Electrochemical Electron Transfer Processes Research Articles

Electronic Spillover from a Metallic Nanoparticle: Can Simple Electrochemical Electron Transfer Processes Be Catalyzed by Electronic Coupling of a Molecular Scale Gold Nanoparticle Simultaneously to the Redox Molecule and the Electrode?

Electrochemical electron transfer (ET) of transition metal complexes or redox metalloproteins can be catalyzed by more than an order of magnitude by molecular scale metallic nanoparticles (NPs), often rationalized by concentration enhancement of the redox molecules in the interfacial region, but collective electronic AuNP array effects have also been forwarded. Using DFT combined with molecular electrochemical ET theory we explore here whether a single molecular scale Au nanocluster (AuC) between a Au (111) surface and the molecular redox probe ferrocene/ferricinium (Fc/Fc+) can trigger an ET rate increase. Computational challenges limit us to AunCs (n up to 147), which are smaller than most electrocatalytic AuCs studied experimentally. AuC-coating thiols are addressed both as adsorption of two S atoms at the structural Au55 bridge sites and as superexchange of variable-size AuCs via a single six-carbon alkanethiyl bridge. Our results are guiding, but enable comparing many AuC surface details (apex, ridge, face, direct vs superexchange ET) with a planar Au(111) surface. The rate-determining electronic transmission coefficients for ET between Fc/Fc+ and AuC are highly sensitive to subtle AuC electronic features. The transmission coefficients mostly compete poorly with direct Fc/Fc+ ET at the Au(111) surface, but Fc/Fc+ 100 face-bound on Au79 and Au147 and ridge bound on Au19 leads to a 2- or 3-fold rate enhancement, in different distance ranges. Single AuCs can thus indeed cause rate enhancement of simple electrochemical ET, but additional, possibly collective AuNC effects, as well as larger clusters and more complete coating layers, also need to be considered.

ELECTROCHEMICAL OXIDATIONS OF ORGANIC SUBSTANCES USING A NEW RUTHENIUM (II) DIARSENIC AQUACOMPLEX AS CATALYST

Various organic functions can undergo oxidation reactions, and the chemical industry increasingly seeks to develop processes based on the Green Chemistry approach in the search for more sustainable practices. Electrochemical electron transfer processes using metal complexes thus appear as an important alternative in these oxidative processes since they exploit numerous reactions in aqueous media and use catalytic amounts of the complex. The purpose of this work was to test the electrocatalytic ability of a new ruthenium (II) diarsenic aqua complex, [Ru(L)(totpy)(OH2)](ClO4)2 (L=Ph2AsCH2CH2AsPh2); (totpy=4´-(4-tolyl)-2,2´:6´,2´´-terpyridine), in electrooxidation experiments of organic substances with different functions. The experiments were conducted at a constant potential of +1.00 V (vs ECS) in a solution of 7:3 phosphate buffer: t-butanol, pH 8.1, with a ratio of 1.00 mmol.L-1 of the aqua complex [Ru(L)(totpy)(OH2)](ClO4)2 to 50.00 mmol.L-1 of each organic substrate. The oxidized organic starting materials and the respective products obtained therefrom were benzyl alcohol (benzaldehyde), benzaldehyde (benzoic acid), benzyl butyl ether (benzaldehyde and benzoic acid) and 1-phenylethanol (acetophenone). Selective reactions with good yields for the products were observed. The electrochemical process used here revealed some benefits over other classic oxidative methods, such as biological advantages and inorganic oxidants, emphasizing the speed, the possibility of using aqueous media in the reactions, selectivity in the formation of products, and the possibility of using small amounts of catalyst.

