Electrocatalysis at polycrystalline silver in base: an example of the complexity of surface active state behaviour

Four low-level redox responses, attributed to the involvement of four different types of active surface state (or site) transitions, were observed within the double layer region in the case of polycrystalline silver electrodes in base; there is now independent evidence for such premonolayer behaviour in data reported by other authors. The potentials for each of these transitions were correlated with the onset/termination potentials of different electrocatalytic processes, usually for oxidations or reductions but in some cases for both types. The data obtained are in good agreement with the IHOAM model of electrocatalysis which was outlined earlier for the other Group 11 metals, gold and copper. Electronic Publication     

The effect of severe thermal pretreatment on the redox behaviour of gold in aqueous acid solution

Severe thermal pretreatment of gold wire electrodes in an inert gas atmosphere resulted in the appearance of dramatic premonolayer oxidation responses, which in some instances commenced at 0.25 V (RHE), for the resulting active gold electrodes in aqueous acid media. Similar behaviour was reported earlier for platinum and gold activated by cathodic pretreatment in acid solution; these active noble metal surfaces are evidently more susceptible to oxidation than bulk copper. Such behaviour was attributed to the effect of surface disorder; many of the metal atoms at the interface are assumed to be in a very active, metastable state possessing quite low lattice stabilization energy. Premonolayer oxidation responses are again correlated with electrocatalytic behaviour and the existence of unusual chemisorption behaviour for molecules reacting at highly active states of metals is outlined. Electronic Publication     

The effect of thermal pretreatment on the electrochemical response for palladium in aqueous media

Previous work on the electrochemistry of palladium in aqueous acid solution demonstrated the existence of two multilayer hydrous oxide reduction peaks, one at ca. 0.24 V and another at ca. 0.55 V vs. RHE, plus the presence of a reversible active surface state transition at ca. 0.24 V. In the present work with thermally activated palladium it was observed that, in agreement with the hydrous oxide reduction behaviour of the system, there is a second active state transition at E≥ca. 0.45 V. In most of its reactions in aqueous acid solution, apart from its unusual capacity to absorb hydrogen, palladium exhibits properties very similar to those of platinum; however, palladium seems to be more prone to dissolution and subsurface oxygen formation. Also the premonolayer oxidation responses of these two metals are often different as the more active state of the palladium surface is not as readily generated as that of platinum. The electrocatalytic properties of palladium, as reported earlier, correlate quite well with the hydrous oxide and premonolayer oxidation behaviour of this electrode system. Electronic Publication     

Green, effective chemical route for the synthesis of silver nanoplates in tannic acid aqueous solution

Silver nanoplates, with average size tunable from 50 to 500 nm, have been synthesized via a simple room-temperature tannic acid (TA) solution-phase chemical reduction method. The synthesis was a seedless process in which tannic acid was used as a reducing as well as a capping agent, and did not need any other surfactant or capping agent to direct the anisotropic growth of the silver nanoparticles. The morphology of silver nanoplates has been confirmed by transmission electron microscopy, the growth process of nanoplates has been studied by UV/vis spectroscopy. Control experiments have been explored for a more thorough understanding of the growth mechanism. It was found that both the concentrations of TA and the pH of solution were the key elements to control the morphology silver nanoplates. The optical in-plane dipole plasmon resonance bands of these silver plates could be tuned from 520 to 1100 nm.     

Morphology and size-controlled synthesis of silver nanoparticles in aqueous surfactant polymer solutions

