币族及铁族金属表面增强红外吸收基底的湿法构造与应用

【作者】 霍胜娟；

【导师】 蔡文斌；

【作者基本信息】 复旦大学 ， 物理化学， 2007， 博士

【摘要】 当分子吸附在纳米岛状结构的金属颗粒表面上，红外吸收信号得到10—1000倍显著的增强，这一现象被称为表面增强红外吸收效应(Surface-enhanced Infrared Absorption Effect，简称SEIRA)。该效应的发现促进了红外光谱在表面化学及分析化学研究中的应用，表面增强红外光谱(SEIRAS)由于其具有表面灵敏度高、选律简单，可以应用于金属—液(气)相界面的分子级别的实时研究。但是SEIRAS技术远不如与其姊妹技术—表面增强拉曼技术(SERS)那样发展迅速、应用广泛，主要原因是：首先，金属纳米颗粒的形状、尺寸，及相互连接程度制约着SEIRA效应的大小，构造表面纳米结构可控的SEIRA活性基底具有挑战性。其次，表面电化学是电化学研究的热点之一，如何制备SEIRA效应强和稳定性高、导电性好，以及电化学响应合理的金属纳米薄膜电极也是开展电化学SEIRAS研究的关键和难点。红外窗口上金属薄膜的传统制备主要采用真空蒸镀和溅射等干法，仪器费用高、耗时，样品容易被污染，光谱重现性差，并且易出现双极和谱峰扭曲的现象。而现有的湿法镀膜技术单调、种类有限，特别是无法获得具有ATR-SEIRA活性的铁族金属(铁，钴，镍)薄膜电极。本论文拟主要针对以上问题，研究如何应用湿法在硅表面上构造各类具有SEIRA活性的新型纳米金属薄膜，拓宽制膜技术和SEIRAS的应用领域，尤其开展铁族金属薄膜电极上的现场ATR-SEIRAS研究。我们提出并实现了制备币族金属薄膜的通用方法—“种子生长法”以及拓展“两步全湿法”到铁族金属膜的制备，并且深入研究了这些电极表面的吸附与反应；我们还结合纳米颗粒的合成和表面组装，在薄硅片上制备了尺寸可调的金和银纳米有序薄膜，探索其作为透射模式SEIRAS基底的可行性。主要研究内容小结如下：1提出一种用于SEIRAS研究的二维有序和结构可控的金纳米薄膜制备方法。构造二维有序可控纳米金属可提供SEIRA机制的研究平台。文献上报道的SEIRA基底，具有表面聚集程度及表面粗糙度难控制的缺点，而近年来单层自组装技术的发展为解决上述问题提供了可能。我们采用硅上羟基化后偶联氨基硅烷粘结一层不同尺寸的纳米金溶胶，制备二维有序可控的SEIRA基底。该方法操作简单，基底易于调控，有利于了解金属纳米结构与SEIRA关系并探索其机制。我们还初步提出种子生长法制备金薄膜，具有较小尺寸的纳米溶胶作为之后化学镀的催化种子层，在含有氯金酸和羟胺的水溶液中，通过调控生长时间可得到不同颗粒大小、聚结度和膜厚度的金膜。这些不同阶段得到的金纳米膜可以用于透射或现场电化学内反射表面红外研究，并呈现可调控的SEIRA效应。上述方法制得的金纳米薄膜具有较强的SEIRA效应，过程无氟无钯，降低了在还原过程中引入其他金属杂质的可能性，它有希望成为理论和实际研究中的一类新型和重要的SEIRAS基底。2具有SEIRA效应的银纳米膜(电极)的“种子生长法”制各及应用我们发展了上一节中的“种子生长法”，用于制备硅上Ag纳米薄膜(含电极)的制备。首先由两种路径获得具有SEIRA活性的银纳米品种，一是基于银溶胶粒子在硅表面的自组装，二基于硅在硝酸银和氢氟酸混合溶液中形成的银纳米粒子层。红外透射实验证明通过该两种路径均能获得较高SEIRA效应的Ag基底。后者更快速方便、效果更佳，有望成为一种红外传感器。我们进而利用“种子生长法”在硅基底上制备银纳米薄膜电极：两种路径形成的银晶种经过无电镀后形成可导电的银膜，两种银膜电极具有和本体电极可比性，均具有较好的SEIRA活性，为现场ATR-SEIRAS研究Ag电极上的吸脱附创造了前提条件。ATR-SEIRAS的增强机理及选律简单，无需对Ag电极进行类似于SERS研究中的ORC粗糙处理，呈现出良好的电极电位稳定性，更方便可靠地判断吡啶分子在银电极表面的吸附构型。3铁族电极ATR-SEIRAS的拓展与应用铁族金属(镍、钴、铁)电极在电催化和腐蚀研究具有重要地位，有必要拓展ATR-SEIRAS方法到其上的现场研究。为此需要在红外窗片上制备具有SEIRA活性的铁族金属薄膜电极。首先在半圆硅柱的反射面上化学镀上一层60nm厚的Au膜底层，后在其上恒电位沉积几无针孔的数十纳米厚的Ni、Fe和Co外层，选择CO为探针分子，检验上述膜的SEIRA效应，并且较详细地研究吸附CO的电氧化动态过程。以镍电极为例，我们不仅实时获得了信噪比高、单向无异常的CO吸收信号，而且检测到氢键遭破坏的界面自由水信号，发现线式及桥式CO吸附模式的转换，观测到CO在镍氧化物上的吸附峰。我们应用ATR-SEIRAS技术从分子水平上获得有机分子特别是缓蚀分子(苯并三氮唑)在铁族电极表面的吸附信息，为腐蚀防护机制研究提供了新的分析工具。我们还利用该项技术确定了模型分子吡啶在Ni、Fe电极上吸附构型。4“种子生长法"制备金、银膜的特殊应用拓展非金属基底上涂覆金属技术(主要蒸镀和溅射)在工业上得到广泛的应用，但是对某些特殊表面，如可作为流动分析用毛细管内壁和遥感用的光纤端面上的金属沉积，以上方法都不适合。采用上述“种子生长法”在较毛细管内壁成功沉积金属，可满足各种形状管道的加工要求，而且所得的金属层均匀连续、厚度可调快速、成本低。“种子生长法”还成功应用于对光纤传感器件的端面镀上金、银膜，反射率也优于蒸镀膜，表明这种新型的“光纤端面金属膜形成技术”具有很好推广前景。总之，本论文工作在解决阻碍SEIRAS拓展与应用的几个重要问题做了一些有益的探索，初步揭示了SEIRAS方法是表面电化学、腐蚀、分析化学和纳米科学研究领域中的重要分析工具。希望本论文工作能为发展SEIRAS成为一种与SERS相媲美的一种普及性研究方法奠定基础。更多还原

