【作者】 李巧霞；

【导师】 蔡文斌；

【作者基本信息】 复旦大学 ， 物理化学， 2008， 博士

【摘要】 金属纳米薄膜具有特殊的光学特性,如表面增强红外吸收（SEIRA）。SEIRA与表面增强拉曼（SER）效应相似,主要起源于纳米结构的金属表面等离子共振所产生的局部电磁场增强。除上述特别光学效应外,在电化学环境中纳米金属薄膜还可能具有某些特殊的催化活性以及耐蚀性等。通过合金化、表面修饰或改变纳米颗粒的尺寸可实现纳米薄膜物理和化学功能的调控,尤其是表面修饰双元金属纳米材料是电催化研究的一个热点。衰减全反射（ATR）表面增强红外吸收光谱（SEIRAS）具有表面信号强、表面选律简单和传质不受阻等优点,适合于电化学界面研究。在电化学ATR-SEIRAS中,工作电极是沉积在红外窗口（主要是Si）上具有光学和化学特性的金属纳米薄膜,因此电化学ATR-SEIRAS是研究功能薄膜电催化和电吸附等界面结构和行为的重要方法。而构建新型功能金属纳米薄膜电极是理解、应用与发展现场ATR-SEIRAS方法的基础。在本论文中,首先为研究双元双层金属纳米薄膜的SEIRA效应和吸附反应特性,构建了厚度可控的Au基Pt肤纳米薄膜,探讨该类结构薄膜红外增强机理。其次,为研制低铂、高效电催化材料,构建了Ru基Pt肤功能纳米薄膜电极并应用于现场ATR-SEIRAS研究,并且探讨了该特殊双元金属电催化的双功能和电子效应。再次,利用SEIRAS简单表面选律和通用湿法构建金属纳米的便利,研究了吡啶在各种金属电极表面上吸附构型,考察了金属价电子结构对其吸附构型的影响。另外,采用扫描探针显微术表征了具有SEIRA效应金属纳米薄膜的形貌。主要研究内容摘要如下:1.厚度可控的Au基/Pt肤纳米薄膜的SEIRA的效应采用自发置换法在Au纳米薄膜上制备不同厚度的Pt肤层薄膜,并检测其红外吸收强度的变化。所谓自发置换法即利用欠电位沉积单层Cu原子作为模版,在含Pt的盐溶液中发生置换反应,经多次循坏可得到不同厚度的Pt肤层。这样制备的催化层具有超薄、厚度可控的特点,为研制超低铂含量的催化剂和研究基底金属电子效应提供了可能。基于上述方法,在四层以内Pt肤层表面粗糙度和Pt上吸附的CO峰强几乎不变,表明了Pt肤层近似外延生长的特性。Pt的SEIRA效应取决于Au基/Pt肤复合纳米膜的形状与尺寸及其电子和光学特性。该研究结果首次从实验上揭示了Pt肤层的SEIRA效应与文献中的有关SERS效应有明显不同:Pt层上SERS信号主要源于基底Au的长程电磁场增强效应的诱导,随层厚度增加而急剧下降;而红外光区,各类金属的光学特性差异小,Pt层上SEIRAS信号受基底金属性质影响很小。同时研究了不同厚度Pt肤层薄膜上预吸附CO电催化氧化行为,发现其比Pt厚膜电极的氧化电位正移了300 mV以上。根据基底与肤层金属间相互作用的d带中心能级理论,在Au基底上肤层Pt的d-带中心上移,加强了d-π^*之间的反馈,使Pt-C键键能增强,从而导致Pt肤电极上CO的吸附强度变强,氧化电位正移。2.Ru基/Pt肤二元金属薄膜的制备及现场ATR-SEIRAS研究为研制低Pt载量的电催化材料及其电催化机制,采用白沉积法制备Ru基/Pt肤二元金属薄膜电极,并应用于电化学SEIRAS研究。自沉积法即在不加外界电压的条件下,一种贵金属（Pt）在另一种贵金属（Ru）基底上自发沉积的现象,即发生一个不可逆的氧化还原置换反应。通常通过自沉积法所得到肤层只有亚单层或几个单层的厚度。常规电化学和现场ATR-SEIRAS测试表明,Ru基/Pt肤电极表现出对CO和CH₃OH很强的催化作用。在含饱和CO的0.1 M HClO₄中,Ru基/Pt肤膜电极比Pt膜电极对CO的氧化电位提前140 mV,在红外光谱图上可观察到Pt-CO和Ru-CO的吸收峰;在电催化氧化甲醇的实验中,Ru基/Pt肤膜电极也表现出很强的催化活性,ATR-SEIRAS测到了甲醇解离产物CO和HCOO^-的红外吸收峰。这些表明Ru基/Pt肤薄膜体现了类似传统合金中Pt-Ru的协同效应,提高了抗CO中毒能力,更重要的是显著降低了贵金属Pt的负载量。本部分工作还以共沉积法制备的Pt-Ru合金及自发置换法制备的Ru基/Pt肤二元薄膜作为比较,分析三者电化学与催化协同效应的异同。3.不同电极表面吡啶吸附的表面增强红外光谱研究吡啶（Py）分子在金属表面的吸附构型由于所处环境和研究方法等的不同,一直未达共识。利用表面增强红外光谱法（SEIRAS）灵敏度高、表面选律简单的优点,采用“两步全湿法”制备Cd及Pt、Ru、Rh和Pd膜电极,应用ATR-SEIRAS技术研究了吡啶在上述各电极的吸附构型。结果显示:吡啶在Cd电极上采取end-on构型,在Pt电极上采取α-pyridyl的edge-on构型;在Ru和Rh电极上主要采取end-on构型并混有少量edge-titled构型;在Pd电极上吸附较弱,混合了ct-pridyl-edge-on和edge-titled两种吸附构型。同时,系统比较吡啶在Au,Ag,Cu,Ni,Cd和Pt族电极上的红外吸收光谱,得出Py的吸附取向沿着（Cd和Cu）、（Ag和Au）、（Ru和Rh）、Ni、Pd、Pt顺序依次形成end-on Py,edge-titled Py和α-pyridyl吸附构型,估算了Py分子在各电极表面的侧向倾斜角度,初步讨论了金属的价层电子结构对吡啶吸附构型的影响。4.SEIRAS用金属纳米薄膜的扫描探针显微表征岛状结构的纳米薄膜是产生SEIRA效应的关键,因此对各类纳米金属薄膜的形貌表征必不可少。扫描探针显微技术可以获得表面信息的三维图象,易于纳米级分辨,且是无损分析。作为团队合作内容,该部分主要开展SEIRAS用金属纳米薄膜形貌表征。对化学镀金随后电镀铂族金属的镀膜样品STM测量发现,化学镀金薄膜颗粒较大,表面较粗糙,而第二步电镀薄膜则颗粒细小致密,形成无针孔的薄膜,与电化学和红外测量结果一致;金、银纳米溶胶在Si表面的自组装及生长可以实现对SEIRA效应的调控,通过AFM测量了不同大小金、银单层纳米颗粒以及后续化学镀膜的三维图象;通过现场电化学AFM实时监控经电化学粗糙前后的Ag膜电极的形貌比较,虽然粗糙后的Ag膜的表观粗糙度增加,但其SEIRA光谱信号却没发生明显改变,佐证了SEIRA活性基底不需要特别进行粗糙化处理的优点。更多还原

【Abstract】 Nanofilm electrodes attract considerable attentions in the field of electrochemical surface science because of their unique surface optical and chemical properties. Regarding the optical property, molecules adsorbed on metal island films exhibit 10-1000 times more intense infrared absorption than would be expected from conventional measurements without metals. This effect is called surface-enhanced infrared absorption （SEIRA）, the enhancement mechanism of which is similar to that of surface-enhanced Raman scattering （SERS）, i.e., mainly originated from the electromagnetic enhancement owing to surface plasmon resonance. In addition to the unique optical functions, nanofilm surfaces have interesting chemical properties, such as enhanced electrocatalytic effect. These properties of metallic nanofilms could be modified through alloying or coating with a second metal, and by decreasing the particle sizes. Owing to high surface sensitivity and simple surface selection