Electrochemical Preparation Of Functional Micro-nano Materials And The Oscillatory Electrocatalytic Oxidation Of Small Bio-organic Molecules

Nanomaterials have attracted considerable attention in diverse fields such as energy, chemical engineering and electronics, due to their unique physicochemical properties that are different from those of common materials. Various methods have been developed to prepare nanomaterials. Thereinto, electrochemical preparation of nanomaterials is a promising technique and has several advantages such as mild condition, easy control and wide application. Meantime, the study of electrooxidation of small bio-organic molecules is significant both for fundamental research on electrochemical ad/desorption and reactions and for potential application in bioanalysis, energy conversion and utilization. The study of oscillatory electrocatalytic oxidation of small bio-organic molecules on nanomaterials has important scientific meaning for recognizing and understanding nanomaterials from the viewpoint of nonlinear science, deepening the whole understanding of the electrochemical processes, enriching the research content of nonlinear kinetics.In this thesis, several novel and convenient electrochemical methods have been developed to prepare some functional nanomaterials such as nanoparticles, micro-nano porous metals and nanoporous hydroxide film. Moreover, electrochemical oscillations during the electrooxidation of small bio-organic molecules on nanostructured materials have been investigated. The main points of this thesis are summarized as follows:1. Researches on the preparation and application of micro-nano structured materials, the electrooxidation of small bio-organic molecules, and the electrochemical oscillations have been reviewed systematically.2. We report here a novel one-step electrochemical method to fabricate three dimensional (3D) micro-nano hierarchical porous gold films (PGFs) by the surface rebuilding of smooth gold substrates in a blank solution of NaOH with square wave potential pulse (SWPP). The potential is controlled such that it involves repeated gold oxidation-reduction and intensive hydrogen evolution, where the hydrogen gas bubbles function as a dynamic template to shape the assembly of the gold nanoparticles from the oxidation-reduction. Particularly, this method is green, convenient, and economical, which enables us to fabricate the 3D porous structure from the metal itself requiring neither Au(â…¢) species and additives in solution nor post-treatment of template removal. The pore formation and evolution have been characterized by scanning electron microscopy (SEM). The as-prepared 3D PGF has multifunction. For example, (i) high electrocatalytic activity toward the oxidation of some fuel/biomolecules like ethanol, glucose, and ascorbic acid; (ii) strong surface-enhanced Raman scattering (SERS) effect with the merits of being stable and easily-renewed; and (iii) interesting transition from superhydrophilicity to superhydrophobicity by decorating with a self-assembled thiol monolayer.3. Macro-nano porous films (PFs) of noble metals of Au, Pd, and AuPd alloy have been successfully fabricated by electrodeposition with hydrogen bubble template. SEM results show that the Au PF prepared in NaOH medium is three-dimension (3D) and the Au, Pd or AuPd alloy PF in HCl medium is two-dimension (2D). X-ray diffraction (XRD) and X-ray energy dispersive spectrum (EDS) results reveal the components of these PFs. The stabilization of hydrogen gas bubbles in NaOH solution and the fast electroreduction of Au(â…¢) species at strong cathodic polarization play key roles in forming the 3D macro-nano porous structures through the bubble-covered electrodeposition. While the uniform 2D macro-nano porous structure of Au, Pd or AuPd alloy is associated with the accumulative filling of metals between bubbles in HCl medium. Electrochemical behaviors show that the as-prepared PFs possess high surface area and exhibit high electrocatalytic activities toward electrooxidation of low-carbon alcohols in alkaline solution. Also the superhydrophobicity is observed on these PFs assembled with a thiol monolayer.4. Pt hydrosols have been synthesized by dispersing a pure Pt wire in a NaOH solution with square wave potential (SWP) or alternating voltage (AV). The rearrangements of superfacial Pt atoms by repeated and fast electro-redox under the potential perturbations (PPs), coupling with the impetus of concurrent hydrogen gas, account for this novel dispersion of bulk Pt. Using this strategy we are able to realize the green and facile synthesis of clean Pt nanoparticles (NPs) requiring no any precursor ions and reducers in aqueous solution under mild conditions. Moreover, the dispersion of bulk Pt has also been carried out by paired electrolysis with two Pt wires. The as-prepared Pt NPs were characterized by SEM, transmission