【Abstract】 Molecules adsorbed on metal island films or metal colloids 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 discovery of which promotes the application of infrared spectroscopy in surface chemistry and analytical chemistry. Owing to its high signal sensitivity and simple surface selection rule, SEIRAS has been regarded as an important spectroscopic method for in situ characterization of surface adsorption and reaction at metal-electrolyte as well as metal-ambient interfaces. However, SEIRAS has not yet been developed to an extent as compared to its counterpart surface enhanced Raman spectroscopy （SERS） in term of its application in surface trace analysis and interfacial electrochemistry, probably due to the following difficulties.The SEIRA-activity greatly depends on the morphology of the nanofilm such as the particle size, proximity and aggregation. So the fabrication of nano-structured metal films with tunable size, shape and proximity is a challenging issue for exploring and exploiting SEIRA effect. Surface electrochemistry is a very important and relatively new interdiscipline that attracts the attention of many chemists. The prerequisite for practicing electrochemical ATR-SEIRAS is to fabricate metallic nanofilm electrodes which possess simultaneously high SEIRA activity, stability and reasonable electrochemical responses. Although dry processes such as evaporation or sputtering in vacuum have predominantly been used in depositing various metals on IR windows in the SEIRAS measurements, several disadvantages exist such as the requirement of a high-cost vacuum evaporator, time-consuming operation, sample contamination by organic species, poor enhancement reproducibility, and the severely distorted spectra. Before our project, the success of the wet process was restricted to very few metals and tactics. In particular, the Fe-group metal nanofilm electrodes with ATR-SEIRA activity was not achieved.To address the problems mentioned above, we aim to develop more flexible wet processes to fabricate new SEIRA-active templates of different kinds, and to broaden the application areas of SEIRAS with an emphasis on extending in situ ATR-SEIRAS to practically important Fe group metal electrodes. In this thesis, we present a ubiquitous "seeded-growth approach" to prepare SEIRA-active coinage metal substrates for ambient and electrochemical applications, and extended the "two-step wet process" to prepare nanofilm electrodes of Fe group metals. With these templates, in situ ATR-SEIRAS investigation of the adsorption and reaction at the desired electrodes were carried out. By means of colloid synthesis and surface self-assembly, structure-tunable Au and Ag nanoparticle films were fabricated on the silicon wafer, the feasibility of these templates in transmission SEIRAS was probed.Main research topics of the thesis are summarized as follows:（1） Fabrication of Structure-Tunable and SEIRA-active Au Nanofilms on SiStructure-tunable Au nanoparticle films provide a platform for studying the mechanism of SEIRA effect. Virtually no reports on controlling surface roughness and aggregation of nanoparticles have been found in SEIRAS measurements, and thus poor reproducibility in SEIRA effect can be expected. The strategy of self-assembly on Si has been adapted to solve the above-mentioned problems. We demonstrate a wet process that can be used to fabricate nanoscale-tunable SEIRA-active template by self-assembling two-dimensional arrays of colloidal Au particles using the APTMS as the organic glue. This process is simple and favorable for controlling the surface morphology of the substrate, in favor of understanding the size and spacing dependent SEIRA effect.The further growth of initial self-assembled Au nuclei with subsequent electroless deposition in a gold-plating bath with hydroxylamine as the reducing agent will increase the mass thickness of the Au nanofilm. By controlling the plating time, the nanoparticle size, interparticle spacing and aggregation state （thus conductivity） can be readily tuned. The Au nanofilm fabricated with the above seeded-growth tactics can serve as good SEIRA templates both in transmission SEIRAS and electrochemical ATR-SEIRAS measurements.This unique wet process does not require corrosive and reactive Fˉand Pd nuclei layers for the deposition to proceed, thus preventing the co-deposition of unwanted metals. It is expected the strategy can be a useful alternate wet process for preparing SEIRA-tunable substrate for both phenomenological and practical viewpoint.