Auto-inhibition of hydrogen gas evolution on gold in aqueous acid solution

It was demonstrated recently that dramatic changes in the redox behaviour of gold/aqueous solution interfaces may be observed following either cathodic or thermal electrode pretreatment. Further work on the cathodic pretreatment of gold in acid solution revealed that as the activity of the gold surface was increased, its performance as a substrate for hydrogen gas evolution under constant potential conditions deteriorated. The change in activity of the gold atoms at the interface, which was attributed to a hydrogen embrittlement process (the occurrence of the latter was subsequently checked by surface microscopy), was confirmed, as in earlier work, by the appearance of a substantial anodic peak at ca. 0.5 V (RHE) in a post-activation positive sweep. Changes in the catalytic activity of a metal surface reflect the fact that the structure (or topography), thermodynamic activity and electronic properties of a surface are dependent not only on pretreatment but also, in the case of the hydrogen evolution reaction, vary with time during the course of reaction. As will be reported shortly, similar (and often more dramatic) time-dependent behaviour was observed for hydrogen gas evolution on other metal electrodes.

Gold nanospikes formed through a simple electrochemical route with high electrocatalytic and surface enhanced Raman scattering activity

We demonstrate a simple electrochemical route to produce uniformly sized gold nanospikes without the need for a capping agent or prior modification of the electrode surface, which are predominantly oriented in the {111} crystal plane and exhibit promising electrocatalytic and SERS properties.

The role of defects, or active states, in surface electrochemistry with particular reference to gold in neutral solution

Metastable, active, or nonequilibrium states due to the presence of abnormal structures and various types of defects are well known in metallurgy. The role of such states at gold surfaces in neutral aqueous media (an important electrode system in the microsensor area) was explored using cyclic voltammetry. It was demonstrated that, as postulated in earlier work from this laboratory, there is a close relationship between premonolayer oxidation, multilayer hydrous oxide reduction and electrocatalytic behaviour in the case of this and other metal electrode systems. Some of the most active, and therefore most important, entities at surfaces (e.g., metal adatoms) are not readily imageable or detectable by high resolution surface microscopy techniques. Cyclic voltammetry, however, provides significant, though not highly specific, information about such species. The main conclusion is that further practical and theoretical work on active states of metal surfaces is highly desirable as their behaviour is not simple and is of major importance in many electrocatalytic processes.

Superactivation of metal electrode surfaces and its relevance to COads oxidation at fuel cell anodes

The inhibiting effect of COads on platinum-based anodes is a major problem in the development of ambient temperature, polyelectrolyte membrane-type fuel cells. One of the unusual features of the response for the oxidative removal of the species in question is that the response observed for this reaction in the positive sweep is highly dependent on the CO admission potential, for example, when the COads is formed in the Hads region it undergoes oxidation at unusually low potentials. Such behaviour is attributed here to hydrogen activation of the platinum surface, with the result that oxide mediators (and COads oxidation) occurs at an earlier stage of the positive sweep. It is also demonstrated, for both platinum and gold in acid solution, that dramatic premonolayer oxidation responses may be observed following suitable preactivation of the electrode surfaces. It is suggested that the defect state of a solid electrode surface is an important variable whose investigation may yield improved fuel cell anode performance.

Investigation of mediated oxidation of ascorbic acid by ferrocenemethanol using large-amplitude fourier transformed ac voltammetry under quasi-reversible electron-transfer conditions at an indium tin oxide electrode

The ability of the technique of large-amplitude Fourier transformed (FT) ac voltammetry to facilitate the quantitative evaluation of electrode processes involving electron transfer and catalytically coupled chemical reactions has been evaluated. Predictions derived on the basis of detailed simulations imply that the rate of electron transfer is crucial, as confirmed by studies on the ferrocenemethanol (FcMeOH)-mediated electrocatalytic oxidation of ascorbic acid. Thus, at glassy carbon, gold, and boron-doped diamond electrodes, the introduction of the coupled electrocatalytic reaction, while producing significantly enhanced dc currents, does not affect the ac harmonics. This outcome is as expected if the FcMeOH (0/+) process remains fully reversible in the presence of ascorbic acid. In contrast, the ac harmonic components available from FT-ac voltammetry are predicted to be highly sensitive to the homogeneous kinetics when an electrocatalytic reaction is coupled to a quasi-reversible electron-transfer process. The required quasi-reversible scenario is available at an indium tin oxide electrode. Consequently, reversible potential, heterogeneous charge-transfer rate constant, and charge-transfer coefficient values of 0.19 V vs Ag/AgCl, 0.006 cm s (-1) and 0.55, respectively, along with a second-order homogeneous chemical rate constant of 2500 M (-1) s (-1) for the rate-determining step in the catalytic reaction were determined by comparison of simulated responses and experimental voltammograms derived from the dc and first to fourth ac harmonic components generated at an indium tin oxide electrode. The theoretical concepts derived for large-amplitude FT ac voltammetry are believed to be applicable to a wide range of important solution-based mediated electrocatalytic reactions.

