The bifurcation point of the oxygen reduction reaction on Au-Pd nanoalloys.

The oxygen reduction reaction is of major importance in energy conversion and storage. Controlling electrocatalytic activity and its selectivity remains a challenge of modern electrochemistry. Here, first principles calculations and analysis of experimental data unravel the mechanism of this reaction on Au-Pd nanoalloys in acid media. A mechanistic model is proposed from comparison of the electrocatalysis of oxygen and hydrogen peroxide reduction on different Au-Pd ensembles. A H2O production channel on contiguous Pd sites proceeding through intermediates different from H2O2 and OOH(σ) adsorbate is identified as the bifurcation point for the two reaction pathway alternatives to yield either H2O or H2O2. H2O2 is a leaving group, albeit reduction of H2O2 to H2O can occur by electrocatalytic HO-OH dissociation that is affected by the presence of adsorbed OOH(σ). Similarities and differences between electrochemical and direct synthesis from H2 + O2 reaction on Au-Pd nanoalloys are discussed.

Nanoparticles and self-organisation: the emergence of hierarchical properties from the nanoparticle soup (i.e., the small is getting bigger). Concluding remarks for Faraday Discussion: Nanoparticle Synthesis and Assembly.

Some four years ago, one of the participants in this Discussion (Prof. Nicholas Kotov) predicted that: "within five years we shall see multiple examples of electronic, sensor, optical and other devices utilizing self-assembled superstructures" (N. A. Kotov, J. Mater. Chem., 2011, 21, 16673-16674). Although this prediction came partially to fruition, we have witnessed an unprecedented interest in the properties of materials at the nanoscale. The point highlighted by Kotov, however, was the importance of self-assembly of structures from well characterised building blocks to yield hierarchical structures, hopefully with predictable properties, a concept that is an everyday pursuit of synthetic chemists. This Discussion has brought together researchers from a wide range of disciplines, i.e., colloid science, modelling, nanoparticle synthesis and organisation, magnetic and optical materials, and new imaging methods, within the excellent traditional Faraday Discussion format, to discuss advances in areas relevant to the main theme of the meeting.

Structure and dielectric properties of electrochemically grown ZrO2 films.

The dielectric properties of electrochemically grown zirconium oxide films by anodisation of zirconium in 1.0 mol dm-3 phosphoric acid solution were investigated in a 3 to 30 V potential range with a view to inducing surface modifications for eventual use in biomedical and electronic applications. The oxide films grown at different potentials were characterised by Atomic Force Microscopy, X-ray photoelectron and Raman spectroscopies; the latter demonstrated the incorporation of phosphate ions into the passive films. Flat band potentials calculated from the Mott-Shottky analysis of the oxides semiconducting properties confirm the bilayer structure of the films. The oxide dielectric permittivity was evaluated from impedance spectroscopy measurements and the film oxide model proposed gave values independent of the oxide growth potential.

Quantized electron-transfer pathways at nanoparticle-redox centre hybrids.

Hexanethiolate gold monolayer-protected clusters (C6-MPCs) with an average core diameter of 1.8 nm and a capacitance of 0.6 aF are synthesised by a two-phase method. These clusters are functionalised with (6-ferrocenyl)-1-hexanethiol by a place exchange reaction at different molar ratios. The average number of ferrocene centres per cluster determined by (1)H NMR is ten, seven and four. Differential pulse voltammetry and cyclic voltammetry measurements for cluster solutions in 0.1 M TBAPF(6)/Tol:AN (2:1) clearly show the response of the Fc(+)/Fc redox couple and of quantized double layer (QDL) charging events of the gold core. A transition from single to multiple electron-transfer response for the redox couple is observed as the number of ferrocene units per cluster is increased. The distances between the redox moieties are estimated considering a homogeneous distribution of the redox sites on the nanoparticle ligand shell. In all the cases, the inter-ferrocene average separation is too large to observe self-exchange reactions and the most likely electron-transfer pathway is by fast rotational diffusion. The oxidation of the ferrocene groups results in an electrostatic switching-off of electron transfers between the electrode and the nanoparticle core.

O2 induced Cu surface segregation in Au-Cu alloys studied by angle resolved XPS and DFT modelling.