We have employed a number of reducing and capping agents to obtain Ag(0) metallic nanoparticles of various sizes and morphologies. The size and morphology were tuned by selecting reducing and capping agents. Spherical particles of 15 and 43 nm diameter were obtained when 1 wt% aqueous starch solution of AgNO₃ precursor salt was reduced by d(+)-glucose and NaOH, respectively, on heating at 70 °C for 30 min. Smaller size particles obtained in the case of d(+)-glucose reduction has been attributed to the slow reduction rate by mild reducing agent d(+)-glucose compared to strong NaOH. Conducting the reduction at ambient temperature of silver salt in liquid crystalline pluronic P123 and L64 also gave spherical particles of 8 and 24 nm, respectively, without the addition of any separate reducing agent. NaOH reduction of salt in ethylene glycol (11 g)/polyvinyl pyrolidone (PVP; 0.053 g) mixture produced large self-assembled cubes of 520 nm when smaller (26–53 nm) star-shaped sharp-edged structures formed initially aggregated on heating the preparation at 190 °C for 1 h. Increasing the amount of PVP (0.5 g) in ethylene glycol (11 g) and heating at 70 °C for 30 min yielded a mixture of spherical and non-spherical (cubes, hexagons, pentagons, and triangle) particles without the addition of an extra reducing agent. Addition of 5 wt% PVP to 1 wt% aqueous starched solution resulted in the formation of a mixture of spherical and anisotropic structures when solution heated at 70 °C for 1 h. Homogeneous smaller sized (29 nm) cubes were synthesized by NaOH reduction of AgNO₃ in 12.5 wt% of water-soluble polymer poly(methyl vinyl ether) at ambient temperature in 30 min reaction time.     

Synthesis and thermal study of octahedral silver nano-plates in polyvinyl alcohol (PVA)

Octahedral silver nano-plates were synthesized from aqueous solution of silver nitrate and polyvinyl alcohol. The colloid formed is dried on glass plates by simple dip-coating method to inhibit the growth of the particles, and to analyze the samples. Samples were characterized by X-ray diffractometer, transmission electron microscope (TEM), thermo-gravimetric analysis (TGA), and differential scanning calorimetry (DSC) techniques. The UV–Vis absorption spectra of these silver nano-plates revealed a high intense plasmon absorption peak near 425 nm. In addition three emission peaks were observed when the excitation was fixed at 222 nm.     

Kinetics of silver nanoparticle growth in aqueous polymer solutions

Silver nanoparticles were prepared in aqueous AgNO₃ solution by using hydroquinone and sodium citrate as reducing agents with neutral polymers poly(vinylpyrrolidone) and poly(vinyl alcohol) as stabilizers. The rate of particle formation was determined with a diode array UV-Vis spectrophotometer. The effects of the polymer concentration on the reaction rate, the size, and the size distribution of the particles formed were studied by transmission electron microscopy. Both the reaction rate and the size of silver nanoparticles decreased with increasing polymer concentration in the range 0.07–0.50 w/v%.     

纳米银自组装电极的制备及对高氯酸二茂铁的电化学响应

采用自组装的方法制备了纳米银粒子修饰金电极,并运用循环伏安法、交流阻抗谱探讨了该电极的电化学特性.研究了高氯酸二茂铁在该修饰电极上的直接电化学行为.实验结果表明,高氯酸二茂铁在该修饰电极上具有良好的电流响应.用示差脉冲法测定高氯酸二茂铁,其氧化峰电流与浓度在4.0×10-6～5.0×10-4 mol/L范围呈良好线性关系,线性方程为:Ip(μA)=0.0236c(μmol/L)-0.0975,线性相关系数为0.9982,检出限为2.3×10-7 mol/L(信噪比为3).     

Analysis of the differential capacity at the silver / lithium perchlorate aqueous solution interface

Analysis of the differential capacity of ideally polarized Ag electrodes without specific adsorption is presented. The capacity was measured for three crystal faces of Ag [(111), (100) and (110)] and for polycrystalline Ag in aqueous solutions at different concentrations of LiClO₄. The inner layer contribution C _i was analysed for all the systems studied according to Grahame's concept. Moreover, using the procedure proposed by Amokrane and Badiali, several contributions to C _i, i.e. the metal (C _m), the solvent (C _s) and dipole orientation (C _dip), were calculated. The influence of the kind of crystal face of the Ag electrode on the values of these contributions is discussed. Received: 21 May 1999 / Accepted: 19 November 1999     

Three-dimensional printed knotted reactors enabling highly sensitive differentiation of silver nanoparticles and ions in aqueous environmental samples

