Nanostructured Au Electrodes Research Articles

Detection of H2O2 using Carbon Nanotubes Covalently Attached to Nanostructured Au Electrode

Detection of hydrogen peroxide (H2O2) has practical significance in various fields, including pharmaceutical, clinical and food industries. The enzyme based H2O2 biosensors allow for the detection at lower potentials, thus avoiding possible interference from reducing agents. However, this type of sensors is inherently less stable, difficult to fabricate and more expensive. Due to high electroactive surface area and electrocatalytic properties, gold nanoparticles and their combination with carbon nanotubes (CNTs) are commonly used in H2O2 sensor design.To avoid fabrication inconveniences and improve stability of a H2O2 sensor, we designed a new hybrid material in which CNTs are covalently attached to a gold surface. First, a highly homogeneous nanostructured gold surface was formed on top of the SiO2 substrate with an intermediate layer of Ti, using E-beam evaporation technique. The average height of the gold nanostructures was 3.9 nm. The gold surface was then electrochemically grafted with aminophenyl groups. Further, plasma-functionalized densified CNT film made from CNT array was attached to the gold surface via amide formation reaction. An introduction of CNTs led to a 40-fold increase in current response. Formation of nanostructured gold surface without actual attachment of nanoparticles to the substrate, as well as covalent bonding of CNTs to the surface, provide a very high stability of the fabricated material, which, in turn, improves the repeatability of measurements.A designed electrode was used for non-enzymatic H2O2 detection. Under optimized parameters of square wave voltammetry and optimum pH, analysis of H2O2 can be performed using 5 independent oxidation peaks. The presence of multiple peaks is due to oxidation of gold, CNTs and H2O2 itself. All peaks increase when H2O2 is added in solution, because of chemical reduction of CNT and gold surfaces, and their consecutive electrochemical oxidation. Using the peak at -0.6 V allows for the H2O2 detection at very low potential, that can minimize interference from various reducing agents. For the -0.6 V peak, the limit of detection was 1.4 mM. Using the peak at -0.05 V allows for much higher sensitivity with the limit of detection of 500 nM.Almost no signal deterioration was observed after 200 measurements, proving high stability of the fabricated electrodes.

(Invited, Digital Presentation) Influence of Hemisphere-Shaped Nanodimples of Gold Electrode on Capacitance in Ionic Liquid

Ionic liquids (ILs) have attracted much attention as promising electrolytes for electrochemical devices due to their wide electrochemical window (stability) and negligible vaporization. For such applications, nanostructured electrodes have advantage on their large surface area leading to accumulation of large density energy at the interface. However, recent reports on IL/electrode interfaces revealed that quite unique structures can be formed, and it is not clear how the interface forms and how it changes by forming an electric double layer (EDL). In this study, we investigated the interfacial capacitance of nanostructured Au electrodes using electrochemical impedance spectroscopy (EIS) and IR measurements.Polystyrene beads were self-assembled in a close packed form on a gold substrate by dipping in polystyrene beads solution and rapid drying. Au electrodeposition on thus prepared surface followed by removal of the beads resulted in the fabrication of a nanostructured Au electrode with periodic dimples. Electrochemical behavior of a ferrocene dissolved ionic liquid (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide (BMI-TFSA)) at the nanostructured and flat Au electrodes were investigated by EIS. We found that the capacitance of the nanostructured electrode was smaller than that of the flat electrode in the whole potential range. This result suggests that the thickness of the electric double layer formed in the nano-sized dimple is thicker than that formed on the flat surface. In the case of nanostructured Au electrode having 160 nm diameter dimples, the large capacitance was observed comparing with other electrodes having the dimples of different sizes. In addition, on the negative-going scan, the capacitance takes maximum at 0.1 V vs. Fc/Fc+, whereas no peak was found on the reverse positive-going scan. This result indicates that the exchange of cation/anion layers smoothly proceeded on the negative-going scan, whereas such a smooth exchange was prevented due to strongly surface-adsorbed BMIM+ on the positive-going scan. We also found that the hysteresis behavior was relatively suppressed in the case of the electrodes having 70 nm and 120 nm diameter dimples. We carried out the In-situ ATR-IR measurement to investigate the structure of ionic liquid molecules at the ionic liquid/electrode interface. The details of the relationships between the arrangement of ionic liquid at the interface and the capacitance of the electrode will be discussed.