（2） Seeded-Growth Approach to Fabrication of Silver Nanoparticle Films on Silicon for SEIRASThe seeded-growth tactics proposed in the above section was extended to fabricate the Ag nanofilm on Si. An Ag seed layer on Si has been attained either by self-assembly of Ag colloids on aminosilanized Si surfaces or deposition of Ag discrete nanoparticles from reduction of Ag⁺ in a HF-bearing solution on Si surfaces. The two kinds of the substrates can serve as good enhancement amplifier in the transmission experiment. It should be noted that the second seeding process is more convenient and reproducible, and thus be promising for potential IR sensor application.The initial immobilizing discrete Ag nanoparticles on Si as catalytic seeds followed by further growth of Ag nanofilms can produce ATR-SEIRA active Ag nanofilm electrodes. Conductive, adhesive and SEIRA-active Ag films served as the working electrode to examine the adsorption configuration of pyridine by in situ ATR-SEIRAS measurement. Owing to simple surface selection rule, stable SEIRA effect with potential excursion, and needless ORC pretreatment in Clˉ-containing electrolytes, the end-on adsorption configuration of pyridine on Ag electrode can be deduced in a straightforward and reliable way.（3） Extension of in situ ATR-SEIRAS to Fe-Group Metal ElectrodesIt is of significant interest to investigate electrocatalytic reaction and corrosion inhibition on Fe-group metal electrodes. The two-step wet process strategy makes it possible for this kind of in situ ATR-SEIRAS study. The strategy combines the initial electroless deposition of an 60-nm-thick Au underlayer on Si with subsequent electrodeposition of the desired Fe group metal overlayers with tens nanometer in thickness. The SEIRA effects of the as-deposited Ni, Fe and Co nanofilms were examined with probe molecule CO. Greatly enhanced IR absorption was detected while maintaining unipolar and normally directed bands. The real-time monitoring of CO electroxidation on Ni nanofilm electrode was cited as an example here. The series of spectroscopic data reveal that the active participation of coadsorbed free H₂O molecules, the conversion of bridge to linear CO and as well as the discovery of CO_Ni^o.Meanwhile, we are interested in applying ATR-SEIRAS to probe the interfacial structures of adsorbed aromatic molecules on Fe-group metal electrodes. In this regard, benzotriazole film formation and its inhibition effect on Fe electrode were investigated by in situ ATR-SEIRAS. In addition, the adsorption configurations of pyridine on Ni and Fe electrodes were clarified.（4） Other Practical Applications of the "Seeded-Growth Fabrication" Tactics Metal coating is of wide interest in the industry, evaporation and sputtering are mostly used for nonconductive substrates of simple shapes. However, in the special cases like the coating on inner wall of a capillary for the flow detection and on the ends of silica fiber for the remote control, the vacuum dry process is not applicable or costly. The successive deposition of metals inside the fine capillary with seeded-growth method proved to be convenient and simple to for capillaries of various shapes, and the resultant Au or Ag layer is uniform and easily-controlled in thickness.The seeded growth process was also successfully applied to deposit Au and Ag film on the end of the fiber sensors with high reflectivity comparable to and even better than that made by sputtering or evaporation. What’s more it is much more cost-effective and material-saving.In summary, the present work was made to explore how to overcome the difficulties which limited the development and application of SEIRAS in the past years. The thesis reveals that SEIRAS, especially ATR-SEIRAS is an important analytical tool not only in surface chemistry, analytical chemistry and nano-science, but also in corrosion and interfacial electrochemistry. We hope that this work has made useful contribution to the development of SEIRAS into a general tool in phenomenological and practical viewpoint.更多还原