Whether silver nanoparticles (AgNPs) persist or release silver ions (Ag⁺) when discharged into a natural environment has remained an unresolved issue. In this study, we employed a low-cost stereolithographic three-dimensional printing (3DP) technology to fabricate the angle-defined knotted reactors (KRs) to construct a simple differentiation scheme for quantitative assessment of Ag⁺ ions and AgNPs in municipal wastewater samples. We chose xanthan/phosphate-buffered saline as a dispersion medium for in situ stabilization of the two silver species, while also facilitating their extraction from complicated wastewater matrices. After method optimization, we measured extraction efficiencies of 54.5 and 32.3% for retaining Ag⁺ ions and AgNPs, respectively, in the printed KR (768-turn), with detection limits (DLs) of 0.86 and 0.52 ng L⁻¹ when determining Ag⁺ ions and AgNPs, respectively (sample run at pH 11 without a rinse solution), and 0.86 ng L⁻¹ when determining Ag⁺ ions alone (sample run at pH 12 with a 1.5-mL rinse solution). The proposed scheme is tolerant of the wastewater matrix and provides more reliable differentiation between Ag⁺/AgNPs than does a conventional filtration method. The concept and applicability of adopting 3DP technology to renew traditional KR devices were evidently proven by means of these significantly improved analytical performance. Our analytical data suggested that the concentrations of Ag⁺ ions and AgNPs in the tested industrial wastewater sample were both higher than those in domestic wastewater, implying that industrial activity might be a main source of environmental silver species, rather than domestic discharge from AgNP-containing products.     

Heterogeneously catalyzed oxidation of butanediols in base free aqueous media

The oxidation of four butanediols under base-free conditions has been investigated using a set of Au, Pd and Pt catalysts prepared using sol-immobilization. The supported nanoparticles are found to be active with bimetallic alloys having much higher activity when compared with the monometallic counterparts. In general the AuPt catalysts are the most active and in all cases the corresponding C4 oxidation products were observed with high selectivity; sequential reaction of these products leads to the formation of acetic acid as an undesired by-product.     

Generation of active surface states of gold and the role of such states in electrocatalysis

The cyclic voltammetry behaviour of gold in aqueous media is often regarded in very simple terms as a combination of two distinct processes, double layer charging/discharging and monolayer oxide formation/removal. This view is questioned here on the basis of both the present results and earlier independent data by other authors. It was demonstrated in the present case that both severe cathodization or thermal pretreatment of polycrystalline gold in acid solution resulted in the appearance of substantial Faradaic responses in the double layer region. Such anamolous behaviour, as outlined recently also for other metals, is rationalized in terms of the presence of active metal atoms (which undergo premonolayer oxidation) at the electrode surface. Such behaviour, which is also assumed to correspond to that of active sites on conventional gold surfaces, is assumed to be of vital importance in electrocatalysis; in many instances the latter process is also quite marked in the double layer region. Received: 26 July 1999 / Accepted: 7 September 1999     

Room temperature synthesis of silver nanowires from tabular silver bromide crystals in the presence of gelatin

Long silver nanowires were synthesized at room temperature by a simple and fast process derived from the development of photographic films. A film consisting of an emulsion of tabular silver bromide grains in gelatin was treated with a photographic developer (4-(methylamino)phenol sulfate (metol), citric acid) in the presence of additional aqueous silver nitrate. The silver nanowires have lengths of more than 50 μm, some even more than 100 μm, and average diameters of about 80 nm. Approximately, 70% of the metallic silver formed in the reduction consists of silver nanowires. Selected area electron diffraction (SAED) results indicate that the silver nanowires grow along the [111] direction. It was found that the presence of gelatin, tabular silver bromide crystals and silver ions in solution are essential for the formation of the silver nanowires. The nanowires appear to originate from the edges of the silver bromide crystals. They were characterized by transmission electron microscopy (TEM), SAED, scanning electron microscopy (SEM), and powder X-ray diffraction (XRD).     