Nanostructured Au Electrode with 100 h Stability for Solar-Driven Electrochemical Reduction of Carbon Dioxide to Carbon Monoxide.

Solar-to-chemical energy conversion is a potential alternative to fossil fuels. A promising approach is the electrochemical (EC) reduction of CO2 to value-added chemicals, particularly hydrocarbons. Here, we report on the selective EC reduction of CO2 to CO on a porous Au nanostructure (pAu) cathode in 0.1 M KHCO3. The pAu cathode anodized at 2.6 V exhibited maximum Faradaic efficiency (FE) for conversion of CO2 to CO (up to 100% at −0.75 V vs reversible hydrogen electrode (RHE)). Furthermore, commercial Si photovoltaic cells were combined with EC systems (PV-EC) consisting of pAu cathodes and IrO2 anodes. The triple-junction cell and EC system resulted in a solar-to-CO conversion efficiency (SCE) of 5.3% under 1 sun illumination and was operated for 100 h. This study provides a PV-EC CO2 reduction system for CO production and indicates the potential of the PV-EC system for the EC reduction of CO2 to value-added chemicals.

Water structure at the multilayers of palladium deposited at nanostructured Au electrodes

In situ ATR-SEIRAS experiments were carried out to describe water structure at the surface of multilayers of Pd modified nanopatterned gold film electrodes in contact with aqueous 0.1 M HClO 4 solution. The results show that the surface water becomes more ordered when the thickness of the Pd film increases. The “ice-like water” band becomes stronger and for the electrode with 5 ML Pd@Au, the ratio of “ice-like water” to “liquid-like water” becomes larger than that in bulk water. Concomitantly, the population of “monomers” or small water clusters diminishes. All surface water bands increase when hydrogen adsorption takes place. The bending band corresponding to “monomers” or “multimers” water is observed chiefly at the potentials of hydrogen adsorption.

An affordable option to Au single crystals through cathodic corrosion of a wire: Fabrication, electrochemical behavior, and applications in electrocatalysis and spectroscopy

Faceting and nanostructuring of polycrystalline gold electrodes by cathodic corrosion in concentrated potassium hydroxide electrolytes has been systematically studied at different electrode potentials. Current-potential curves for the restructured Au electrodes in 0.1 M H2SO4 show characteristics of Au(111) in the double-layer and oxidation region. Thus, the modified Au electrodes adopt properties typically known for well-defined single crystal surfaces. Besides the preferential surface faceting, the electrochemically active surface area (EASA) is enhanced as a function of potential, concentration and time. Scanning electron micrographs show the formation of triangular pits and nanostructures with a specific orientation confirming the formation of (111)-facets. In this way, the behavior of single crystals is accompanied with the properties of nanoparticles which are of utmost interest in electrocatalysis and surface-enhanced Raman spectroscopy (SERS). The electrocatalytic activity of the newly formed “Au(111)” surface from an Au wire has been tested towards the hydrogen evolution reaction (HER) and for the formic acid oxidation reaction (FAOR). The study of electrocatalytic reactions at these nanostructured electrodes allows to identify active centers, which are absent for extended single crystal surfaces. Adsorbed pyridine on the nanostructured Au electrodes directly shows SERS activity, while untreated polycrystalline Au is SERS-inactive. The use of cathodic corrosion of simple wires is a paradigm of SERS-applications in electrochemistry with clean Au electrodes that provide properties of Au(111) single crystals.

Surface Mechanism of Catalytic Electrodes in Lithium-Oxygen Batteries: How Nanostructures Mediate the Interfacial Reactions.