Long-range interfacial electron transfer and electrocatalysis of molecular scale Prussian Blue nanoparticles linked to Au(111)-electrode surfaces by different chemical contacting groups

We have explored interfacial electrochemical electron transfer (ET) and electrocatalysis of 5–6 nm Prussian Blue nanoparticles (PBNPs) immobilized on Au(111)-electrode surfaces via molecular wiring with variable-length, and differently functionalized thiol-based self-assembled molecular monolayers (SAMs). The SAMs contain positively (−NH3 +) or negatively charged (–COO–) terminal group, as well an electrostatically neutral hydrophobic terminal group (–CH3). The surface microscopic structures of the immobilized PBNPs were characterized by high-resolution atomic force microscopy (AFM) directly in aqueous electrolyte solution under the same conditions as for electrochemical measurements. The PBNPs displayed fast and reversible interfacial ET on all the surfaces, notably in multi-ET steps as reflected in narrow voltammetric peaks. The ET kinetics can be controlled by adjusting the length of the SAM forming linker molecules. The interfacial ET rate constants were found to depend exponentially on the ET distance for distances longer than a few methylene groups in the chain, with decay factors (β) of 0.9, 1.1, and 1.3 per CH2, for SAMs terminated by −NH3 +,–COO–, and–CH3, respectively. This feature suggests, first that the interfacial ET processes follow a tunneling mechanism, resembling that of metalloproteins in a similar assembly. Secondly, the electronic contact of the SAM terminal groups that anchor non-covalently the PBNP are crucial as reported for other types of molecular junctions. Highly efficient PBNP electrocatalysis of H2O2 reduction was also observed for the three linker groups, and the electrocatalytic mechanisms analyzed.

Facile gamma radiolytic synthesis of synergistic Co3O4-rGO nanocomposite: direct use in photocatalytic water splitting

Nanostructured Co3O4 was synthesized through a gamma (γ)-radiolysis technique using alcoholic (iso-propanol) salt solutions of cobalt ions with and without suspended graphene oxide (GO) nanoparticles, respectively. Formation of Co3O4 accompanied with GO reduction takes place simultaneously (in situ) upon γ-ray exposure carried out at a rate of 5.1 KGy h−1. Reduction of GO and formation of Co3O4 were confirmed by XRD, Raman and UV–vis spectroscopy. XRD and HRTEM results supports the embedding of nano-crystalline Co3O4 in an amorphous matrix. Relatively larger crystallites of cobalt oxide obtained in the presence of rGO proved to be a decisive supporting material for the directional growth of Co3O4. Electrochemical characterization established the fact that rGO is indeed considered as a proficient medium for electrochemical electron transfer process. Photo-assisted H2 generation studies using Co3O4 and Co3O4-rGO nano-composite yielded 3 and 30 μmol h−1 g−1 of hydrogen (H2) generation, respectively, supports the action of rGO as an electron trap.

Direct Probing of the Structure and Electron Transfer of Fullerene/Ferrocene Hybrid on Au(111) Electrodes by in Situ Electrochemical STM

The electron donor-acceptor dyads are an emerging class of materials showing important applications in nonlinear optics, dye-sensitized solar cells, and molecular electronics. Investigation of their structure and electron transfer at the molecular level provides insights into the structure-property relationship and can benefit the design and preparation of electron donor-acceptor dyad materials. Herein, the interface adstructure and electron transfer of buckyferrocene Fe(C60Me5)Cp, a typical electron donor-acceptor dyad, is directly probed using in situ electrochemical scanning tunneling microscopy (STM) combined with theoretical simulations. It is found that the adsorption geometry and assembled structure of Fe(C60Me5)Cp is significantly affected by the electrochemical environments. In 0.1 M HClO4 solution, Fe(C60Me5)Cp forms well-ordered monolayers and multilayers on Au(111) surfaces with molecular dimer as the building block. In 0.1 M NaClO4 solution, typical six-fold symmetric close-packed monolayer with vertically adsorbed Fe(C60Me5)Cp is formed. Upon electrochemical oxidation, the oxidized Fe(C60Me5)Cp shows higher brightness in an STM image, which facilitates the direct visualization of the interfacial electrochemical electron transfer process. Theoretical simulation indicates that the electrode potential-activated, one-electron transfer from Fe(C60Me5)Cp to the electrode leads to the change of the delocalization character of the frontier orbital in the molecule, which is responsible for the STM image contrast change. This result is beneficial for understanding the structure and property of single electron donor-acceptor dyads. It also provides a direct approach to study the electron transfer of electron donor-acceptor compounds at the molecular level.