Standard potentials of the silver,silver bromide electrode and the thermodynamic properties of hydrobromic acid in 1,2-dimethoxyethane and its aqueous mixtures

The standard potentials of the silver, silver bromide electrode have been determined in 1,2-dimethoxyethane (DME) and in nineteenDME + water solvents from the e.m.f. measurements of cells of the type Pt|H₂(g, 1 atm)|HBr (m), solvent|AgBr|Ag at intervals of 5°C from 5 to 45°C. The molality of HBr covered the range from 0.01 to 0.1 mol kg^–1. In solvents of highDME content, where the dielectric constant is small, it was necessary to correct for ion-pair formation. The temperature variation of the standard potential has been used to evaluate the standard thermodynamic functions for the cell reaction, and the standard quantities for the transfer of HBr from water to the respective solvents. The results have been discussed both in relation to the acid-base nature of the solvent mixtures and also their structural effects on the transfer process.

---

Standardpotentiale der Silber, Silberbromid-Elektrode und thermodynamische Eigenschaften von H Br in 1,2-Dimethoxyethan und 1,2-Dimethoxyethan—Wasser-Mischungen

Zusammenfassung Die Standardpotentiale der Silber, Silberbromid-Elektrode wurden in 1,2-Dimethoxyethan (DME) und in 19 verschiedenenDME—Wasser-Gemischen aus EMK-Messungen der Zelle Pt|H₂(g,1 atm)|HBr (m), Lsgsm.|AgBr|Ag in Temperaturintervallen von 5°C zwischen 5 und 45°C bestimmt. Die Molalität von HBr deckte den Bereich von 0,01 bis 0,1 mol kg^–1. Bei Lösungen mit höheremDME-Gehalt — und damit niedrigen Dielektrizitätskonstanten —war es nötig, für die Bildung von Ionenpaaren eine Korrektur einzuführen. Über die Temperaturvariation wurden die thermodynamischen Größen für die Zellenreaktion und die Standardgrößen für den Transfer von HBr aus Wasser in das jeweilige Lösungsmittel bestimmt. Die Ergebnisse werden sowohl im Zusammenhang zur Säure-Base-Natur de Lösungsmittelmischungen als auch in bezug auf strukturelle Effekte im Transferprozeß diskutiert.

     

Some unusual features of the configuration of polymer loops in a melt

A diversity of structural features can be imposed on a melt of polymer loops by controlling the linking number between each pair of loops. For the simple case of unlinked loops, considered in this paper, new statistical weighting factors have been found which depend on the global configurational properties of writhe and self inductance. The separate effect of these terms is extreme: the writhe term favours rod-like statistics, whereas the inductance favours globular configurations. Together the two terms tend to cancel each other in a way which is reminiscent of the screened potential in polymer chain melts. The result is that the exponent which describes the size of an unlinked loop in a melt of unlinked loops is perturbed from the gaussian result of 0.5 to 0.45.     

Sensitivity enhancement of SPR biosensor with silver mirror reaction on the Ag/Au film

Based on well-known silver mirror reaction the Ag film was formed on Au film modified by self-assembled monolayer (SAM) of 1,6-hexanedithiol (HDT). The sensitivity of the biosensor based on this Ag/Au film is enhanced compared to that based on Au film. When the surface plasmon resonance (SPR) biosensor based on this Ag/Au film was used to determine human IgG, the range of concentrations of human IgG that could be determined is 0.30-40.00 μg mL⁻¹. The lowest concentration (0.30 μg mL⁻¹) that could be detected was about 8 times lower than that obtained by the biosensor without modification by Ag film (2.50 μg mL⁻¹), which demonstrated that the biosensor based on Ag/Au film could make the resonant wavelength move to longer wavelength following with the sensitivity enhancement of the SPR biosensor.     

