Electrocatalysis of redox reactions by metal nanoparticles on graphite electrodes

A selection of graphitic materials of both scientific as well as commercial importance has been modified by deposition of various metals at very low coverages under overpotential or underpotential conditions. Nanoparticles were found with some metals. The changes in the electrocatalytic activity of the supporting electrode by the metal modification were studied using electrochemical impedance measurements of a fast redox system. The carbon/solution interface was characterized with surface Raman spectroscopy, electrochemical impedance measurements, and cyclic voltammetry. Electronic Publication     

Surface chemistry of quantum-sized metal nanoparticles under light illumination

Size reduction of metal nanoparticles increases the exposure of metal surfaces significantly, favoring heterogeneous chemistry at the surface of the nanoparticles. The optical properties of metal nanoparticles, such as light absorption, also exhibit a strong dependence on their size. It is expected that there will be strong coupling of light absorption and surface chemistry when the metal nanoparticles are small enough. For instance, metal nanoparticles with sizes in the range of 2–10 nm exhibit both surface plasmon resonances, which can efficiently produce high-energy hot electrons near the surface of the nanoparticles under light illumination, and the Coulomb blockade effect, which favors electron transfer from the metal nanoparticles to the surface adsorbates. The synergy of efficient hot electron generation and electron transfer on the surface of small metal nanoparticles leads to double-faced effects: (i) surface (adsorption) chemistry influences optical absorption in the metal nanoparticles, and (ii) optical absorption in the metal nanoparticles promotes (or inhibits) surface adsorption and heterogeneous chemistry. This review article focuses on the discussion of typical quantum phenomena in metal nanoparticles of 2–10 nm in size, which are referred to as “quantum-sized metal nanoparticles”. Both theoretical and experimental examples and results are summarized to highlight the strong correlations between the optical absorption and surface chemistry for quantum-sized metal nanoparticles of various compositions. A comprehensive understanding of these correlations may shed light on achieving high-efficiency photocatalysis and photonics.

Size reduction of metal nanoparticles increases the exposure of metal surfaces significantly, favoring heterogeneous photochemistry at the surface of the nanoparticles.     

Electron transfer reactions of cytochrome c at metal electrodes

The heterogeneous electron transfer reactions of cytochromec occurring at platinum, gold and mercury electrodes are shown to be quasi-reversible. In each case the electrodes have not been modified and the cytochromec samples are native. This work extends previous work and demonstrates that biological molecule electron transfer reactions can be studied at clean metal surfaces to gain fundamental knowledge of the mechanisms of these reactions.     

Dynamics of adiabatic electron transfer reactions at metal electrodes

We have investigated the dynamics of adiabatic electron transfer reactions at metal electrodes using a Hamiltonian suggested by Schmickler (J. Electroanal. Chem., 204 (1986) 31). We show that in the adiabatic limit the problem reduces to that of dynamics of a single variable, the shift of the ionic orbital caused by its interaction with the solvent. This variable is identified as the reaction co-ordinate for the problem and we show that in certain limits, it obeys a non-linear Volterra type integral equation, with a stochastic inhomogeneous term. For an inhomogenous term with the autocorrelation function decaying exponentially, this may be converted into a differential equation for Brownian motion. This equation can be analysed to obtain the rate, through the associated Fokker-Planck equation. The rate so obtained, has a correction to the pre-exponential factor obtained by Schmickler. A possible extension to inner sphere reactions is also discussed.     

Plasmonic photocatalysts: harvesting visible light with noble metal nanoparticles

The efforts to produce photocatalysts operating efficiently under visible light have led to a number of plasmonic photocatalysts, in which noble metal nanoparticles are deposited on the surface of polar semiconductor or insulator particles. In the metal-semiconductor composite photocatalysts, the noble metal nanoparticles act as a major component for harvesting visible light due to their surface plasmon resonance while the metal-semiconductor interface efficiently separates the photogenerated electrons and holes. In this article, we survey various plasmonic photocatalysts that have been prepared and characterized in recent years.     

Characterization of surface water on Au core Pt-group metal shell nanoparticles coated electrodes by surface-enhanced Raman spectroscopy

We utilized the strategy of 'borrowing SERS activity', by chemically coating several atomic layers of a Pt-group metal on highly SERS-active Au nanoparticles, to obtain the first SERS (also Raman) spectra of surface water on Pt and Pd metals, and propose conceptual models for water adsorbed on Pt and Pd metal surfaces.     