Surface segregation effects on polycrystalline Au-Cu alloys (Au(0.80)Cu(0.20), Au(0.85)Cu(0.15) and Au(0.90)Cu(0.10)) were studied at room temperature by angle resolved XPS (ARXPS) and density functional theory (DFT) before and after exposure to O(2). Au surface enrichment was found as predicted from calculations showing that this process is energetically favourable, with a segregation energy for Au in a Cu matrix of -0.37 eV atom(-1). Surface enrichment with Cu was observed after exposure to O(2) due to its dissociative adsorption, in agreement with DFT calculations that predicted an energy gain of -1.80 eV atom(-1) for the transfer of Cu atoms to a surface containing adsorbed oxygen atoms, thus leading to an inversion in surface population.

Single atom hot-spots at Au-Pd nanoalloys for electrocatalytic H2O2 production.

A novel strategy to direct the oxygen reduction reaction to preferentially produce H(2)O(2) is formulated and evaluated. The approach combines the inertness of Au nanoparticles toward oxidation, with the improved O(2) sticking probability of isolated transition metal "guest" atoms embedded in the Au "host". DFT modeling was employed to screen for the best alloy candidates. Modeling indicates that isolated alloying atoms of Pd, Pt, or Rh placed within the Au surface should enhance the H(2)O(2) production relative to pure Au. Consequently, Au(1-x)Pd(x) nanoalloys with variable Pd content supported on Vulcan XC-72 were prepared to investigate the predicted selectivity toward H(2)O(2) production for Au alloyed with Pd. It is demonstrated that increasing the Pd concentration to 8% leads to an increase of the electrocatalytic H(2)O(2) production selectivity up to nearly 95%, when the nanoparticles are placed in an environment compatible with that of a proton exchange membrane. Further increase of Pd content leads to a drop in H(2)O(2) selectivity, to below 10% for x = 0.5. It is proposed that the enhancement in H(2)O(2) selectivity is caused by the presence of individual surface Pd atoms surrounded by gold, whereas surface ensembles of contiguous Pd atoms support H(2)O formation. The results are discussed in the context of exergonic electrocatalytic H(2)O(2) synthesis in Polymer Electrolyte Fuel Cells for the simultaneous cogeneration of chemicals and electricity, the latter a credit to production costs.

Role of axially coordinated surface sites for electrochemically controlled carbon monoxide adsorption on single crystal copper electrodes.

The adsorption of CO on low index copper single crystals in electrochemical environments has been investigated. The results, analysed through a combination of in situ infrared spectroscopy, DFT and cyclic voltammetry, reveal a unique adsorption behaviour when compared to previous studies on copper and the more widely studied noble metal surfaces. By employing small, weakly specifically adsorbed electrolytes, it is shown that carbon monoxide is adsorbed over a much wider electrode potential range than previously reported. The electrochemical Stark shift (δν/δE) observed is similar for the three Cu(hkl) surfaces examined despite different surface coverages. Most notably, however, is an electrochemical feature observed at ca. -1.0 V (vs. Ag/AgCl) on the (110) surface. It is proposed that this voltammetric feature arises from the reduction/oxidation of Cu(δ+) surface sites involved in the binding of carbon monoxide with the participation of the electrolyte anion. This provides additional specific sites for CO adsorption. DFT calculations support the proposed presence of low-coordination copper sites stabilised by electrolyte anions. An experimental electron transfer rate constant of 4.2 s(-1) to the Cu(δ+) surface sites formed was found. These new observations concerning the surface electrochemistry of CO on Cu indicate that the electrocatalytic behaviour of Cu electrodes in processes such as CO(2) reduction need to be re-evaluated to take account of the rich adsorption behaviour of CO, including the co-adsorption of the electrolyte anion to these sites.

Electrocatalytic oxygen reduction on functionalized gold nanoparticles incorporated in a hydrophobic environment.

The electrocatalytic properties of gold nanoparticles covalently capped with a monolayer film of 1,4-decylphenyl groups for oxygen reduction in an alkaline solution have been studied. Functionalized nanoparticles were adsorbed on a film of the same capping ligand previously grafted to a glassy carbon electrode. The molecular film-nanoparticle assembly was characterized by cyclic voltammetry and XPS. It is shown that although the attachment of the capping ligand to the electrode surface blocks direct electron transfer, the metal centers of the incorporated nanoparticles provide sites for electron tunneling from the electrode surface thus leading to sites where oxygen reduction can take place. Rotating disk voltammetry shows that the oxygen reduction reaction follows mainly a peroxide formation channel on these nanostructured surfaces. The capping ligand greatly influences the reduction mechanism by establishing a local hydrophobic environment at the reaction centers within the film.