The active site behaviour of electrochemically synthesised gold nanomaterials

Even though gold is the noblest of metals, a weak chemisorber and is regarded as being quite inert, it demonstrates significant electrocatalytic activity in its nanostructured form. It is demonstrated here that nanostructured and even evaporated thin films of gold are covered with active sites which are responsible for such activity. The identification of these sites is demonstrated with conventional electrochemical techniques such as cyclic voltammetry as well as a large amplitude Fourier transformed alternating current (FT-ac) method under acidic and alkaline conditions. The latter technique is beneficial in determining if an electrode process is either Faradaic or capacitive in nature. The observed behaviour is analogous to that observed for activated gold electrodes whose surfaces have been severely disrupted by cathodic polarisation in the hydrogen evolution region. It is shown that significant electrochemical oxidation responses occur at discrete potential values well below that for the formation of the compact monolayer oxide of bulk gold and are attributed to the facile oxidation of surface active sites. Several electrocatalytic reactions are explored in which the onset potential is determined by the presence of such sites on the surface. Significantly, the facile oxidation of active sites is used to drive the electroless deposition of metals such as platinum, palladium and silver from their aqueous salts on the surface of gold nanostructures. The resultant surface decoration of gold with secondary metal nanoparticles not only indicates regions on the surface which are rich in active sites but also provides a method to form interesting bimetallic surfaces.

Exploiting the facile oxidation of evaporated gold films to drive electroless silver deposition for the creation of bimetallic Au/Ag surfaces

Gold is often considered as an inert material but it has been unequivocally demonstrated that it possesses unique electronic, optical, catalytic and electrocatalytic properties when in a nanostructured form.[1] For the latter the electrochemical behaviour of gold in aqueous media has been widely studied on a plethora of gold samples, including bulk polycrystalline and single-crystal electrodes, nanoparticles, evaporated films as well as electrodeposited nanostructures, particles and thin films.[1b, 2] It is now well-established that the electrochemical behaviour of gold is not as simple as an extended double-layer charging region followed by a monolayer oxide-formation/-removal process. In fact the so-called double-layer region of gold is significantly more complicated and has been investigated with a variety of electrochemical and surface science techniques. Burke and others[3] have demonstrated that significant processes due to the oxidation of low lattice stabilised atoms or clusters of atoms occur in this region at thermally and electrochemically treated electrodes which were confirmed later by Bond[4] to be Faradaic in nature via large-amplitude Fourier transformed ac voltammetric experiments. Supporting evidence for the oxidation of gold in the double-layer region was provided by Bard,[5] who used a surface interrogation mode of scanning electrochemical microscopy to quantify the extent of this process that forms incipient oxides on the surface. These were estimated to be as high as 20% of a monolayer. This correlated with contact electrode resistance measurements,[6] capacitance measurements[7] and also electroreflection techniques...

Plasmonic effect of gold nanoparticles in organic solar cells

Light trapping, due to the embedding of metallic nanoparticles, has been shown to be beneficial for a better photoabsorption in organic solar cells. Researchers in plasmonics and in the organic photovoltaics fields are working together to improve the absorption of sunlight and the photon–electron coupling to boost the performance of the devices. Recent advances in the field of plasmonics for organic solar cells focus on the incorporation of gold nanoparticles. This article reviews the different methods to produce and embed gold nanoparticles into organic solar cells. In particular, concentration, size and geometry of gold nanoparticles are key factors that directly influence the light absorption in the devices. It is shown that a careful choice of size, concentration and location of gold nanoparticles in the device result in an enhancement of the power conversion efficiencies when compared to standard organic solar cell devices. Our latest results on gold nanoparticles embedded in on organic solar cell devices are included. We demonstrate that embedded gold nanoparticles, created by depositing and annealing a gold film on transparent electrode, generate a plasmonic effect which can be exploited to increase the power conversion efficiency of a bulk heterojunction solar cell up to 10%.