Molecular electrodes at the exposed edge of metal/insulator/metal trilayer structures

Producing reliable electrical contacts of molecular dimensions has been a critical challenge in the field of molecule-based electronics. Conventional thin film deposition and photolithography techniques have been utilized to construct novel nanometer-sized electrodes on the exposed vertical plane on the edge of a thin film multilayer structure (metal/insulator/metal). Via thiol surface attachment to metal leads, an array of paramagnetic, cyanide-bridged octametal complexes, [(pzTp)FeIII(CN)3]4[NiII(L)]4[O3SCF3]4 (1) [(pzTp) = tetra(pyrazol-1-yl)borate; L = 1-S(acetyl)tris(pyrazolyl)decane], were covalently linked onto the electrodes forming a dominant conduction pathway. A series of molecule-based devices were fabricated using Ni, NiFe, Ta, and Au as metal electrodes separated by insulating Al2O3 spacers, followed by treatment with 1. A series of control experiments were also performed to demonstrate that the conduction path was through tethered metal clusters. The molecular current was analyzed via the Simmons tunnel model, and calculations are consistent with electron tunneling through the alkane ethers to the central metal core. With a Ni/Al2O3/Au molecular electrode, the tether binding was found to be reversible to the top Au layer, allowing for a new class of chemical detection based on the steric bulk of coordinating analytes to disconnect the molecular current path. Simple and economical photolithography/liftoff/self-assembly fabrication techniques afford robust molecular junctions with high reproducibility (>90%) and long operational lifetimes (>1 year).     

Electron transfer of hemoglobin at electrodes modified with colloidal clay nanoparticles

Nanostructured sodium montmorillonite was prepared via a colloidal chemical approach and deposited onto glassy carbon electrodes (GCE). Subsequently, hemoglobin was spontaneously adsorbed onto the clay membrane-modified electrode. The colloidal clay nanoparticles and the adsorbed protein were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The electrochemical impedance behavior of the system was studied using a microlithographically fabricated interdigitated microsensor electrode (IME). The interaction of the clay nanoparticles with hemoglobin was investigated by UV-VIS spectroscopy and electrochemical methods. The heme protein adsorbed in this way displayed a well-defined electrode process and the electron transfer was confirmed to originate from its heme site. Furthermore, nitric oxide affects the hemoglobin electrochemistry.     

Electrocatalytic oxidation of some biologically important compounds at conducting polymer electrodes modified by metal complexes

Conducting poly(3-methylthiophene) electrodes were electrochemically prepared. The resulting polymer films were modified with an inorganic complex, ferrocene. The incorporation of the ferrocene/ferrocenium moiety into the polymer film resulted in enhanced charge transfer towards the oxidation of some organic molecules of biological interest. The electrochemical response of the complex-containing polymer electrode was compared to that of the unmodified polymer electrode and that of the substrate. Apparent diffusion coefficients of the redox species were estimated from the cyclic voltammetric data for different biological molecules at the ferrocene-containing polymer electrode. Infra-red spectroscopic measurements for the “as-grown” films revealed the presence of the inorganic complex within the polymer. The modified polymer electrode showed noticeable enhancement for the charge transfer across the film interface and can be used as an electrochemical sensor for biological compounds. Received: 3 June 1997 / Accepted: 7 July 1997     

血红蛋白在纳米金修饰电极上的电化学研究

氧化还原蛋白在电极上的直接电化学研究不但能获得有关蛋白质和酶的热力学和动力学性质等重要信息,为开发新型生物传感器和生物反应器提供理论指导,而且对了解它们在生命体内的电子转移机理和生理作用机制具有重要意义。血红蛋白(Hb)是以血红素为辅基的蛋白质,在生物体中的主要     

Interference of surface plasmon resonances causes enhanced depolarized light scattering from metal nanoparticles

We show that the strongly depolarized light scattering from noble metal particles is a result of interference of two surface plasmon resonances on the same particle. The maximum depolarization occurs between two resonances. Under favorable conditions the anisotropy of the scattering light can be much lower than what is possible for dielectric particles. This explanation is discussed in relation to earlier published experimental measurements. Comparison of experimental results with theoretical calculations provides information on the shape distribution of metallic particles in the suspension.     

