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.     

Structural Evolution of Pt,Au and Cu Anodes by Electrolysis up to Contact Glow Discharge Electrolysis in Alkaline Electrolytes**

Applying a voltage to metal electrodes in contact with aqueous electrolytes results in the electrolysis of water at voltages above the decomposition voltage and plasma formation in the electrolyte at much higher voltages referred to as contact glow discharge electrolysis (CGDE). While several studies explore parameters that lead to changes in the I–U characteristics in this voltage range, little is known about the evolution of the structural properties of the electrodes. Here we study this aspect on materials essential to electrocatalysis, namely Pt, Au, and Cu. The stationary I–U characteristics are almost identical for all electrodes. Detailed structural characterization by optical microscopy, scanning electron microscopy, and electrochemical approaches reveal that Pt is stable during electrolysis and CGDE, while Au and Cu exhibit a voltage-dependent oxide formation. More importantly, oxides are reduced when the Au and Cu electrodes are kept in the electrolysis solution after electrolysis. We suspect that H₂O₂ (formed during electrolysis) is responsible for the oxide reduction. The reduced oxides (which are also accessible via electrochemical reduction) form a porous film, representing a possible new class of materials in energy storage and conversion studies.     

Formation of supramolecular nanobelt arrays consisting of cobalt(II) "picket-fence" porphyrin on Au surfaces

Adlayers of cobalt(II) 5,10,15,20-tetrakis(alpha,alpha,alpha,alpha-2-pivalamidophenyl)porphyrin (CoTpivPP) were prepared by immersing either Au(111) or Au(100) substrate in a benzene solution containing CoTpivPP molecules, and they were investigated in 0.1 M HClO4 and 0.1 M H2SO4 by cyclic voltammetry and in situ scanning tunneling microscopy (STM). The adlayer structure and electrochemical properties of CoTpivPP are compared to those of 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II) (CoTPP). Characteristic nanobelt arrays consisting of CoTpivPP molecules were produced on both Au(111) and Au(100) surfaces. The stability of the nanobelt arrays was controlled by manipulating the electrode potential. On the other hand, the formation of nanobelt arrays consisting of O2-adducted CoTpivPP molecules depended upon the crystallographic orientation of Au. The state of O2 trapped in the cavity of CoTpivPP was distinctly observed in STM images as a bright spot in the nanobelt array formed on reconstructed Au(100)-(hex) surface, but not on Au(111) surface. This result suggests that the arrangement of underlying Au atoms plays an important role in the formation of nanobelt arrays with the sixth ligand coordination.     

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.     

Spectroelectrochemical Measurements at Colloidal Au Multilayer Optically Transparent Electrodes

Application of thin colloidal Au multilayers on glass substrates as optically transparent electrodes is reported herein. Evaluation of the electrode by scanning electron microscopy and as an optically transparent thin layer electrode (OTTLE) illustrate its use for spectroelectrochemical applications. A spectroelectrochemical Nernst plot acquired for the oxidation of o‐tolidine yielded a value of E°=0.605 V (vs. Ag/AgCl) and n=1.88. Both of these values are in excellent agreement with previously published values obtained using a traditional Au minigrid OTTLE. The colloidal Au multilayer electrode is expected to offer many advantages over the minigrid OTTLE in specific applications. These advantages include direct control over microscopic surface topography, large electrochemical surface area, applicability to many different electrode geometries, and increased microscopic surface roughness for SERS and SPR applications.     

Single Potential Scan Methods for Nanoporous Gold Formation on Ultramicroelectrode Surfaces

Nanoporous gold (NPG) has been extensively investigated because of their applications. Here, we report a straightforward method for the preparation of NPG on micrometer-scale electrodes. Well-defined NPG structures were formed on Au surfaces by a single potential scan within 100 seconds. This method is applied to Au surfaces with small dimensions regardless of the electrode geometry, whereas it is not applicable to conventional millimeter-scale electrodes. The effects of electrode sizes and scan rates on NPG formation were systematically examined, and the amperometric glucose detection with 20-μL sample volumes using an ultramicroelectrode (UME) with NPG surfaces was demonstrated.     