rule, ATR-SEIRAS is regarded as an important spectroscopic method for in situ characterization of surface adsorption and reaction at metal-electrolyte as well as metal-ambient interfaces. In ATR-SEIRAS, it is essential to fabricate novel functional nanofilms on the reflecting surface of a Si prism.The first part of this thesis concerns with the application of spontaneous replacement method to the preparation of a Pt skin layer on Au nanofilm electrode, the overlayer thickness-dependent SEIRA effect, and its comparison to that of the corresponding SERS effect. The CO adsorption and oxidation on different Pt coating layers is also investigated spectroscopically. After that, targeting on effective electrocatalyts with low Pt loadings, a new bimetallic structure consisting of Ru nanofilm substrate and Pt overlayer was constructed as the working electrode in ATR-SEIRAS measurements. Thirdly, by taking advantages of the versatile two-step wet process in fabricating nanofilms electrodes, and the straightforward surface selection rule of SEIRAS, pyridine adsorption configuration on various metal electrodes has been systematically reinvestigated, indicating that the valence electronic structures of metals significantly affect the adsorption geometries. The last part is a description of the teamwork involved, in which the morphologies of SEIRA-active nanofilms on Si were characterized with scanning probe microscopy. The main topics and results of the thesis are summarized as follows:1. Nanostructured Pt-coated Au Thin Films Exhibit Films-dependent and Tunable SEIRA EffectsSEIRA-active platinum overlayers can be prepared by the redox replacement of underpotential-deposited Cu on Au film by electroless deposited on silicon. With the numbers of the cycles （upd - redox replacement cycle）, the thickness of Pt overlayers increases accordingly. Using CO as a probe adsorbate, the intensity of SEIRAs is somewhat weaker when the films after one cycle have a few exposed Au sites. Whereas after several UPD redox cycles, the pinhole-free Pt overlayer exhibits significantly SEIRA effect. In addition, the SEIRA enhancement factor remains nearly the same up to four Pt monolayers, and decreases as the thickness of the Pt layer increase further. The thickness dependence is quite different from that for surface enhanced Raman scattering （SERS）, which may be ascribed to the similarity （large difference） in optical dielectric properties of these two metals in the mid IR （visible） frequencies. The CO adlayer oxidation on a thin Pt overlayer on-Au electrode requires a higher potential as compared to that on a thick Pt overlayer on Au electrode. Based on the d-band center energy, the interaction between Au and Pt induces a stronger CO adsorption on Pt sites. The UPD-redox replacement strategy provides a promising approach to the preparation of efficient film electrocatalysts with ultralow Pt loadings.2. Preparation of Pt-on-Ru films with ultra low-Pt loadings for in situ ATR-SEIRAS on electrocatalysisA new spontaneous deposition approach was developed to fabricate Pt skin layers on Ru nanoparticle films （simplified hereafter as Ru/Pt films） on a Au-coated Si substrate for in situ ATR-SEIRAS application. In this approach, immersion of a nominally reduced Ru film into a Pt salt solution without external potential control leads to the irreversible redox replacement of a Ru surface layer by a Pt skin layer. This Ru/Pt films structure greatly reduces the Pt loading without sacrificing high electrocatalytic property towards the oxidation of CO and CH3OH. Alternatively, UPD Cu-based redox replacement was used to prepare a