The use of catalysts is the key to boost electrode reactions in lithium-oxygen (Li-O2) batteries. In-depth understanding of the nanoscale catalytic effect at electrode/electrolyte interfaces is of great significance for guiding a design of functionally optimized catalyst. Here, using electrochemical atomic force microscopy, we present the real-time imaging of interfacial evolution on nanostructured Au electrodes in a working battery, revealing that the nanostructure of Au is directly related to the catalytic activity toward oxygen reduction reaction (ORR)/oxygen evolution reaction (OER). In situ views show that nanoporous Au with a size of ∼14 nm for ligaments and ∼5 nm for nanopores promote the nucleation and growth of discharge product Li2O2 with large size at a high discharge voltage, yet densely packed Au nanoparticles with a diameter of ∼15 nm could catalyze Li2O2 to fully decompose via the top-bottom approach at a low charge potential. In addition, the difference in the nucleation potential of Li2O2 on the electrode with hybrid nanostructures could result in an uneven distribution of discharge products, which is alleviated at a large discharge rate and the capacity of the battery is improved significantly. These observations provide deep insights into the mechanisms of Li-O2 interfacial reaction catalyzed by nanostructured catalysts and strategies for improving Li-O2 batteries.

(Invited) Cathodic Corrosion of Polycrystalline Au Electrodes: Applications in Electrocatalysis and Surface Spectroscopy

Faceting and nanostructuring of polycrystalline Au electrodes by cathodic corrosion in concentrated aqueous alkali metal hydroxide electrolytes have been studied systematically. Scanning electron micrographs show the formation of octahedral Au nanocrystals and triangular pits by using NaOH and KOH solutions, respectively. These Au surfaces with preferential (111)-texture and a unique nanostructure are of special interest for new developments and applications in electrocatalysis and surface-enhanced Raman spectroscopy (SERS). Cathodic corrosion of Au leads to a single-crystal-like electrochemical behavior with characteristic features of Au(111) electrodes. The electrocatalytic behavior of this new type of nanostructures will be presented for the hydrogen evolution reaction (HER) and for the formic acid oxidation reaction (FAOR). The Raman activity of adsorbed pyridine on the nanostructured Au electrodes was studied with SERS. While untreated polycrystalline Au is not SERS-active, the use of cathodic corrosion opens up a way for in-situ SERS-applications in electrochemistry. Figure 1

CO2 Reduction to CO on Au: Facet Dependent Activity and Selectivity

Due to the drastic drop in renewable electricity prices, electrochemical CO2 reduction has received renewed interest. While CO2 reduction can lead to a wide variety of products, almost all go through a CO intermediate, thus understanding the 2 electron transfer reaction of CO2 to CO is important not just for CO production, but also for products such as ethylene, ethanol, methane and others. Au has been shown to be the most active electrocatalyst for CO production, however major fundamental knowledge on this reaction has yet to be discovered. While most works focus on highly active nanostructured Au electrodes, there has yet to be a comprehensive study on the intrinsic electrochemical activity of individual Au facets.This talk will discuss our work on the electrochemical conversion of CO2 to CO on single crystal Au <100>, <111>, <110>, and <211> facets as well as polycrystalline Au. Our results show a 25 fold difference between the least and most active crystal facets. All facets follow approximately the same Tafel slope between the tested potentials of -0.6 V to -0.8 V vs. RHE. Pb underpotential deposition (UPD) was used to deposit a monolayer of Pb on the Au gold surface to determine surface area. However, the potential at which Pb UPD deposits is facet dependent. By setting a correct potential we could selectively deposit lead on a given facet. While PB UPD favored depositing on the most active sites, we still could gain fundamental catalysis knowledge. On polycrystalline Au, 20% of a monolayer of Pb yielded a 50% decrease in CO production, but only a 30% decrease in H2 production. 50% of a Pb monolayer on Au yielded a 95% decrease in CO production, but only a 50% decrease on H2 production. The decrease in CO production as a function of Pb coverage indicated that not only could we passivate the most active sites, but also that the facet dependent activity was quite different for CO production versus the competing H2 evolution side reaction. To further utilize Pb UPD to isolate sites we reinvestigated one of our least active facets, Au <111>. As expected, STM images show that our single crystal is not completely flat and there is a small percentage of step sites. We used selective Pb UPD to deposit 4% of a monolayer in the attempts to passivate these sites. This led to a 50% decrease in CO activity, but the same H2 evolution activity. Covering the Au with 15% of a monolayer led to a 90% drop in CO activity, but the same H2 evolution activity. These results suggest that our intrinsic activity of our least active facets is probably much lower than measured and is dominated by a small number of highly active defect sites. While these results focus on Au, no single crystal is defect free. Thus these results may lead us to also question whether there are other situations where the non-optimal facets for a given reaction are actually much less active than originally thought.