Pb deposition on I-coated Au(111). UHV-EC and EC-STM studies

This article concerns the growth of an atomic layer of Pb on the Au(111)( radical3 x radical3)R30 degrees -I structure. The importance of this study lies in the use of Pb underpotential deposition (UPD) as a sacrificial layer in surface-limited redox replacement (SLRR). SLRR reactions are being applied in the formation of metal nanofilms via electrochemical atomic layer deposition (ALD). Pb UPD is a surface-limited reaction, and if it is placed in a solution of ions of a more noble metal, redox replacement can occur, but limited by the amount of Pb present. Pb UPD is a candidate for use as a sacrificial layer for replacement by any more noble element. It has been used by this group for both Cu and Pt nanofilm formation using electrochemical ALD. The I atom layer was intended to facilitate electrochemical annealing during nanofilm growth. Two distinctly different Pb atomic layer structures are reported, studied using in situ scanning tunneling microscopy (STM) with an electrochemical flow cell and ultrahigh vacuum surface analysis combined directly with electrochemical reactions (UHV-EC). Starting with the initial Au(111)( radical3 x radical3)R30 degrees -I, 1/3 monolayer of I on the Au(111) surface, Pb deposition began at approximately 0.1 V. The first Pb UPD structure was observed just below -0.2 V and displayed a (2 x radical3)-rect unit cell, for a structure composed of 1/4 monolayer each of Pb and I. The I atoms fit in Pb 4-fold sites, on the Au(111) surface. The structure was present in domains rotated by 120 degrees. Deposition to -0.4 V resulted in complete loss of the I atoms and formation of a Pb monolayer on the Au(111), which produced a Moiré pattern, due to the Pb and Au lattice mismatch. These structures represent two well-defined starting points for the growth of nanofilms of other more noble elements. It is apparent from these studies that the adsorption of I- on Pb is weak, and it will rinse away. If Pb is used as a sacrificial metal in an electrochemical ALD cycle and adsorbed I atoms are employed for electrochemical annealing, I atoms will need to be applied each cycle.     

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.     

The structure, growth and reactivity of electrodeposited Ru/Au(111) surfaces

In order to elucidate electronic effects on the oxidation of CO on small Ru clusters, we investigated this reaction on well defined Ru/Au(111) model systems via parallel in-situ STM studies of the structure and electrochemical deposition of Ru on Au(111) in H₂SO₄ solution and cyclic voltammetry of CO monolayer oxidation on these surfaces. The Ru deposit consists of nanoscale islands, which coalesce with increasing coverage. The Ru saturation coverage depends on the deposition potential, resulting in Ru submonolayer (>0.1 V), (defective) monolayer (≥−0.1 V), and multilayer films (<−0.1 V). At potentials >0.6 V irreversible formation of Ru oxide/hydroxide species is observed, which can be partly reduced in the range 0.4 to 0.0 V. CO stripping commences at ≈0.1 V and occurs over a broad potential range. From the stripping charge a local CO coverage on the Ru monolayer islands of 0.7 ML was estimated. The observed influence of the morphology of the Ru deposit on the CO stripping voltammetry is explained by (local) variations in the CO adsorption energy due to electronic modifications of the Ru film.     

金核银壳纳米粒子薄膜的制备及SERS活性研究

采用柠檬酸化学还原法制备金溶胶, 通过自组装技术在石英片表面制备金纳米粒子薄膜, 在银增强剂混合溶液中反应获得金核银壳纳米粒子薄膜. 用紫外-可见吸收光谱仪和原子力显微镜(AFM)研究了不同条件下制备的金核银壳纳米粒子薄膜的光谱特性和表面形貌, 并以结晶紫为探针分子测量了金核银壳纳米粒子薄膜的表面增强拉曼光谱(SERS). 结果表明, 金纳米粒子薄膜的分布、银增强剂反应时间的长短对金核银壳纳米粒子薄膜的形成均有重要影响. 制备过程中, 可以通过控制反应条件获得一定粒径的、具有良好表面增强拉曼散射活性的金核银壳纳米粒子薄膜.     