Pt-skin layer-coated Ru film electrode. The two Ru/Pt film electrodes were comparable to the codeposited Pt-Ru alloy film electrode in terms of high electrocatalytic behavior, although delicate differences were detected both in electrochemical and spectroscopic responses for these three films. It was assumed that the electronic effect may contribute in an increasing share to the enhanced electrocatalysis of a Ru/Pt bimetallic film in addition to the bifunctional effect, as compared to a Pt-Ru alloy film. 3. ATR-SEIRAS Investigation of Pyridine Adsorption on Different ElectrodesSurface-enhanced IR absorption spectroscopy in attenuated-total reflection （ATR-SEIRAS） configuration has been extended for the first time to a cadmium electrode with the adsorption of pyridine as the model system. In the potential range of-1.2 to -0.8 V （vs. SCE）, only in-plane vibrations belonging to the A₁ mode were detected whereas those to the B₁ mode virtually were not, suggesting that pyridine coordinates vertically or at a titling angle to the Cd electrode surface via its nitrogen end without edge-tilted rotation of its ring plane.ATR-SEIRAS was also applied to investigate adsorption configurations of pyridine （Py） on platinum, palladium, ruthenium, and rhodium nanofilm electrodes. The results reveal that a-pyridyl species predominantly form on Pt electrodes by assuming an edge-on configuration with its ring N and a-C atoms bonding to the Pt surface, while on Ru and Rh electrodes pyridine molecules essentially remain intact by adopting end-on （a slightly edge-tilted） configuration through bonding with its N lone pair electrons. Py adsorption on a Pd electrode may lie in between the above two cases; both a-pyridyl species and edge-tilted intact pyridine could be significantly present.Further comparison of the typical adsorption configurations of Py on the transitional metal electrodes, such as Pt, Ru, Rh, Pd, Ag, Au, Cu, Cd, and Ni film electrodes, a rough trend of forming end-on Py, edge-tilted Py, and edge-on a-pyridyl was suggested in a sequence from （Cd and Cu）, （Ag and Au）, （Ru and Rh）, and Ni, Pd, and Pt. The valence electron structures of metals may affect substantially the adsorption configuration.4. Morphological Characterization of Metallic Nanofilms on Si Using Scanning Probing Microscopy （SPM）SEIRA-activity greatly depends on the morphology of a metallic nanofilm. As nondestructive methods, scanning tunneling microscopy （STM） and atomic force microscopy （AFM） can easily provide the nanostructure informations for SEIRA-active metal films on Si. As part of a teamwork, it is my duty to carry out SPM characterization of surface morphologies. STM reveals that the Au underfilm from chemical deposition on Si was rougher and the particle size was larger, and further growth of a second transition metal overfilm via electrodeposition results in densely packed smaller particles, in accordance with the pinhole-free nature of the overfilm as indicated further by electrochemical and spectroscopic measurements. Self-assembled monolayers of Au and Ag colloid particles on Si wafer can be clearly observed by using AFM, as well as their subsequent chemical growth. Larger particles and increasing coalescence result in larger and finally distorted SEIRA effect. In-situ AFM was applied to characterize the morphology of a Ag nanofilm before and after it was subjected to electrochemical annealing. The electrochemical annealing leads to the dissolution of very small particles, but the SEIRA effect didn’t change significantly, indicating that the electrochemical roughening has virtually no effect on SEIRA, unlike on SERS.更多还原