Surface-Morphology-Dependent Electrolyte Effects on Gold-Catalyzed Electrochemical CO2 Reduction

The electrocatalytic property of a flat or an oxide-derived nanostructure Au electrode was investigated using surface sensitive analysis methods such as impedance spectroscopy and Kelvin probe force microscopy (KPFM) when electrochemical conversion of carbon dioxide (CO2) to carbon monoxide (CO) was performed with either KHCO3- or NaHCO3-based neutral electrolyte. A strong dependence on the cation of the electrolyte was exhibited on the flat Au electrode surface. CO selectivity and capacitance dispersion are significantly higher with the KHCO3 electrolyte. On the other hand, the nanostructured Au electrodes, having much more improved activity and durability of CO2 reduction, showed much less electrolyte-dependent catalytic activity. The difference in CO selectivity with KHCO3 and NaHCO3 electrolytes can be explained by the difference in hydration level and consequent adsorption strength of the cations on the flat Au metal electrodes, implying that ion-pairing interactions between the metal, cations, CO2, ...

Determination of the Local Electric Field at Au/SAM Interfaces Using the Vibrational Stark Effect

A comprehensive understanding of physical and chemical processes at biological membranes requires the knowledge of the interfacial electric field which is a key parameter for controlling molecular structures and reaction dynamics. An appropriate approach is based on the vibrational Stark effect (VSE) that exploits the electric-field dependent perturbation of localized vibrational modes. In this work, 6-mercaptohexanenitrile (C5CN) and 7-mercaptoheptanenitrile (C6CN) were used to form self-assembled monolayers (SAMs) on a nanostructured Au electrode as a simple mimic for biomembranes. The C≡N stretching mode was probed by surface enhanced infrared absorption (SEIRA) spectroscopy to determine the frequency and intensity as a function of the electrode potential. The intensity variations were related to potential-dependent changes of the nitrile orientation with respect to the electric field. Supported by electrochemical impedance spectroscopy, molecular dynamics simulations, and quantum chemical calculations...

(Invited) Influence of Nano-Structuring of Substrate Materials on Local Structure of Ionic Liquid

Ionic liquids (ILs) have attracted much attention as promising electrolytes for electrochemical devices due to their wide electrochemical window (stability) and negligible vaporization. For such applications, nanostructured electrodes have advantage on their large surface area leading to accumulation of large density energy at the interface. However, recent reports on IL/electrode interfaces revealed that quite strange structures can be formed and it is not clear how the interface forms and how it changes by forming the characteristic local structure. In this study, we investigated the local structure and physicochemical properties of the interfacial properties of nanostructured materials using XPS and electrochemical impedance spectroscopy (EIS). Polystyrene beads were self-assembled in a close packed form on a gold substrate by dipping in polystyrene beads solution and rapid drying. Au electrodeposition on thus prepared surface followed by removal of the beads resulted in the fabrication of a nanostructured Au electrode with periodic 100 nm-sized Au dimples. Electrochemical behavior of a ferrocene dissolved ionic liquid (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide (BMI-TFSA)) at the nanostructured Au electrode was investigated by EIS. It is obvious that the capacitance of the nanostructured electrode was smaller than that of the flat electrode in the whole potential range. This result suggests that the thickness of the electric double layer formed in the nano-sized dimple is thicker than that formed on the flat surface. The dimple size dependence on the capacitance will also be discussed. On the other hand, it is an interesting subject to study how ILs and solutes behave in the nano-micro sized pore. We investigated the properties of IL(BMI-TFSA) and solute ions (Au3+, Ag+, Mg2+ etc.) confined in the porous silica whose diameter was 1.8 nm using XPS, IR and TEM. It was revealed that the molar ratio of all the ions existing in the pore is largely different from that in the bulk. Interestingly, the molar ratio of Au3+ to BMI+ (or TFSA-) was very large comparing with that of as prepared BMI-TFSA solution. The result of N-1s XPS spectra revealed that cation and anion of IL were organized in the nanopores. Such characteristic local structure in nanospace may affect the catalytic reaction in the pore.