三氧化钼与金纳米粒子复合薄膜光致变色特性的研究

MoO3具有光致变色特性,在大屏幕显示、高密度存储和灵巧窗等研究领域中具有潜在的应用前景^[1-3]。但由于MoO3薄膜中相当一部分光生载流子被复合,其参与变色反应的利用率很低,导致MoO3薄膜的变色效率较低,使其难以实现实用化。半导体表面修饰贵金属后,在半导体／金属界面上可形成Schottky势垒,这将有助于光生载流子的有效分离,从而提高其利用率。据此,本文采用在MoO3薄膜表面修饰金纳米粒子的方法,制备了MnO3和金纳米粒子的复合薄膜（MoO3/Au),变色试验结果表明,MoO3/Au复合薄膜的紫外和可见光变色效率相对MoO3薄膜均有较大的提高。     

Electrochemical redox control of ferrocene using a supramolecular assembly of ferrocene-linked C(60) derivative and metallooctaethylporphyrin array on a Au(111) electrode

Supramolecular assembled layers of ferrocene-linked C(60) derivative (C(60)Fc) and various metal ions coordinated to octaethylporphyrin (MOEP) were formed on the surface of a Au(111) single-crystal electrode by immersing the Au substrate successively into a benzene solution containing MOEP and one containing C(60)Fc molecules. The MOEPs used were zinc(II) (ZnOEP), cobalt(II) (CoOEP), copper(II) (CuOEP), and iron(III) chloride (FeClOEP) of OEP (2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine). The molecules of C(60)Fc directly attached to the Au(111) electrode showed poorly defined electrochemical redox response, whereas a clear electrochemical redox reaction of the ferrocene group in the C(60)Fc molecule was observed at 0.78 V versus reversible hydrogen electrode on ZnOEP, CoOEP, and CuOEP adlayers, but not on the FeClOEP adlayer. Adlattices of the underlying layer and the top layer of C(60)Fc were determined by in situ scanning tunneling microscopy. Adlayer structures of MOEP were independent of the central metal ion; that is, MOEP molecules were arranged hexagonally with two different orientations. Highly ordered C(60)Fc arrays were formed with 1:1 composition on the ZnOEP-, CoOEP-, and CuOEP-modified Au(111) surface, whereas a disordered structure of C(60)Fc was found on the FeClOEP-modified Au(111) surface. The presence of Cl ligand was found to prevent the formation of supramolecularly assembled layers with C(60)Fc molecules, resulting in an ill-defined unclear electrochemical response of the Fc group. The well-defined electrochemical response of the Fc group in C(60)Fc was clearly due to the control of orientation of C(60)Fc molecules.     

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.     

Electrochemical Behavior and Amperometric Detection of 4‐Chlorophenol on Nano‐Au Thin Films Modified Glassy Carbon Electrode

An amperometric sensor based on nano‐Au thin films was fabricated, by means of which a fast response to 4‐chlorophenol (4‐CP) can be achieved in the range of mM concentrations. The nanostructured Au thin film was prepared on glassy carbon electrodes by a template‐free, double‐potential step electrodeposition technique. Its structural feature can be controlled well by adjusting the deposition time. The amperometric detection of 4‐CP was performed at +0.85 V with a linear detection range from 0.2 to 4.8 mM and a detection limit of 0.11 mM (S/N=3). Besides, the effect of concentrations on the electrochemical behavior of 4‐CP on the Au thin film was investigated by linear sweep voltammetry, differential pulse voltammetry and electrochemical impedance spectroscopy.     