Kinetics of electrocatalytic reduction of oxygen and hydrogen peroxide on dispersed gold nanoparticles.

The electrocatalytic properties of Au nanoparticles of mean size between 4.2 to 9.5 nm have been investigated for the oxygen reduction reaction (ORR). The particles were prepared on dispersed Vulcan XC-72R carbon black by reduction of a gold salt and by deposition of polymer stabilised gold sols. These were then attached to a glassy carbon disc electrode from their dispersion in a Nafion solution. The dependence on particle size of activity and selectivity for H(2)O(2) formation and the rate constants for oxygen and hydrogen peroxide reduction have been investigated using Rotating (Ring) Disc Electrode measurements. The electrocatalytic activity showed a maximum for a mean particle size of 5.7 nm and decreased significantly with particle size. The number of electrons exchanged per O(2) molecule increased from a value close to 2 to 3.4 as the potential was made more negative. The oxygen reduction selectivity for H(2)O(2) production was higher for mean particle sizes below 6 nm.

Direct electron transfer to a metalloenzyme redox center coordinated to a monolayer-protected cluster.

A strategy for establishing electrical contact to the metal center of a redox metalloenzyme, galactose oxidase (GOase), by coordination of a linker attached to a monolayer-protected gold cluster is presented. The cluster-enzyme hybrid system was first prepared in solution and characterized by high-angle annular dark-field scanning transmission electron microscopy. Electrochemical communication between a gold electrode and GOase was achieved by first modifying the electrode surface with a biphenyl dithiol self-assembled monolayer followed by reaction with gold clusters capped with thioctic acid. GOase was then immobilized by replacement of the H(2)O molecule at the Cu(II) exogenous site by coordination of a carboxylate-terminated gold cluster. This chemical attachment ensured electrical contact between the redox center and the electrode, leading to direct mediatorless electron transfer to the protein. Hybrid systems can find applications in biosensors and biofuel cells and for studying electrochemically the catalytic mechanism of reactions for which free radicals and electron-transfer reactions are involved. The present results can be extended to other metalloenzymes.

Electrochemical reduction of oxygen on nanoparticulate gold electrodeposited on a molecular template.

The reduction of oxygen on gold electrodeposited on an organic template has been investigated. The template consisted of reduced 4-nitrophenyl groups attached to glassy carbon (GC) by the electrochemical reduction of the corresponding diazonium compound. The electrode modified by this Au nanostructured film shows electrocatalytic properties for the oxygen reduction reaction (ORR) different from those of bulk Au, GC or GC grafted with 4-nitrophenyl groups. The reduced nitrophenyl film inhibits the O2 reduction reaction. A two-step reduction mechanism with production of hydrogen peroxide in the first step and water in the second was observed in alkaline solutions. The standard heterogeneous rate constants for this two-electron transfer sequential reaction (EE reduction mechanism) have been calculated using non-linear regression analysis (NLR).

All dressed up, but where to go? Concluding remarks for FD 140.

The aim of this meeting was to bring together experimentalists and theoreticians to discuss the interplay between recent developments in theoretical and computational tools with experimental results in electrocatalysis. Intense and rewarding discussions in aspects of very topical electrochemical research arose during the three days of this Faraday Discussion. A closer collaboration between experimentalists and theoreticians is a prerequisite for successful development of the field and this meeting definitely aided mutual understanding of these areas of research. Why was this meeting important and timely? It is an interesting observation that, since we are immersed in rapid fundamental changes in the way scientific research is carried out, we sometimes cease to notice how much this has changed. Clear examples of this are the better availability of advanced instrumental and computational techniques and the instantaneous access to scientific information through the Internet. Thus, we are in the middle of revolutionary changes in scientific research resulting from three new fundamental advances. The first refers to the possibility of carrying out quantum chemical calculations at a high level of theory and using these for the modelling of complex electrochemical reactions to give an insight into interfacial structure and reaction mechanisms. In some cases, this is providing quantitative information of preferred reaction channels that it would have not been possible to obtain a decade ago. The second refers to the new instrumental techniques that have become available. The third is developments in materials science.