(Invited) Behavior of Ionic Liquid and Solute Molecules at Electrode/Ionic Liquid Interface

Ionic liquids (ILs) have attracted keen attention due to its applicability to various kinds of electrochemical devices such as Li-ion battery, wet-type solar battery, etc. For most of such applications, the interface between ionic liquid and electrodes plays an important role in their performance. However, we still lack enough knowledge on the interface structure and spatial distribution of chemical species during the electrochemical processes. 1. Nanostructured electrode In an application of electrochemical devices, nanostructured electrodes have advantage on their large surface area leading to accumulation of large density energy at the interface. However, recent reports on IL/electrode interfaces revealed that quite strange structures can be formed and it is not clear how the interface forms and how it changes by forming an electric double layer (EDL). We investigated the interfacial capacitance of nanostructured Au electrodes with periodic 100 nm-sized dimples using electrochemical impedance spectroscopy (EIS). It was revealed that the capacitance of the nanostructured electrode was largely different from that of the flat electrode in the whole potential range. This result suggests that the thickness of the electric double layer formed in the nano-sized dimple is different from that formed on the flat surface. The dimple size dependence on the capacitance was also observed. 2. ILs confined in Nanopore It is known that ILs confined in small spaces exhibit characteristic properties which are different from those of bulk ILs. Therefore, it is an interesting subject to study how ILs and solutes behave in the nano-micro sized pore. We investigated the properties of IL(BMI-TFSA) and solute ions (Au3+, Ag+, Mg2+ etc.) confined in the porous silica whose diameter was 1.8 nm using XPS, IR and TEM. It was revealed that the molar ratio of all the ions existing in the pore is largely different from that in the bulk. Interestingly, the molar ratio of Au3+ to BMI+ (or TFSA-) was very large comparing with that of as prepared BMI-TFSA solution. The result of N-1s XPS spectra revealed that cation and anion of IL were organized in the nanopores. Such characteristic local structure in nanospace may affect the catalytic reaction in the pore. So, we tried to synthesize Au nanoparticles via a reductive reaction in an ionic liquid confined in a nanopore using an X-ray irradiation technique[1,2]. High density Au particles whose size was limited by the pore diameter were observed (Fig. 1 left). This result is very interesting and benefits not only for the formation of the high density carrier but also for the development of the porous electrode used for various kinds of electrochemical devices and/or techniques such as battery, electrochemical deposition, impurity removing devices and so on, because condensation of reactant ions in the nanopore leads to the effective electrochemical (or chemical) reaction. 3. Diffusion behavior of solute ions at IL/electrode interface It is known that most of ILs show extremely low vapor pressure, which enables us to apply various techniques that need vacuum conditions such as photoelectron spectroscopy. We recently developed the electrochemical photoelectron spectroscopy (EC-PES) to carry out in-situ XPS and UPS measurements under electrochemical condition using 3-electrodes type cell (Fig. 1 right). By observing the spatial distribution of concentration of contained ions close to the electrode (The mole fraction of solute and ionic liquid ions was plotted against the distance from the edge of the working electrode), we firstly succeeded in revealing the extraordinary diffusion behavior of solute ion (Ag+) in ionic liquid (BMI-TFSA). Under the applied potential of 0 V vs. Pt, the concentration of Ag+ ion was uniform. Under the applied potential of -1.2 V vs. Pt, a drastic change was observed: the concentration of Ag+ ion decreased at the edge of electrode, and gradually increased at larger distance from the electrode. This indicates that the diffusion layer was formed by the reductive reaction of Ag+ (Ag+ + e- => Ag) at the electrode. The thickness of the observed diffusion layer was 240 μm, which was much larger than the value usually supposed. By the numerical simulation, we revealed that the metal ions diffused by the hopping-like mechanism, in which the apparent diffusion coefficient increased with increasing of the concentration of ‘holes’ (vacancy formed between IL molecules) acting as hopping sites. [1] A. Imanishi, M. Tamura, S. Kuwabata, Chem. Commun. (2009)1775. [2] A. Imanishi, S. Gonsui, T. Tsuda, S. Kuwabata, K. Fukui, Phys. Chem. Chem. Phys., 13(2011)14823. Figure 1

Contributors to Enhanced CO2 Electroreduction Activity and Stability in a Nanostructured Au Electrocatalyst.