The effect of electrode material on the electrochemical formation of porous copper surfaces using hydrogen bubble templating

The electrodeposition of copper onto copper, gold, palladium and glassy carbon (GC) electrodes via a hydrogen bubble templating method is reported. It is found that the composition of the underlying electrode material significantly influences the morphology of the copper electrodeposit. Highly ordered porous structures are achieved with Cu and Au electrodes, however on Pd this order is disrupted and a rough randomly oriented surface is formed whereas on GC a bubble templating effect is not observed. Chronopotentiograms recorded during the electrodeposition process allows bubble formation and detachment from the surface to be monitored where distinctly different potential versus time profiles are observed at the different electrodes. The porous Cu surfaces are characterised with scanning electron microscopy, X-ray diffraction and cyclic voltammetric measurements recorded under alkaline conditions. The latter demonstrates that there are active sites present on electrodeposited copper whose coverage and reactivity depend on the underlying electrode material. The most active Cu surface is achieved at a Pd substrate for both the hydrogen evolution reaction and the catalytic reduction of ferricyanide ions with thiosulphate ions. This demonstrates that the highly ordered porous structure on the micron scale which typifies the morphology that can be achieved with the hydrogen bubbling template method is not required in producing the most effective material.

Gold nanoparticles modified electrodes for biosensors

Biomolecules are chemical compounds found in living organisms which are the building blocks of life and perform important functions. Fluctuation from the normal concentration of these biomolecules in living system leads to several disorders. Thus the exact determination of them in human fluids is essential in the clinical point of view. High performance liquid chromatography, flow injection analysis, capillary electrophoresis, fluorimetry, spectrophotometry, electrochemical and chemiluminescence techniques were usually used for the determination of biologically important molecules. Among these techniques, electrochemical determination of biomolecules has several advantages over other methods viz., simplicity, selectivity and sensitivity. In the past two decades, electrodes modified with polymer films, self-assembled monolayers containing different functional groups and carbon paste have been used as electrochemical sensors. But in recent years, nanomaterials based electrochemical sensors play an important role in the improvement of public health because of its rapid detection, high sensitivity and specificity in clinical diagnostics. To date gold nanoparticles (AuNPs) have received arousing attention mainly due to their fascinating electronic and optical properties as a consequence of their reduced dimensions. These unique properties of AuNPs make them as an ideal candidate for the immobilization of enzymes for biosensing. Further, the electrochemical properties of AuNPs reveal that they exhibit interesting properties by enhancing the electrode conductivity, facilitating electron transfer and improving the detection limit of biomolecules. In this chapter, we summarized the different strategies used for the attachment of AuNPs on electrode surfaces and highlighted the electrochemical determination of glucose, ascorbic acid (AA), uric acid (UA) and dopamine derivatives using the AuNPs modified electrodes.

Organic photovoltaics using thin gold film as an alternative anode to indium tin oxide

Indium Tin Oxide (ITO) is the most commonly used anode as a transparent electrode and more recently as an anode for organic photovoltaics (OPVs). However, there are significant drawbacks in using ITO which include high material costs, mechanical instability including brittleness and poor electrical properties which limit its use in low-cost flexible devices. We present initial results of poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester OPVs showing that an efficiency of 1.9% (short-circuit current 7.01 mA/cm2, open-circuit voltage 0.55 V, fill factor 0.49) can be attained using an ultra thin film of gold coated glass as the device anode. The initial I-V characteristics demonstrate that using high work function metals when the thin film is kept ultra thin can be used as a replacement to ITO due to their greater stability and better morphological control.

Direct Growth of Gold Nanorods on Gold and Indium Tin Oxide Surfaces: Spectral, Electrochemical, and Electrocatalytic Studies

