Novel Functional Metal Film Electrodes-their Fabrication And Application In ATR Surface-Enhanced IR Absorption Spectroscopy

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.