An investigation of the surface enhanced Raman scattering (SERS) from a new substrate of silver-modified silver electrode by magnetron sputtering

'Pure' silver nanoparticles on silver electrode were prepared by magnetron sputtering. The silver-modified silver electrode has good stability and the silver nanoparticles on silver electrode have homogeneous size distribution. Compared with the silver colloid modified silver electrode, there were no any extraneous component ions on the electrode, for the modified silver nanoparticles are prepared by magnetron sputtering. Synchronously, we obtained much higher quality SERS spectra of adenine molecules on the silver electrode modified by magnetron sputtering (SEMMS), and the study of the adsorption behavior of adenine on the silver-modified silver electrode by surface enhanced Raman scattering (SERS) indicated that the silver-modified silver electrode was highly efficient substrates for SERS investigation. From the rich information on the SEMMS obtained from high-quality potential-dependent SERS, we may deduce the adsorption behavior of adenine and the probable SERS mechanism in the process. The probable reasons are given.     

Detecting cells on the surface of a silver electrode quartz crystal microbalance using plasma treatment and graft polymerization

This paper utilizes a silver electrode quartz crystal microbalance (QCM) mass sensor to detect the physiology of cells. This study also investigates the plasma surface modification of silver electrode QCMs through deposition of hexamethyldisilazane (HMDSZ) films as a protection film. To improve the cell growth, this paper also performs post-treatments by surface-grafting acrylic acid (AAc), acrylamide (AAm), and oxygen plasma treatment onto the QCM electrodes. Experimental results indicate that plasma deposition is a useful technique to protect the surface of silver electrodes. This technique extends the unpeeling time of silver electrodes from 1 to 7 days. The hydrophilic silver electrode QCM surface modified by AAm exhibited a better storage time effect than other post-treatments.     

Synthesis and characterization of silver nanowires with zigzag morphology in N,N-dimethylformamide

Zigzag silver nanowires with a uniform diameter of 20±5 nm were prepared by reducing silver nitrate (AgNO₃) with N,N-dimethylformamide (DMF) in the presence of tetrabutyl titanate (TBT) and acetylacetone (AcAc) at 373 K for 18 h. X-ray and selected area electron diffraction (XRD and SAED) patterns reveal that the prepared product is made of pure silver with face centered cubic structure. Transmission electron microscopy (TEM) investigations suggest that the amount of silver nanowires is enhanced with increase in reaction time, and the end-to-end assemblies of silver nanorods are observed during the reaction process. After 18 h reaction, silver nanowires with zigzag morphology are obtained. In this paper, a possible growth process of silver nanowires with this interesting shape is described. Silver nanoparticles with small sizes were obtained by reducing Ag⁺ ions with DMF, providing seeds for homogeneous growth of silver nanorods. With the extending reaction time, the synthesized silver nanorods were connected in an end-to-end manner, and the interface between the connections of two nanorods gradually disappeared. The final product shows zigzag morphology with various angles. The angles between two connecting straight parts of zigzag nanowires exhibit an alterable range of 74-151°. These silver nanowires show tremendous potential applications in future nanoscale electronic circuits.     

Bandpass reaction cell inductively coupled plasma mass spectrometry for the determination of silver and cadmium in samples in the presence of excess Zr, Nb and Mo

An inductively coupled plasma quadrupole mass spectrometer (ICP-QMS) equipped with a Dynamic Reaction Cell™ (DRC) was used to reduce the oxide interferences for the accurate determination of Ag and Cd in samples in the presence of excess Zr, Nb and Mo. The effect of the operating conditions of the DRC system was studied to obtain the best signal to noise ratios for and . The potentially interfering polyatomic ions determination were decreased in intensity to an insignificant level by using O₂ as the reaction gas. Furthermore, a rejection parameter q (Rpq) of 0.6 was used to filter out unwanted precursors of interfering species from the ion beam to eliminate interferences created in the reaction cell. The detection limit was ca. 2 and 3 pg ml⁻¹ for and , respectively, with the DRC ICP-MS method. The method was applied to the determination of Ag and Cd in NIST SRM 2709 San Joaquin soil reference material as well as NIST SRM 1568a rice flour reference material. The precision between sample replicates was better than 5.7% and the analysis results were in good agreement with the expected values.