Observable consequences of formation of Au anions from deposition of Au atoms on ultrathin oxide films

Charging of metal atoms or clusters on oxide surfaces has important consequences on their chemical and physical properties. Recently it is has been shown that negatively charged gold atoms and clusters form spontaneously from neutral Au atoms deposited on ultrathin MgO films. The formation of anions on the surface remains difficult to prove experimentally. Also theoretically, the discrimination between neutral and charged adsorbed species is not straightforward. In this paper we perform an accurate analysis of the observable consequences of the formation of Au anions on an oxide surface. To this end we consider the following properties: spin distribution, density of states, Bader charges, substrate relaxation, simulated scanning tunneling microscopy images, work function changes, CO vibrational frequency, electric field effects, and core level shifts. Most of these properties are accessible experimentally, at least in principle. Taken individually, these properties do not necessarily provide conclusive evidence about the charged nature of the adsorbate. Taken together, they offer a complete and unambiguous characterization of the formation of Au anions.     

Regularities of Electrochemical Formation and Properties of Oxide Films on Titanium–Iron Alloys

Regularities governing the formation and properties of oxide films on titanium–iron alloys containing 10, 20, 32, 45, 60, or 80 at. % Fe are studied by the voltammetry and chronoamperometry methods coupled with local measurements of activation potentials. Dependences of the oxide film thickness on potential are found. Activation potentials for samples with oxide films formed in air and at 5 V decrease with increasing iron content in alloys.     

Amino group position dependent orientation of self-assembled monomolecular films of tetraaminophthalocyanatocobalt(II) on Au surfaces

Self-assembled monomolecular films of 1,8,15,22-tetraaminophthalocyanatocobalt(II) (4alpha-CoIITAPc) and 2,9,16,23-tetraaminophthalocyanatocobalt(II) (4beta-CoIITAPc) on Au surfaces were prepared by spontaneous adsorption from solution. These films were characterized by cyclic voltammetry and Raman spectroscopy. Both the surface coverage (Gamma) and intensity of the in-plane stretching bands obtained from Raman studies vary for these monomolecular films, indicating different orientations adopted by them on Au surfaces. The 4alpha-CoIITAPc-modified electrode exhibits an E1/2 of 0.35 V, while the 4beta-CoIITAPc-modified electrode exhibits an E1/2 of 0.19 V, corresponding to the CoII/CoIII redox couple in 0.1 M H2SO4. The Gamma estimated from the charge associated with the oxidation of Co(II) gives (2.62 +/- 0.10) x 10-11 mol cm-2 for 4alpha-CoIITAPc and (3.43 +/- 0.14) x 10-10 mol cm-2 for 4beta-CoIITAPc. In Raman spectral studies, the intensity ratio between in-plane phthalocyanine (Pc) stretching and the Au-N stretching was found to be 6.6 for 4beta-CoIITAPc, while it was 1.6 for 4alpha-CoIITAPc. The obtained lower Gamma and intensity ratio values suggest that 4alpha-CoIITAPc adopts nearly a parallel orientation on the Au surface, while the higher Gamma and intensity ratio values suggest that 4beta-CoIITAPc adopts a perpendicular orientation. The electrochemical reduction of dioxygen was carried out using these differently oriented Pc's in phosphate buffer solution (pH 7.2). Both the Pc's catalyze the reduction of dioxygen; however, the 4alpha-CoIITAPc-modified electrode greatly reduces the dioxygen reduction overpotential compared to 4beta-CoIITAPc-modified and bare Au electrodes.     

