【作者】 黄炜；

【导师】 李则林；

【作者基本信息】 湖南师范大学 ， 分析化学， 2009， 博士

【摘要】 纳米材料由于其特殊的结构以及由此产生的一系列纳米效应,决定了其具有不同于普通材料的新异性质,广泛应用于能源、化工、电子等领域。目前,人们已能利用各种方法来制备纳米材料。其中,电化学方法制备纳米材料具有反应条件温和、可控性好、适用范围广等优点,是一种有前途的纳米材料制备方法。同时,生物/有机小分子的电氧化研究不仅在电化学吸脱附、电化学反应等表面过程具有基础理论研究价值,而且能促进与之相关的生物分析、能源转化利用等方面的实际应用。纳米材料上生物/有机小分子的振荡电催化氧化研究,对于认识和理解纳米材料上的电极过程,丰富非线性动力学,均具有重要的科学意义。本学位论文发展了一些新颖、便捷的电化学方法,用来制备诸如纳米粒子、微/纳米多孔金属和纳米多孔氢氧化物薄膜等功能材料,发现和系统研究了纳米多孔薄膜电极材料上多种生物/有机小分的振荡电催化氧化反应与机理。主要内容如下:1.系统综述了微/纳米材料的制备和应用,生物/有机小分子的电催化氧化及电化学振荡相关的文献。2.报道了一种在NaOH空白液中,通过方波电势脉冲技术对光亮Au基底实施表面重建来便捷制备多功能三维梯度多孔Au膜的新方法。制备过程涉及Au的氧化-还原和强析氢反应。Au表面经氧化-还原产生Au纳米粒子,在氢气泡动态模板的导向作用下组装成多孔结构。该方法绿色、方便、经济,既不需要在溶液中引入Au（Ⅲ）物种和添加剂,也无需去模板处理,即可在Au表面构建三维多孔结构。用扫描电镜（SEM）表征了孔的形成与演化。该三维多孔Au膜具有多功能:①对乙醇、葡萄糖和抗坏血酸等燃料/生物分子的电氧化表现出高的电催化活性;②三维多孔Au膜具有强且稳定的表面增强拉曼散射（SERS）效应,且拉曼活性易更新;③当多孔膜表面修饰单层硫醇后,由超亲水转变为超疏水。3.用氢气泡模板法直接电沉积,成功制备了贵金属Au、Pd和AuPd合金多孔膜。NaOH介质中氢气泡高的稳定性,以及强阴极条件下Au（Ⅲ）物种高的还原速率,使得Au以跨泡方式电沉积,形成三维微/纳米多孔结构。而在HCl介质中,Au、Pd或AuPd以气泡间隙填充方式电沉积,形成二维微/纳米多孔结构。用SEM表征了上述多孔结构。X-射线衍射（XRD）和能量散射谱（EDS）揭示了多孔膜的组成。电化学测试表明贵金属多孔膜具有高比表面积,对低碳醇的电氧化表现出高的电催化活性。而且,多孔膜表面修饰硫醇后具有超疏水性。4.发展了一种利用方波电势脉冲或交流电技术在NaOH空白溶液中电化学分散纯Pt丝新方法,制备出Pt的水溶胶。在电势扰动过程中,Pt表面经氧化-还原产生的Pt原子重排形成团簇和纳米粒子,在析氢作用下,分散于底液中。运用上述策略,溶液中无需任何前驱体离子和还原剂,且在温和的条件下,实现了干净Pt粒子的绿色、简易合成。而且,采用两电极系统对两根Pt丝实施成对电解,可同时产生Pt溶胶。用SEM、TEM和XRD对Pt纳米粒子进行了表征。所制备的Pt纳米粒子对乙醇的电催化氧化具有高活性,并有一定的SERS效应。5.通过阴极电沉积法制备了纳米多孔Ni（OH）₂薄膜（NHNF）。系统研究了该多孔薄膜上多种氨基酸（α-丙氨酸、丙氨酸、赖氨酸、甘氨酸、丝氨酸、精氨酸）和甘肽的振荡电催化氧化反应。用时间分辨和电势调制拉曼光谱在分子水平上原位监测了上述电催化过程。实验结果表明:①NHNF作为有效的电子传递体,对上述生物分子的电氧化有较高的电催化活性:②α-氨基酸的电氧化经历脱羧和氨基转化为腈的过程;③该电氧化受扩散传质控制。首次观察到上述生物分子在NHNF上电氧化时的电势和电流振荡。周期性析氧对振荡的产生起关键作用,即极限扩散控制下反应物分子在电极表面的氧化耗尽引发析氧,由此又产生搅拌对流作用使其表面浓度得以恢复,析氧停止,如此循环,产生振荡。6.在所制备的三维多孔Au薄膜（PGF）上,通过阴极沉积方法分别修饰Ni或Pt,制备出Ni/PGF或Pt/PGF修饰电极。分别研究了碱性下Ni/PGF对三种生物分子（抗坏血酸、葡萄糖和甘氨酸）和酸性下Pt/PGF对甲酸的的振荡电催化氧化过程。结果表明,Ni/PGF对生物分子的电氧化有较高的电催化活性,电氧化受扩散传质控制,且首次发现这些体系中均能产生电势振荡。其中,葡萄糖和抗坏血酸的电势振荡分为小振幅和大振幅两种:前一振荡出现在析氧前,主要与强吸附中间体的形成与去除有关,为电化学反应与表面过程耦合型;后一振荡伴随周期性析氧,主要源于表面浓度的极限扩散消耗与析氧引起表面浓度的对流恢复,归于电化学反应与扩散对流传质耦合型。甘氨酸的电氧化仅出现第二类振荡。Pt/PGF电极对甲酸的电催化活性随铂修饰量增加而降低,涉及表面过程为主的电势振荡则变得容易发生。这主要由于低Pt量时甲酸氧化以直接过程为主,毒化中间体CO_ad的形成受到抑制,体系无明显负反馈;高Pt量时甲酸氧化以间接过程为主,解离吸附的CO_ad的毒化作用使负反馈增强,且与强的正反馈(CO_ad的去除)匹配,在特定电流范围内产生电势振荡。7.运用循环伏安交叉环判据,观测到生物分子在Pt、Ag电极上的电氧化振荡。其中,半胱氨酸在Pt上进行阳极氧化时,在较低的电势范围内出现振荡。考察并分析了溶液中各组分（半胱氨酸、H⁺、Cl^-）浓度对振荡的影响。半胱氨酸可经半胱氨酸自由基氧化成胱氨酸或经半胱氨酸自由基氧化成RSO₃^-而振荡的产生可能源于一对正负反馈,即自由基在铂表面的形成与去除。一定浓度的Cl^-有助于平衡正负反馈而产生振荡。多种生物分子（甲硫氨酸、精氨酸、赖氨酸、抗坏血酸和葡萄糖）在Ag电极上电氧化也出现电化学振荡,且均与析氧过程有关,推测振荡主要源于生物分子的极限扩散氧化和析氧引起对流恢复。电势振荡和析氧过程可改变银电极表面形貌,继而对振荡也有一定影响。更多还原

【Abstract】 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（Ⅲ） 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（Ⅲ） 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.更多还原