electron microscopy (TEM) and XRD. The Pt NPs exhibit high electrocatalytic activity toward the ethanol electrooxidation and detectable SERS signals for adsorbed pyridine.5. Systematic investigations have been carried out with respect to the electrocatalytical oxidation of several amino biomolecules in alkaline solutions on a nanoporous thin film electrode of electrodeposited nickel hydroxide nanoflakes (NHNFs). The amino biomolecules studied here include five amino acids (β-alanine, alanine, lysine, glycine, serine and arginine) and one dipeptide (glycylglycine). Potential-dependent and temporal-resolved in situ Raman spectra, together with electrochemical measurements, have been employed to reveal the electrocatalytical processes at the molecular level for the first time. Experimental results show that (i) the NHNFs act as an effective electron mediator with high electrocatalytical activity toward these biomolecules, (ii) amino group is transferred into nitrile group and decarboxylation occurs simultaneously for theα-amino acids, and (iii) the electrooxidation reaction rate of amino biomolecules is diffusion-controlled. Moreover, oscillations both in potential and in current have been observed for the first time during the electrocatalytic oxidation of these biomolecules on the film electrode of NHNFs. Periodic oxygen evolution plays a key role in the oscillations. It is triggered off and shut up, respectively, while the surface concentration of amino biomolecules depletes to zero by diffusion-limited oxidation and is replenished by the convection-enhanced flow from the gas release.6. Ni or Pt modified PGF (Ni/PGF or Pt/PGF) electrode, as prepared by cathodic electrodeposition method, has been used to investigate the oscillatory electrocatalytic oxidation of biomolecules including ascorbic acid (AA), glucose (Glu) and glycine (Gly) in alkaline medium, and of formic acid (FA) in acidic medium, respectively. Experimental results show that the Ni/PGF exhibits high electrocatalytic activity toward biomolecules and the reactions is diffusion-controlled. Two different types of potential oscillations have been observed during the electrooxidation of AA or Glu. One occurs before oxygen evolution due to the formation and removal of strongly adsorptive intermediate, and belongs to the type of coupling of charge transfer with surface steps. The other arises accompanying periodic oxygen evolution from the coupling of charge transfer with diffusion and convection mass transfer. Only the second type of potential oscillations can be observed during glycine electrooxidation. The Pt/PGF electrode that decorated with a tiny amount of Pt shows excellent performance toward the electrooxidation of formic acid. However, the electrocatalytic activity decreases with the amount of Pt increasing, while the potential oscillations involving surface processes occur easily all the better. Unlike on the surface with low amount Pt where a direct path (CO_ad-free path) is essential, the indirect path predominates on the surface with high amount Pt during formic acid decomposition. For the latter, the formation and removal of CO_ad constitutes one pair of strong negative and positive feedbacks, which produces oscillations within certain current ranges.7. New electrochemical oscillations have been found by means of the CV-based criterion during the electrooxidation of biomolecules on Pt and Ag electrodes. The electrochemical oscillations in the electrooxidation of cysteine on Pt electrode in acidic chloride-containing solution are studied by cyclic voltammetry, chronopotentiometry, chronopotentiometry with current ramp and impedance spectra. It is spectulated that cysteine is first oxidized to cysteine radical (RS·), and then parallelly dimerized to cystine or oxidized to RSO₃^-. A big crossing cycle, which means a pair of overlapping positive and negative feedbacks within a bistable range, is found within a low potential range when cysteine is oxidized in a solution containing enough H⁺ and Cl^- Oscillations may result from the overlapping positive and negative feedback, namely formation and removal of RS·on the surface of Pt electrode, while the presence of Cl^- is in favor of the balance of positive and negative feedback. Potential oscillations have also been observed on Ag electrode during the electrooxidation of several biomolecules including methionine, arginine, lysine, ascorbic acid and glucose. Both these oscillations are related to oxygen evolution. The oscillations may originate from the depletion and replenishment of biomolecules surface concentration by diffusion-limited oxidation and by convection-enhanced mass transfer from oxygen evolution, respectively. The repeated redox and oxygen evolution reactions on Ag surface during oscillations can result in complicated changes of surface morphology, and hence can influence the oscillations to certain extent.