Studies On Coordination Reaction Between Tetraphenylporphyrin And Nickel On TiO₂（110） Surface And The Change Of Molecular Conformation

Porphyrins are ubiquitous in nature and have high potential for applications in material science, medicine, biological chemistry, etc. The research of the interface between organic molecules and wide bandgap semiconductor is both fundamentally important and of technical and practical applications for the development of sensors, solar cells and catalysts. Metalloporphyrins and porphyrin arrays bound to TiO2 has attracted considerable attention in this area due to the following advantages:(a) the ability to act as active center in photo-catalytic reaction in gas sensors and nanoporous catalytic materials; (b) the close match of their photoabsorption properties to the solar spectrum makes them appealing candidates for efficient photovoltaic applications. However, to our knowledge, porphyrin metalation on oxide surfaces has not been reported before. The research on model surface is helpful to understand the fundamental physical problem in real device. Using the modern surface science techniques to explain the surface reaction between organics and oxide is not only providing theoretical basis for the construction and optimization of relative organic photovoltaic devices, but also providing experimental foundation for the development of organic electronic. In this thesis, we select Tetraphenylporphyrin (2HTPP) and Nickel(II) Tetraphenylporphyrin (NiTPP) to study the system of the direct synthesis of NiTPP on rutile TiO2(110) and the change of molecular conformation by synchrotron radiation photoemission spectroscopy (SRPES), X-ray photoemission spectroscopy (XPS), scanning tunneling microscopy (STM) and Density Functional Theory (DFT).The main experimental results of this dissertation can be described in the following:(1) The reduced and stoichiometric TiO2(110)-lxl surfaces were prepared to compare the effect of 2HTPP molecules on these surfaces. The thermal stability experiments of 2HTPP and NiTPP show that these two kinds of molecules have good thermal stability. Their monolayers can be prepared by annealing their multilayers to 550 K. The molecules are stable up to 610 K. Increasing the temperature, the molecules will decompose and only carbon leaves on the surface.(2) The in-situ metalation of 2HTPP with Ni on TiO2(110)-1×1 surface to NiTPP has been investigated by XPS, STM and DFT. If Ni is deposited onto 2HTPP (sub)monolayers on TiO2, the reaction proceeds already at room temperature, while elevated temperatures are required when Ni is deposited first. The reaction from 2HTPP to NiTPP is accompanied by changes of the molecular conformation from saddle-shaped to a four-fold symmetry. The formed NiTPP molecules undergo a～45° rotation relative to 2HTPP, which is consistent with DFT calculations.(3) With combination of SRPES, STM and DFT, the coordination reaction between 2HTPP and Ni on reconstructed TiO2(110)-1×2 surface has been studied. The metalation can be realized at room temperature irrespective of the deposition order of Ni and 2HTPP, which however leads to different metalation degrees. Changing the deposition order, the maximum degree of metalation from post-deposited Ni is higher than that of pre-deposited Ni. Compared to TiO2(110)-1×1 surface, Ni clusters on TiO2(110)-1×2 surface are formed with smaller size and higher density at sites vicinal to cross-links or single-links, which enlarged the maximum metalation degree of 2HTPP reacting with post-deposited Ni. Furthermore, the adsorption and diffusion of Ni atoms on the added Ti2O3 rows make the metalation of 2HTPP with pre-deposited Ni possibly occur at room temperature. The metalation from 2HTPP to NiTPP is accompanied by molecular conformation changes from tilted two-lobed along the [001] direction to a four-lobed feature with two diagonal phenyl groups along the [001] direction, which is supported by DFT-based STM simulations.