Photoswitching electron transport properties of an azobenzene containing thiol-SAM.

The influence of conformational and electrical properties of azobenzene molecules on the electron transfer barrier properties of their SAMs was studied by SECM and ellipsometry.

Dependence of the critical temperature on molecular parameters.

At the critical temperature the surface tension between coexisting liquid and vapor phases must be zero, and the repulsive contributions associated with cavity formation must exactly counterbalance those from interactions of a molecule in the cavity and the bulk. An expression for the critical temperature of pure fluids in terms of the parameters of scaled particle theory (SPT) has been obtained, and the calculated critical temperatures are compared with experimental data for a range of pure fluids. These include noble and diatomic gases, short and medium length hydrocarbons, aromatic compounds, halogenated compounds, oxygen-containing compounds, and water. Considering the simplicity of this approach, a remarkably good correlation between calculated and experimental values is found for most of these fluids.

Optical switching of coupled plasmons of Ag-nanoparticles by photoisomerisation of an azobenzene ligand.

The optical switching of coupled plasmons of silver nanoparticles derivatised with a photoisomerisable azobenzene ligand is presented. It is shown that nanoparticle clusters, linked with an azobenzene dithiol molecule, display switchable optical properties. The photoisomerisation of the linker molecule was used to vary the separation between nanoparticles, which was monitored by changes in the UV-Vis-spectra of the plasmon band of adjacent nanoparticles. A red-shift due to the appearance of a coupled longitudinal plasmon band was observed resulting from the formation of nanoparticle clusters. The maximum absorbance wavelength of this secondary plasmon band was altered by isomerisation of the linker and the spectral changes observed were in good agreement with theory and earlier measurements for gold. Evidence of energy transfer between a nanoparticle and an azobenzene terminated monothiol attached to it was also observed in the UV-Vis spectra.

Precision control of single-molecule electrical junctions.

There is much discussion of molecules as components for future electronic devices. However, the contacts, the local environment and the temperature can all affect their electrical properties. This sensitivity, particularly at the single-molecule level, may limit the use of molecules as active electrical components, and therefore it is important to design and evaluate molecular junctions with a robust and stable electrical response over a wide range of junction configurations and temperatures. Here we report an approach to monitor the electrical properties of single-molecule junctions, which involves precise control of the contact spacing and tilt angle of the molecule. Comparison with ab initio transport calculations shows that the tilt-angle dependence of the electrical conductance is a sensitive spectroscopic probe, providing information about the position of the Fermi energy. It is also shown that the electrical properties of flexible molecules are dependent on temperature, whereas those of molecules designed for their rigidity are not.

Quantum chemical modelling of the rate determining step for oxygen reduction on quinones.

Two inner-sphere electrocatalytic channels for quinone-mediated reduction of molecular oxygen to form hydrogen peroxide have been addressed by means of density functional theory. Each of the channels comprises an initial rate determining chemical step and a subsequent electrochemical reduction step by which peroxide is produced. The reduction mechanism was determined for 9,10-anthraquinone and 9,10-phenanthrenequinone and the quantum chemical results are compared with experimental results. Two distinctly different structures were determined for the critical chemical step depending on whether the catalytic site is present as HQ* or Q*-. While a superoxo species is formed on HQ*, a van der Waals (vdW) type compound is formed on Q*-. It is shown that the Gibbs energy of activation for the semiquinone/oxygen reaction is largely determined by the entropy term. The results explain the experimentally observed pH dependence of the O2 reduction rate on quinone functionalised electrodes.

Synthesis of metal nanoparticles stabilized by metal-carbon bonds.

A new method for the preparation of metal nanoparticles in organic media is proposed. This is based on the formation of metal-carbon bonds after reduction of the corresponding diazonium derivative of the capping ligand. The particles are very stable due to the strong metal-ligand covalent bond, and the proposed method appears to be an alternative for the preparation of monolayer-protected metal nanoparticles when the formation of metal-sulfur or metal-nitrogen bonds needs to be avoided.