[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: The synthesis of AuxCu(1−x) nanoalloys (NAs) by a two-phase method employing amines acting simultaneously as coordinating ligands, phase transfer agents as well as nanoparticle (NP) stabilisers, is presented. The value of x was varied between 0 and 1 and dodecylamine (DDA) and hexadecylamine (HDA) were used as ligands. The nanoparticles were characterised by elemental analysis, UV-vis and IR spectroscopies, X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), scanning TEM (STEM) and X-ray photoelectron spectroscopy (XPS). Alloying with Au imparts significant stability to Cu nanoparticles. Interestingly, some degree of surface segregation for Cu in the synthesised AuxCu(1−x) NAs is revealed from the TEM analysis, contrary to expectations based on previously calculated segregation energies. XRD analysis demonstrates a high degree of crystallinity of the cores although the crystallite sizes obtained from the Scherrer equation are smaller than TEM measurements. A mild heat treatment is sufficient to enhance the overall particle crystallinity resulting in crystallite size estimates from XRD comparable to those obtained from TEM.
Journal of Materials Chemistry 05/2012; 22(21):10514-10524. · 5.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
Physical Chemistry Chemical Physics 04/2012; 14(20):7448-55. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: Alloying of metals offers great opportunities for directing reactivity of catalytic reactions. For nanoalloys, this is critically dependent on near-surface composition, which is determined by the segregation energies of alloy components. Here Au–Pd surface composition and distribution of Pd within a Au0.7Pd0.3 nanoalloy were investigated by monitoring the electrocatalytic behavior for the oxygen reduction reaction used as a sensitive surface ensemble probe. A time-dependent selectivity toward the formation of H2O2 as the main oxygen reduction product has been observed, demonstrating that the applied potential history determines surface composition. DFT modeling suggests that these changes can result both from Pd surface diffusion and from exchange of Pd between the shell and the core. Importantly, it is shown that these reorganizations are controlled by surface adsorbate population, which results in a potential-dependent Au–Pd surface composition and in remarkable structural memory effects.Keywords: segregation; nanoparticle; ORR; H2O2; bimetallic; alloy; adsorption
Journal of Physical Chemistry Letters 01/2012; · 6.59 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
Journal of the American Chemical Society 12/2011; 133(48):19432-41. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: The most common differential pulse techniques are analyzed and their responses are compared. Depending on whether or not the initial conditions are re-established before the application of each double pulse we distinguish between double pulse and multipulse differential techniques. The nomenclature employed in the literature is clarified to avoid future confusion.The differences and similarities between the double pulse and multipulse responses are studied for electrode processes of any degree of reversibility, analysing the advantages and disadvantages of each method. The effect of the electrode size is also discussed, establishing the conditions under which the voltammograms obtained by different techniques are coincident or not.
Journal of electroanalytical chemistry 08/2011; 659(1):12-24. · 2.67 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
Physical Chemistry Chemical Physics 01/2011; 13(12):5242-51. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: Platinum nanoparticles have been prepared by potentiostatic multipulse electrodeposition with controlled nucleation and growth on freshly cleaved and electrochemically oxidized highly oriented pyrolytic graphite. The influence of the applied potential sequence on the size distribution was investigated. For short electrolysis times, the deposition of nanoparticles takes place via a progressive nucleation mechanism. A narrow size distribution was obtained by controlling independently the nucleation and growth steps, and particles with heights between 52 and 1.4 nm could be prepared by altering the pulse parameters. Anodic oxidation of the substrate had a large influence on the particle size, resulting in the preparation of particles 1.4 nm in height. XPS demonstrated that Pt particles of small size were readily oxidized. The rate of electrochemical methanol oxidation showed a dependence on the particle size, and no oxidation of methanol could be observed for the smaller sizes investigated.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: 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.
Physical Chemistry Chemical Physics 07/2010; 12(28):8042-52. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: The reduction of oxygen in alkaline solution has been studied on highly oriented pyrolytic graphite (HOPG) electrodes modified with various quinones using a rotating disk electrode (RDE). The electrode surface was modified by adsorption of quinones from a 0.1 M KOH solution. The oxygen reduction activity of these electrodes was considerably higher than that for unmodified HOPG and characteristic current maxima for oxygen reduction was observed. All quinones studied catalysed the two-electron reduction of oxygen to hydrogen peroxide. The peak potentials for oxygen reduction were in good correlation with the redox potentials of the quinones that were found from the cyclic voltammograms in oxygen-free solutions. The results obtained give further evidence that oxygen reduction is catalysed by the semiquinone radical and that the redox potential of the quinone is the most important factor determining its electrocatalytic activity for oxygen reduction.