Interfacial Electrochemical Electron Transfer Processes in Bacterial Biofilm Environments on Au(111)

We have studied Streptococcus mutans (S. mutans) biofilm growth and growth inhibition on Au(111)-surfaces using atomic force microscopy (AFM) and interfacial electrochemistry of a number of redox probe molecules. AFM of the biofilm growth and growth inhibition on both mica and Au(111)-surfaces was followed by sampling at given times, drying the samples naturally, and imaging. The electrochemical investigations were based on single-crystal Au(111)-electrode surfaces to exclude polycrystallinity as a cause of inhomogeneous voltammetric broadening on the biofilm covered electrode surfaces. The redox couples were chosen for their positive ([Ru(NH(3))(6)](3+/2+), [Co(terpy)(2)](3+/2+), terpy = 2,2',2''-terpyridine) or negative ([Fe(CN)(6)](3-/4-), [IrCl(6)](3-/4-)) electrostatic charge. [Co(NH(3))(6)](3+/2+) and [Co(phen)(3)](3+/2+) (phen = 1,10-phenanthroline) were other inhibition factors investigated. The positively and negatively charged redox probe couples displayed antagonistic inhibition and voltammetric patterns. [Ru(NH(3))(6)](3+/2+) and the homologous compound [Co(NH(3))(6)](3+/2+) were the only probe compounds to effect growth inhibition. On the other hand, cyclic voltammetry (CV) of both [Ru(NH(3))(6)](3+/2+) (positively charged, biofilm growth inhibitor) and [Co(terpy)(2)](3+/2+) (positively charged, no biofilm growth inhibition) displayed fully reversible CV on biofilm covered electrodes, almost indistinguishable from CV at bare Au(111)-electrode surfaces. In comparison, CVs of [Fe(CN)(6)](3-/4-) and [IrCl(6)](3-/4-) (both negatively charged and no growth inhibition) were distorted from planar diffusion behavior on bare Au(111)-electrode surfaces toward spherical diffusion behavior on S. mutans biofilm covered Au(111)-electrode surfaces. DNAase treatment of the biofilm covered Au(111)-electrode surface partly restores planar diffusion CV of [Fe(CN)(6)](3-/4-) and [IrCl(6)](3-/4-). This is reflected in a decrease of the growth rate and the appearance of molecular-scale structures near the bacterial edges as imaged by AFM after DNAase treatment. A rationale for the different voltammetric behavior of positively and negatively charged redox probe molecules based on electrostatic properties of the local surface environment is offered.

An approach to optimised calculations of the potential energy surfaces for the case of electron transfer reactions at a metal/solution interface

An effective computational scheme to construct the adiabatic potential energy surfaces (APES) along the reaction coordinates for an electron transfer reaction occurring by two steps at a metal electrode is considered in the framework of the Anderson–Newns model. Two Theorems have been proved which predict the existence of all possible solutions of the Anderson–Newns equations at arbitrary values of the key parameters and point out the region for each solution. Asymptotic formulas for solutions near multiple roots have been derived and combined in an effective way with numerical routines. The analysis of some important properties of the APES, which can be of interest for modelling the electrochemical electron transfer processes, is presented as well. The APES describing the reduction of Zn(II) and In(III) aqua-complexes at a mercury electrode have been built and discussed.

Monte Carlo simulation of electrochemical electron transfer processes

A Monte Carlo simulation of electrochemical electron transfer (ET) is performed at small overpotentials with the use of the Dogonadze, Kuznetsov and Vorotyntsev model (J. Electroanal. Chem. 25 (1970) App. 17). The ET process is considered as a random walk in a set of Gibbs energy surfaces (GES) corresponding to various electronic states. The method allows one to calculate the transition probability at arbitrary electronic coupling of the redox couple with the metal electrode, i.e. it covers the whole range between non-adiabatic and adiabatic limits.

Spatial, time and energy resolved electrochemical electron transfer processes

New ideas and perspectives for the study of electrochemical electron transfer processes are discussed. Schemes for the study of electron transfer processes in space, energy and time domains are considered. Among them: through-metal-scanning reaction microscopy/spectroscopy (TMSRM), coherent generation of ultra-short electron pulses, tunnel electron current in bridged STM junctions. Schemes for the observation of the inverted region in electrochemical systems are suggested. New phenomena are predicted related with coherent electron transfer processes.