Dollar Family And Ferrous Metal Surfaces To Enhance The Wet Structure And Application Of Infrared Absorption Of The Substrate

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.