[show abstract][hide abstract] ABSTRACT: The properties of electrochemically grafted gold electrodes surface have been investigated. Electrografting with 1- and 2-naphthyl, biphenyl and 4-bromophenyl groups yielded strongly attached layers and for the latter, electrochemical quartz crystal microbalance (EQCM) and atomic force microscopy (AFM) measurements showed that a multilayer film of bromophenyl groups was formed. This was confirmed by X-ray photoelectron spectroscopy (XPS). The blocking of electron transfer for several redox probes caused by the aryl-modification was investigated using cyclic voltammetry (CV). Oxygen reduction was studied using a rotating disk electrode (RDE) and the data also revealed a strong inhibition of this process by the attached aryl groups. The stability of these functionalised surfaces points to the strength of the Au–C bonds formed.
Journal of Electroanalytical Chemistry. 01/2010; 641:90-98.
[show abstract][hide abstract] ABSTRACT: 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.
Journal of the American Chemical Society 08/2009; 131(29):10229-36. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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).
Physical Chemistry Chemical Physics 06/2009; 11(18):3463-71. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: Glassy carbon electrodes were grafted with carboxyphenyl groups by reduction of 4-carboxyphenyldiazonium tetrafluoroborate and these modified electrodes were characterised by cyclic voltammetry and ac impedance measurements. Cu(II) was reacted with the carboxyphenyl groups in the film to give a surface voltammetric response for the immobilised Cu(II)/Cu(I) couple. The results indicated an ECEC mechanism, in which the chemical steps correspond to the change of coordination environment following the electron transfer steps. The relaxation half-life time for the Cu(I) species formed after electron transfer was estimated at (140±11)s. The large value of the peak width of ∼200mV was analysed by modelling the voltammograms and the large value of the full width at half maximum (FWHM) could be explained by dispersion in the formal potentials of Cu centres present in a variety of environments in the films studied. An ECEC mechanism (scheme of square) is proposed for the electron transfer reaction considering that the chemical step after reduction of the Cu(II) complex corresponds to conformational changes within the attached layer. Experimental data clearly show that the oxidation of the reduced film can take place from different Cu(I) complexes formed along the reduction to the fully relaxed Cu(I) species.
[show abstract][hide abstract] ABSTRACT: The electrochemical reduction of oxygen on glassy carbon (GC) electrodes modified with in situ generated diazonium cations of anthraquinone (AQ) has been studied using the rotating disk electrode (RDE) technique. The electrografting of the GC electrodes was carried out in two different media: in acetonitrile and in an aqueous acidic solution (0.5M HCl). 1- and 2-Aminoanthraquinone were used as starting compounds for the formation of the corresponding diazonium derivatives. The anthraquinone diazonium cations were generated by reaction of the aminoanthraquinones with tert-butyl nitrite and sodium nitrite in acetonitrile and in 0.5M HCl, respectively. For comparison purposes, the previously synthesised and crystallised diazonium tetrafluoroborates of anthraquinone were used for the GC surface modification. Cyclic voltammetry was employed to determine the surface concentration of AQ in O2 free 0.1M KOH. The electrocatalytic behaviour towards O2 reduction was similar for all the AQ-modified electrodes studied. The kinetic parameters of oxygen reduction were determined using a surface redox catalytic cycle model. The rate constant of the reaction between the semiquinone radical anion of AQ and molecular oxygen was virtually independent of the point of attachment of the quinone to the electrode surface.
[show abstract][hide abstract] ABSTRACT: A new strategy for the self-organization of bimetallic structures using an inorganic chemistry coordination reaction through the ligand shells of silver and gold nanoparticles is demonstrated. Silver nanoparticles functionalized with thioctic acid and nitrilotriacetic-cobalt(II) as the metal center were reacted with gold nanoparticles functionalized with imidazole moieties used as the Co(II) specific ligand. TEM and UV−vis spectroscopy demonstrate the attachment through the ligand shells of the particles.
Journal of Physical Chemistry C - J PHYS CHEM C. 06/2008; 112(27).