Facile solid-state synthesis of oxidation-resistant metal nanoparticles at ambient conditions

A simple and scalable method for the synthesis of metal nanoparticles in the solid-state was developed, which can produce nanoparticles in the absence of solvents. Nanoparticles of coinage metals were synthesized by grinding solid hydrazine and the metal precursors in their acetates and oxides at 25 °C. The silver and gold acetates converted completely within 6 min into Ag and Au nanoparticles, respectively, while complete conversion of the copper acetate to the Cu sub-micrometer particles took about 2 h. Metal oxide precursors were also converted into metal nanoparticles by grinding alone. The resulting particles exhibit distinctive crystalline lattice fringes, indicating the formation of highly crystalline phases. The Cu sub-micrometer particles are better resistant to oxidation and exhibit higher conductivity compared to conventional Cu nanoparticles. This solid-state method was also applied for the synthesis of platinum group metals and intermetallic Cu₃Au, which can be further extended to synthesize other metal nanoparticles.     

Accumulation and voltammetric determination of complexed metal ions at zeolite-modified sensor electrodes

Chemically modified electrodes based on zeolite-containing graphite pastes were constructed and evaluated as sensor electrodes for the voltammetric determination of traces of metallic species in solution. Zeolite molecular sieves with pore sizes of 3, 4, 5, and 10 A were all suitable for chemical deposition and subsequent voltammetric quantitation of traces of Cu(II), Cd(II), and Zn(II). The highest sensitivity was obtained using the zeolite with the 10 A pore size. The detection limit obtained for Cu(II) was 0.3 muM following a 2 min chemical deposition. The detection limits for Cd(II) and Zn(II) following a 4 min chemical deposition were 87 nM and 145 nM, respectively. The effects on the zinc signal of coexisting metallic species in the ammonia deposition medium were studied. While the addition of Hg(II) gave rise to increasing zinc signals, the addition of Ag(I), Cu(II), Cd(II), Ni(II), and Co(II) resulted in decreasing zinc signal amplitudes. Most notably, the magnitude of the interference from these latter metal ions correlated well with the coordination numbers of their ammonia complexes. Thus the electrode acted as a device which was sensitive to the size and shape of the interfering metal complex.     

Fabrication of organosilane-modified electrodes for metal ion detection at the molecular level

Organosilane-modified (island-type) electrodes of 5 and 10 microm were fabricated and used to detect trace amounts of metal ions using atomic force microscopy. The smaller electrode had a lower limit of detection due to the enhancement in electron tunneling through metal ions that are adsorbed between the conductive-tip (mobile) and the surface (fixed) electrode. To simulate the detection of metal ions at concentrations below 10(-3) mM, a sectional adsorption mechanism is proposed, which satisfactorily explains the adsorption of Cu(II) and Hg(II) to the amine and thiol moieties of the 5 microm-sized electrode.     

UPD metal electrocatalysis of formic acid oxidation at teflon-bonded carbon substrate electrodes

Teflon-bonded porous electrodes have been prepared for the study of the upd metal effect on the electrocatalysis of formic acid oxidation. The influence of upd lead on finely divided platinum with porous carbon as substrate was tested in fuel cell anodes for the oxidation of formic acid in 1 M HClO₄ electrolyte.In order to investigate the increase in activity through the use of upd lead, the co-adsorption of lead and a strongly adsorbed intermediate was followed by a flux cell technique. From this study it follows that the third-body effect is of minor importance. Obviously upd lead itself catalyses the direct oxidation to CO₂.     

Sustainable green catalysis by supported metal nanoparticles

The recent progress of sustainable green catalysis by supported metal nanoparticles is described. The template synthesis of metal nanoparticles in ordered porous materials is studied for the rational design of heterogeneous catalysts capable of high activity and selectivity. The application of these materials in green catalytic processes results in a unique activity and selectivity arising from the concerted effect of metal nanoparticles and supports. The high catalytic performances of Pt nanoparticles in mesoporous silica is reported. Supported metal catalysts have also been applied to biomass conversion by heterogeneous catalysis. Additionally, the degradation of cellulose by supported metal catalysts, in which bifunctional catalysis of acid and metal plays the key role for the hydrolysis and reduction of cellulose, is also reported. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 224–235; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200900004     

Theoretical study of electromagnetic scattering by metal nanoparticles

Progress in near-field optical spectroscopy research on metal nanoparticles demands a better understanding of the role of particle-particle and tip-sample interactions. In this perspective, we investigate theoretically, at a very moderate level of sophistication, the optical behavior of simple silver nanoparticle aggregates, in terms of a formalism involving a multipolar expansion of the fields involved, along with a simplified model for the optical behavior of nanostructures previously developed. In particular, the tip-sample interaction is taken into account roughly, treating the tip as an additional, single particle, characterized by proper dielectric behavior.