The formation of a nanostructure is a popular strategy for catalyst applications because it can generate new surfaces that can significantly improve the catalytic activity and durability of the catalysts. However, the increase in the surface area resulting from nanostructuring does not fully explain the substantial improvement in the catalytic properties of the CO2 electroreduction reaction, and the underlying mechanisms have not yet been fully understood. Here, based on a combination of extended X-ray absorption fine structure analysis, X-ray photoelectron spectroscopy, and Kelvin probe force microscopy, we observed a contracted Au-Au bond length and low work function with the nanostructured Au surface that had enhanced catalytic activity for electrochemical CO2 reduction. The results may improve the understanding of the enhanced stability of the nanostructured Au electrode based on the resistance of cation adhesion during the CO2 reduction reaction.

Epinephrine sensing at nanostructured Au electrode and determination its oxidative metabolism

Epinephrine (EP) is a neurotransmitter and hormone involved in an enormous number of processes in living organisms, however its highly active oxidation metabolites can cause hard damages and lead to serious health consequences. This paper described a relatively simple method of fabrication of Au nanotube array electrodes for electrochemical determination of EP, which could be easily transferred to non-laboratory conditions. On the basis of Tafel plot, some parameters of EP oxidation process like number of electrons involved in the rate-determining step (nα=2) and electron transfer coefficient (α=0.45) were evaluated for the studied electrode. The linear sweep voltammetry (LSV) revealed a very good linear response in the EP concentration range of 60–600μM EP, however to 1000μM its linearity differs insignificantly. Using differential pulse voltammetry (DPV), the calibration curve for EP determination is close to linear in the range of 10–150μM, but the detection limit of 2.8μM was evaluated for a lower concentration range 10–60μM. The proposed DPV method was successfully applied to the determination of EP in a pure, ascorbic acid containing and real samples. The DPV results were evaluated and compared with those obtained by the high-performance liquid chromatography (HPLC) method. Combining the ROXY™ EC System with mass spectrometry creates an analytical tool for oxidative products of EP and conjugation of EP reactive potential metabolites characterized for I and II stage of biotransformation. The presence of leucoadrenochrome-o-semiquinone radicals confirmed by mass spectrometry implies that the ECE (electron transfer-chemical reaction-electron transfer) mechanism of EP oxidation at the Au electrode is more feasible.

Effect of nanocomposite Au–YSZ electrodes on potentiometric sensor response to NOx and CO

The solid-state potentiometric sensor responses of high surface area nanocomposite and low surface area gold sensing electrodes were compared. The composite nature of the high surface area gold and stabilized zirconia electrode preserved average gold particle size of 62nm after calcination and sintering. The electrochemical sensor response of up to 300ppm CO, NO2 or NO gas in the presence of 3% O2 (balance N2) was measured. Sensors using the low surface area Au electrodes had a larger sensitivity to all test gases, more than twice the sensitivity to CO and NO2, than the high surface area nanostructured composite electrode. The superior heterogeneous catalytic activity of the nanostructured Au electrode, as determined through temperature programmed reaction, is believed to reduce the charge transfer potentiometric sensitivity to the test gases. In comparison, the low surface area Au sensing electrode did not favor heterogeneous catalysis of CO or NO2 at 500°C, the temperature of sensor operation.

Aqueous CO2 Reduction at Very Low Overpotential on Oxide-Derived Au Nanoparticles

Carbon dioxide reduction is an essential component of many prospective technologies for the renewable synthesis of carbon-containing fuels. Known catalysts for this reaction generally suffer from low energetic efficiency, poor product selectivity, and rapid deactivation. We show that the reduction of thick Au oxide films results in the formation of Au nanoparticles ("oxide-derived Au") that exhibit highly selective CO(2) reduction to CO in water at overpotentials as low as 140 mV and retain their activity for at least 8 h. Under identical conditions, polycrystalline Au electrodes and several other nanostructured Au electrodes prepared via alternative methods require at least 200 mV of additional overpotential to attain comparable CO(2) reduction activity and rapidly lose their activity. Electrokinetic studies indicate that the improved catalysis is linked to dramatically increased stabilization of the CO(2)(•-) intermediate on the surfaces of the oxide-derived Au electrodes.