The surfactant-assisted seed-mediated growth method was used for the formation of gold nanorods (GNRs) directly on gold (Au) and indium tin oxide (ITO) surfaces. Citrate-stabilized similar to 2.6 nm spherical gold nanoparticles (AuNPs) were first self-assembled on ITO or Au surfaces modified with (3-mercaptopropyl)-trimethoxysilane (MPTS) sol-gel film and then immersed in a cationic surfactant growth solution to form GNRs. The growth of GNRs on the MPTS sol gel film modified ITO surface was monitored by UV-visible spectroscopy. The ITO surface with the attached spherical AuNPs shows a surface plasmon resonance band at 550 nm. The intensity of this absorption band increases while increasing the immersion time of the AuNP-modified ITO surface into the growth solution, and after 5 h, an additional shoulder band around 680 nm was observed. The intensity of this shoulder band increases, and it was shifted to longer wavelength as the immersion time of the AuNP-modified ITO surface into the growth solution increases. After 20 h, a predominant wave at 720 nm was observed along with a band at 550 nm. Further immersion of the modified ITO surface into the growth solution did not change the absorption characteristics. The bands observed at 550 and 720 nm were characteristics of GNRs, corresponding to transverse and longitudinal waves, respectively. The AFM images showed the presence of GNRs on the surface of the MPTS sol gel modified ITO surface with a typical length of similar to 100-120 nm and a width of similar to 20-22 nm in addition to a few spherical AuNPs, indicating that seeded spherical AuNPs were not completely involved in the GNRs' formation. Finally, the electrocatalytic activity of the surface-grown GNRs on the MPTS sol gel film modified Au electrode toward the oxidation of ascorbic acid (AA) was studied. Unlike a polycrystalline Au electrode, the surface-grown GNR-modified electrode shows two well-defined voltammetric peaks for AA at 0.01 and 0.35 V in alkaline, neutral, and acidic pHs. The cause for the observed two oxidation peaks for AA was due to the presence of both nanorods and spherical nanoparticles on the electrode surface. The presence of spherical AuNPs on the MPTS sol gel film oxidized AA at more positive potential, whereas the GNRs oxidized AA at less positive potential. The observed 340 mV less positive potential shift in the oxidation of AA suggested that GNRs are better electrocatalysts for the oxidation of AA than the spherical AuNPs.

About the transparent electrode of the organic photovoltaic cells

Electrodes and the nature of their contact with organic materials play a crucial role in the realization of efficient optoelectronic components. Whether the injection (organic light-emitting diodes - OLEDs) or collection (organic photovoltaic cells - OPV cells) of carriers, contacts must be as efficient as possible. To do this, it is customary to refer to electrode surface treatment and/or using a buffer layer all things to optimize the contact. Efficiency of organic photovoltaic cells based on organic electron donor/organic electron acceptor junctions can be strongly improved when the transparent conductive anode is coated with a buffer layer (ABL). We show that an ultra-thin gold (0.5 nm) or a thin molybdenum oxide (3-5 nm) can be used as efficient ABL. However, the effects of these ABL depend on the highest occupied molecular orbital (HOMO) of different electron donors of the OPV cells. The results indicate that, in the case of metal ABL, a good matching between the work function of the anode and the highest occupied molecular orbital of the donor material is the major factor limiting the hole transfer efficiency. Indeed, gold is efficient as ABL only when the HOMO of the organic donor is close to its work function Phi(Au). MoO3 has a wider field of application as ABL than gold. The role of the oxide is not so clearly understood than that of Au, different models proposed to interpret the experimental results are discussed.

Study of the Adsorption of Fibrinogen on Gold-Coated Silicon Wafer by an Impedance Method

In 0.1 mol/l KH2PO4–Na2HPO4 (pH 7.80) buffer solution, the potential of zero charge (PZC) and the open circuit potential of gold-coated silicon were determined to be about −0.6 and +0.10 V (vs SCE), respectively. The open circuit potential was higher than the PZC, which indicated that the surface of the gold-coated electrode had a positive charge. The ellipsometry experiment showed that the adsorption of fibrinogen onto the gold-coated silicon wafer surface arrived at a saturated state when the adsorption time exceeded 50 min. The percentage of surface without adsorbed protein, θ, was about 63%. This means that the proportion of surface actually occupied by fibrinogen was only about 37% after the adsorption arrived at saturation. The solution/protein capacitance value was determined in an impulse state around −0.59 V (vs SCE) and was stable (4.2×10−5 F) at other potentials.

Adsorption of human serum albumin onto gold: a combined electrochemical and ellipsometric study

Human serum albumin adsorption onto gold surfaces was investigated by electrochemical and ellipsometric methods. Albumin adsorption onto gold was confirmed by the change of the open circuit potential of gold and by the ellipsometric parameter variation during albumin immobilization. In both experiments the parameters reached stable values within 10-15 min. The albumin adsorption layer thickness measured with the ellipsometer was about 1.5 nm. The adsorption of albumin Under applied potential was also investigated and it was found that both positive and negative applied potential promote albumin adsorption. Changes in the optical parameters of bare gold and albumin adsorbed onto gold surface under applied potential were investigated with in Situ ellipsometry. The similarity and reversibility of the optical changes showed that adsorbed albumin was stable on the gold surface Under the applied potential range (-200-600 mV). The cyclic voltammograms of K3Fe(CN)(6) on the modified gold surface showed that albumin Could partly block the oxidation and reduction reaction. (C) 2004 Elsevier Inc. All rights reserved.