify the adsorption of SWNTs on the gold electrodes.The results proved that SWNTs could enhance the direct electron transfer proecss between the electrodes and redox proteins.