Van der Waals interactions at metal/organic interfaces at the single-molecule level

Van der Waals (vdW) interaction, and its subtle interplay with chemically specific interactions and surface roughness at metal/organic interfaces, is critical to the understanding of structure-function relations in diverse areas, including catalysis, molecular electronics and self-assembly. However, vdW interactions remain challenging to characterize directly at the fundamental, single-molecule level both in experiments and in first principles calculations with accurate treatment of the non-local, London dispersion interactions. In particular, for metal/organic interfaces, efforts so far have largely focused on model systems consisting of adsorbed molecules on flat metallic surfaces with minimal specific chemical interaction. Here we show, through measurements of single-molecule mechanics, that pyridine derivatives can bind to nanostructured Au electrodes through an additional binding mechanism beyond the chemically specific N-Au donor-acceptor bond. Using density functional theory simulations we show that vdW interactions between the pyridine ring and Au electrodes can play a key role in the junction mechanics. These measurements thus provide a quantitative characterization of vdW interactions at metal/organic interfaces at the single-molecule level.

Ultrathin Transparent Au Electrodes for Organic Photovoltaics Fabricated Using a Mixed Mono‐Molecular Nucleation Layer

AbstractA rapid, solvent free method for the fabrication of highly transparent ultrathin (∼8 nm) Au films on glass has been developed. This is achieved by derivatizing the glass surface with a mixed monolayer of 3‐mercaptopropyl(trim­ethoxysilane) and 3‐aminopropyl(trimethoxysilane) via co‐deposition from the vapor phase, prior to Au deposition by thermal evaporation. The mixed mono­layer modifies the growth kinetics, producing highly conductive films (∼11 Ω per square) with a remarkably low root‐mean‐square roughness (∼0.4 nm) that are exceptionally robust towards UV/O3 treatment and ultrasonic agitation in a range of common solvents. As such, they are potentially widely applicable for a variety of large area applications, particularly where stable, chemically well‐defined, ultrasmooth substrate electrodes are required, such as in organic optoelectronics and the emerging fields of nanoelectronics and nanophotonics. By integrating microsphere lithography into the fabrication process, we also demonstrate a means of tuning the transparency by incorporating a random array of circular apertures into the film. The application of these nanostructured Au electrodes is demonstrated in efficient organic photovoltaic devices where it offers a compelling alternative to indium tin oxide coated glass.

Electrochemical and density functional studies of the catalytic ethylene oxidation on nanostructured Au electrodes

Electrocatalytic oxidation of ethylene on gold was investigated by means of differential electrochemical mass spectroscopy (DEMS), quartz crystal microbalance (QCM) and DFT calculations. The product analysis indicates a selectivity of the polycrystalline gold towards formation of acetaldehyde. The oxidation process on fresh surface prepared by surface gold oxide reduction, on the other hand, forms mainly carbon dioxide. The oxidation process encompasses metal dissolution. DFT based analysis of the ethylene oxidation on Au clusters was employed to interpret the experimental data. DFT calculations support experimental findings and indicate possible reaction mechanisms of catalytic reactions. DFT calculations point to the different reactivity on individual types of surfaces and different types of clusters.

Electrophoretic deposition of uniformly distributedTiO2 nanoparticles using an anodic aluminum oxide template for efficient photolysis

In this study, a novel method for fabricating a high efficiency titanium dioxidephotocatalyst is presented. Radio frequency (RF) magnetron sputtering was employed togrow 3D nano-structured Au thin films using the barrier-layer side of anodic aluminumoxide (AAO) as the template. The template was prepared by immersing this side ofan AAO film into a 30 wt% phosphoric acid solution to modify the surface ofthe barrier layer in such a manner that a contrasting surface was obtained. Theelectrophoretic deposition method was then implemented to uniformly depositTiO2 nanoparticles on the 3D nano-structured Au electrode. The efficiency of theproposed photocatalyst, in terms of the photolysis efficiency, was measured usingproton-motive-force driving chloroplastmimic photovoltaics. Experiments were carriedout to demonstrate the photolysis efficiency of the proposed 3D nano-structuredTiO2 photocatalyst, which showed a 10-fold increase over that of Degussa P25 planeTiO2. The photocurrent could be further enhanced by the deposition of a sensitized dye, such asN3, which extended the absorption spectrum from the UV range to the visible light range.