Adsorption of molecular thin films on metal and metal oxide surfaces

Abstract: Metal and metal oxides are widely used in industry, and to optimize their performance their surfaces are commonly functionalized by the formation of thin films. Self-assembled monolayers (SAMs) are deposited on metals or metal oxides either from solution or by gas deposition. The gas deposition enables the preparation of SAMs under very well controlled conditions in ultrahigh vacuum (UHV).Thiols with polar terminal groups are utilized for creating the responsive surfaces which can interact electrostatically with other adsorbates. Surface charge affects wetting and adhesion, and many other surface properties. Polar terminal groups in thiols could be used to modify these factors. Mixed SAMs can provide more flexible surfaces, and selection of particular terminal groups in the mixed SAMs could change the resulting surface properties under the influence of factors such as pH, temperature, and photo-illumination. However, in order to control these phenomena by mixed polar-terminated thiols, it is necessary to understand the composition and conformation of the mixed SAMs and their response to these factors. In this work, mixtures of thiols with carboxylic and amino terminal groups were studied. Carboxylic and amino terminal groups of thiol interact with each other via hydrogen bonding in solution and form a complex. Complexes adsorb to the surface in non-conventional orientations. Unmixed SAMs from each type, either carboxylic terminated thiols or amino terminated thiols adsorbed on gold in standing up orientation while SAMs from complexes are in an axially in-plane orientation. The orientation of mixtures causes greater hydrophobicity. Thiolated surfaces with complexes are less responsive to the pH changes than for the unmixed thiolated surface with either carboxylic or amino termination. Contact angle changes significantly with pH change for the unmixed thiolated surfaces but there is no change in the contact angle with water on the mixed SAMs. Selenol is an alternative to replace thiols for particular applications such as contact with biological matter which has a better compatibility with selenol than sulfur. However, the Se-C bond is weaker than the S-C bond which limits the application of selenol. Understanding the selenol adsorption mechanism on gold surfaces could shed some light on Se-C cleavage and so is investigated in this work. Se-C cleavage happens in the low coverage areas on the step since atoms at steps have lower coordination making them more reactive than atoms on the terraces. At higher dosage, the herring bone structure of the gold is lifted up, and at full coverage there is a smooth layer on the terraces. Another area where the self-assembly of molecules is of importance is for dye sensitized solar cells, which are based on the adsorption of the dye onto metal oxides surfaces such as TiO2.The interface between the self-assembled dye monolayer and the substrate is an important factor to consider when designing dyes and surfaces in dye sensitized solar cells (DSSCs). The quality of the self-assembled monolayers of the dye on the TiO2 surface has a critical influence on the efficiency of the DSSCs.  Creation of just a monolayer of dye on the surface could lead to an efficient current of photo-excited electrons to the TiO2 and degeneration of the dye by redox. On the other hand, multilayer formation or aggregation of dyes on the TiO2 surface causes indirect contact between the dye molecules and the surface, which reduces the efficiency of the DSSCs. Therefore, it is very important to investigate the amount of dye adsorption on the surface. In this work, T-PAC dye showed island growth with some ad-layer that is not in contact with the surface, whereas the MP13 dye adsorption occurs through laminar growth. Results for the sublimated samples were similar to those for the sample with deposition from a dye solution, except for the existence of water in the latter sample.Cuprite (Cu2O) is an important initial and common corrosion product on copper under atmospheric conditions. Copper could be a good replacement for noble metal as catalysts for methanol dehydrogenation. Knowledge about the structure of Cu2O(100) and Cu2O(111) surfaces could be used to obtain a deeper understanding of methanol dehydrogenation mechanisms with respect to adsorption sites on the surfaces. In this work, a detailed study was done of Cu2O(100) surface which revealed the possible surface structures as the result of different preparation conditions. Studies of the structure of Cu2O(100) and Cu2O(111) surfaces show that Cu2O(100) has a comparatively stable surface and reduces surface reactivity. As a consequence, dehydrogenation of methanol is more efficient on the Cu2O(111) surface. The hydrogen produced from methanol dehydrogenation is stored in oxygen adatom sites on both surfaces.