Lightweight ITO Electrodes Decorated with Gold Nanostructures for Electrochemical Applications

Gold nanostructures were fabricated on a transparent indium tin oxide (ITO) coated PET substrate by an electrodeposition technique from a potassium gold (III) chloride solution for two different types of applications. It was found that the optical transparency of lightweight ITO electrodes could be maintained by depositing isolated gold nanostructures while opening up the use of these electrodes for inner sphere electron reactions, such as hydroquinone oxidation, which are not possible at ITO electrodes. For practical applications the adhesion of gold to the ITO electrode was improved by modifying the ITO surface with 3‐mercaptopropyl‐trimethoxysilane (MPS). Compared to Au/ITO, the Au/MPS/ITO electrode showed vastly improved electrochemical activity toward various electron transfer reactions when subjected to mechanical stress. The biosensing properties of the Au/MPS/ITO electrode was also investigated by studying the detection of immobilized DNA on the Au/MPS/ITO electrode via electrochemical impedance spectroscopy (EIS).     

Silver UPD ultra-thin film modified nanoporous gold electrode with applications in the electrochemical detection of chloride

Nanoporous noble metals are usually expected to exhibit much higher surface areas than smooth ones, making them of particular importance in many electrochemical applications. This paper describes a simple electrochemical method to modify a nanoporous Au (NPG) surface by using an under potentially deposited (UPD) Ag adlayer. The NPG electrode was obtained by the dealloying of Zn from Au_xZn_1−x in a 40-60 mol% zinc chloride-1-ethyl-3-methylimidazolium chloride (ZnCl₂-EMIC) ionic liquid. The Ag UPD modified nanoporous gold (NPG/Ag(UPD)) electrode possessed dual properties, including an intrinsic high surface area from the nanoporous structure and the characteristics of the Ag UPD adlayer. The potential utility of using NPG/Ag(UPD) for sensors was demonstrated by its excellent sensitivity and selectivity in the electrochemical determination of chloride ions. An atomic scale metal monolayer obtained in the UPD process was selected as a sensing agent. The long-term storability and operational stability of the electrode were strongly demonstrated. Specifically, two couples of redox waves at ∼552 mV and ∼272 mV, respectively, were observed in the cyclic voltammograms (CVs) of the NPG/Ag(UPD) after the adsorption of chloride ions. The first couple of redox waves was related to the UPD and silver stripping and the second couple of redox waves was induced by the adsorption of Cl⁻. The Cl⁻ adsorption process on the NPG/Ag(UPD) electrode followed the transient Langmuir adsorption kinetic model. The ratio of the integrated charges for these two anodic stripping peaks was selectively used to determine dilute chloride ion levels. The calibration curve was linear in the Cl⁻ concentration range of 0.5-30.0 μM.     

Anisotropic in-plane gradients of poly(acrylic acid) formed by electropolymerization with spatiotemporal control of the electrochemical potential

Laterally varying thickness gradients of poly(acrylic acid) (PAA) were formed by Zn(II)-catalyzed electropolymerization of acrylic acid (AA) in the presence of an in-plane electrochemical potential gradient applied to Au working electrodes. In the static potential gradient (SPG) approach, two ends of a Au working electrode were clamped at distinct potentials for the duration of the electropolymerization process, thereby generating a time-independent in-plane electrochemical potential gradient, V(x). A dynamic potential gradient (DPG) approach was also used, in which the two end potentials were varied in time, while maintaining a constant voltage offset, to generate an in-plane electrochemical potential gradient, V(x,t). Because the kinetics of heterogeneous electron transfer vary with the local overpotential, these two methods produce PAA films with laterally varying thickness gradients, although they exhibit different spatial characteristics. X-ray photoelectron spectroscopy (XPS) and surface plasmon resonance (SPR) imaging were used to characterize the PAA gradients. The in-plane thickness variations of PAA gradients formed by both SPG and DPG approaches agree with predictions of the Butler-Volmer equation at small absolute overpotentials, while at large (negative) overpotentials, mass transport dominates, and the thickness reaches a plateau value independent of local potential. DPG-produced PAA gradients are generally broader than SPG gradients with the same initial potential and comparable effective growth time, indicating that the DPG approach is more suitable for formation of thicker gradients.