Zhao, Xiang (Ivan); (2016) Studies of Adsorption of Ringed Carboxylic Acids on the Rutile (110) Surface through Density Functional Theory (DFT). Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Contemporary applications of titanium dioxide (TiO2) can be found in many areas, most notably those in development of dye-sensitized solar cells (DSSCs), spurring extensive research on both its bulk and surface properties, as well as adsorption of countless substances onto the latter. Of these studies, theoretical studies have been done through ab initio computational methods based on the framework of DFT. In our studies, we have performed density functional theoretic (DFT) ab initio simulations of TiO2 rutile and anatase bulk crystalline structures, the (110) surface, as well as the adsorption of benzoic acid, phenylalanine and simple zinc porphyrin (ZnPP), consisting of just a simple porphyrin ring and a carboxylic acid group linked to the meso-side, onto the rutile (110) surface, examining all our result in comparison with experimental observations. It is hoped that through these studies, involving increasingly complicated adsorbates, we can either account for experimental results for studies on these same systems, through theoretical and computational models and explanations, or refine the latter to achieve these goals. We also seek to find the relationship between optimized physical structures of these adsorbate-rutile substrate complexes, the energetics and increasingly importantly towards the end, how these influence the complexes’ electronic structures, especially in the case of ZnPP, whose derivatives have been studied as potential dye candidates in DSSCs. In carrying out our ab initio simulations, we have used generalized gradient approximation (GGA) based on the Perdue-Wang 91 (PW91) functionals and DFTD2 (dispersive) methods for optimizations of physical structures of the adsorbatesurface complexes, before performing calculations of the electronic DOS structures for the resultant adsorbate-TiO2 complexes. Through our studies with benzoic acid, we have found that contrary to proposed adsorption configurations from experimental findings through STM, which suggest alignment of the benzene rings along the [1¯10] directions when adsorbed at saturation coverage on the unconstructed (110) surface, the other proposed possible model involving alignment of the rings along the [001] direction has been found to be more energetically stable. Conversely, our DFT calculations have indicated for adsorption on the (1 × 2)-reconstructed rutile (110) surface, the [001]-aligned mode has been found to be more energetically stable, contrary to the proposed [1¯10] alignment of the benzene rings. However, as increasing the tunneling current in our simulated STM has resulted in the carboxylate groups instead of the benzene rings being imaged, this can also possibly indicate need to carry out STM studies at different tunneling current settings to further refine our accounts for how benzoic acid adsorbs on the surfaces of rutile. We have also noted shifts in adsorbate HOMO positions with respect to the valence band maxima, as the benzene ring is rotated by 90◦ Our studies on phenylalanine’s adsorption on the rutile (110) surface have been carried out, in close comparison with result from X-ray photoelectron spectroscopy (XPS) studies. The most stable mode of adsorption has been once more that of the bidentate dissociative (BD) mode, with an adsorption energy value of nearly 2.0 eV/adsorbate. However, in terms of coverage and adsorption patterns, the saturation coverage as produced experimentally was not the most energetically stable, with the patterns at half or even a quarter of the saturation coverage being more so when Van der Waals’ forces have been taken into account. In terms of the orientation of the benzene ring, BD adsorption resulted in an inclination of 35◦ to the (110) normal, as opposed to the 25◦ reported experimentally. Further contractions in the TiO2 valence-conduction bands have been observed, as we study BD adsorption of phenylalanine, at less saturated coverages. Finally, we have carried out DFT studies on the adsorption of the simple ZnPP on the rutile (110) surface, and it has been found that changes in the alignment of the porphyrin ring, in addition to having effects on adsorption energetics, also alter the electronic band structures and the associated energy levels, both before and after excitation. It has been found at the most saturated coverage corresponding to one ZnPP per 8 Ti5c sites, where a porphyrin ring can still undergo a complete rotation about the [110] axis unhindered, adsorption energy is maximized at around 2.4 eV/ZnPP when the porphyrin ring is at about 40-50◦ from either the [001] and the [1¯10] directions, in comparison to 2.2-2.3 eV/ZnPP when the porphyrin rings are nearly aligned with the these directions. Furthermore and much more significantly, we have only found the [001]-aligned configuration to be producing a ZnPP-TiO2 electronic DOS structure that makes the system suitable for DSSC applications.

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