Studies On Dynamic Processes Of Adsorption And Photocatalytic Reactions Of Molecules On Rutile TiO₂ （110） Surface Using STM

The photocatalysis of TiO₂ attracted a wide study interest in the last decades for its potential application in solar energy conversion and environmental remediation. Scanning tunneling microscopy （STM） has been a powerful technique to study the photochemistry at a single-molecule scale for understanding the interaction between adsorbates and substrates and identifying the reactivity at specific sites, benefit from its high resolution in the real space. In this thesis, the adsorption behaviors and the reactions are studied for various molecules, like H₂O, CH₃OH, and HCHO, on rutile TiO₂（110） surface, especially on their dynamic processes at a single-molecule scale using STM.In chapter 1,1 give a brief review on the properties of TiO₂ and some processes in the photocatalytic reactions, including the properties and structures of rutile TiO₂ （110） surface, the mechanism of photocatalytic reactions. In this chapter, I also introduce the method using STM to characterize the dynamic processes of molecules on various surfaces and their mechanisms.In chapter 2, the proton/H-atom transfer processes in the H₂O/TiO₂（110） system are systematic studied through the high resolution STM images and the corresponding I-t spectra. I find that the hydrogen in OHt can transfer between the O of OHt and the adjacent bridge O （Ob） under the assist of inelastic tunneling electrons induced vibrational excitation. While at 80 K water molecule can dissociate reversibly through thermal excitation, one of the H-atom/proton transfer to the adjacent Ob and back, which proved the pseudo-dissociative adsorption of water at Tisc site. The inelastic tunneling electrons can accelerate the H-atom/proton transfer process or further excite both the two H-atom/proton in water molecule transfer away. The proton transfer for H2O on TiO₂（110） surface is crucial to understand the elementary reaction of water.In chapter 3, I systematically investigate the photocatalytic reaction of methanol en the TiO₂（110） surface at various conditions, using STM jointed with temperabve-programmed dcsorption （TPD） techniques. The STM and TPD results show that the photocatalytic reaction is indeed initiated from the molecular methanol at the 5-fold coordinated Ti sites, reflecting the photoactive nature of methanol. Comparison with the reaction of methanol triggered by the injected electron or hole from the STM tip, the photocatalytic reaction of methanol is ascribed to be oxidized by photogenerated hole. The formaldehyde yield from the TPD results is much smaller by a factor of 2/3 than the amount of dissociated methanol from the STM results at 80 K. This observation can be assigned to the reverse reaction during the TPD measurement, and may explain the lower yield of formaldehyde using molecular methanol than using methoxy. From the fractal-like reaction kinetics of methanol, I can associate the coverage-dependence of the spectral dimensions with the change for the diffusion of holes across the surface from a one-dimensional to a two-dimensional behavior because of the increased scattering species at higher coverage. I further investigate the propagation of carriers on the rutile TiO₂（110） surface through the nonlocal reaction of the adsorbed methanol molecules during the carriers injection process. The diffusion of hole across the surface is anisotropic and the decay length along [1-10] direction is larger than that along [001] direction. The results provide a clear picture for the photocatalytic reaction of molecular methanol and may rationalize the different observations performed at various conditions.In chapter 4, I investigate the dynamic processes of formaldehyde （HCHO） molecules on five-fold-coordinated titanium （Ti5c） sites of rutile TiO₂（110） surface, using STM joint with density functional theory simulations. The results under the mild scanning conditions （0.6 V,2 pA） show that the adsorbed HCHO molecules at Ti5c sites present as two types of protrusions, either centered at Ti5c rows or centered at bridging oxygen （Ob） rows in the STM images, corresponding to the monodentate adsorption configuration through O-Ti5c bond and to the bidentate adsorption configuration through both of O-Ti5c and C-Ob bonds, respectively, which can be well supported by the simulated images. I also observe that the monodentate adsorption tends to spontaneously switch to the bidentate adsorption. These results confirm the existence of the energetically more favored bidentate adsorption for HCHO at Ti5c sites. I obtain that the energy barriers are approximately 0.28 and 0.75 eV for the adsorbed HCHO molecules switching from the monodentate adsorption to the bidentate adsorption and reversely switching from the bidentate adsorption to the monodentate adsorption, respectively, from the measurements of their dynamic processes. These findings can well elucidate the missing signature of the energetically more favored bidentate configuration in some previous experiments, and provide insightful understanding of formaldehyde on TiO₂（110） surface. The formation of HCHO dimer and the interaction between HCHO and Ov pair are also been studied.