Characterization Of Catalytic And Photocatalytic Reactions On Rutile TiO₂（110）Surface At The Single-molecule Level

Since Fujishima and Honda discovered the photocatalytic water splitting on TiO2electrode in1972, it has triggered a lot of study interests because of its important applications in solar energy conversion and environmental treatments. In the heterogeneous catalysis, the surface and the interface of a catalyst play a key role in the photocatalytic reactions. It is important to study the TiO2surface and its interactions with adsorbates for further understanding of the mechanisms of the photocatalysis. High-resolution scanning tunneling microscopy (STM) is a useful technique to directly image the surface structures, which provides a way to directly characterize the chemical reactions on the TiO2surfaces.In this thesis, I systematically investigate the adsorption and dissociation behaviors of molecules, such as O2, CO2, H2O and CH3OH on TiO2(110)-1×1surface at low temperatures. By combining the light illumination in situ, the photocatalytic reactions of molecular adsorbates on TiO2(110)-1×1surface have been characterized.In chapter1, a brief review of the photocatalysis is introduced. It includes the discovery of photocatalysis and the band structures of TiO2. Some important processes are discussed, such as the photon absorption and excitation to generate electron-hole pairs, the thermalization of charge carriers, the trapping of electrons and holes and their recombination, the charge transfer and the redox reactions. Some other issues in photocatalysis are also discussed, such as quantum yield of photo-reactions and the dependence on the light intensity.In chapter2, I describe the use of STM technique in characterization of chemical reactions on surface, especially the photocatalytic reactions using STM by illuminating the sample surfaces. Some previous results on the adsorption and reaction of molecules on the TiO2(110) surface are reviewed.In chapter3,I focus on the studies of the adsorption of the molecular oxygen either at the bridge-bonded oxygen vacancies (Ov) or at the hydroxyls (OHb) on TiO2(110)-1×1surface. Using an in situ O2dosing method, it is able to directly verify the exact adsorption sites and the dynamic behaviors of the molecular O2. It has been identified that at a low coverage of O2, the singly adsorbed molecular O2at Ov can be dissociated through an intermediate state as driven by the STM tip. However, a singly adsorbed molecular O2at the OHb can survive from such a tip-induced effect, which implies that the singly adsorbed O2at the OHb is much stable than that at the OV-It is interesting to observe that at high coverages with the Ov fully filled by excess O2dosing, the singly adsorbed O2at Ov tend to be non-dissociative even under higher bias voltage of2.2V. Such a non-dissociative behavior is most likely attributed to the presence of two or more O2molecules simultaneously adsorbed at a Ov with a more stable configuration than a singly adsorbed molecular O2at an Ov-Furthermore, the activities of molecular O2under light illumination and in CO oxidation are also discussed.In chapter4,I describe the measurement the CO2reduction by electron attachment, which can be used to determine the redox potential of CO2on the TiO2surface in ultra-high vacuum. In the experiments, the dissociation of CO2molecules is found to be activated by electron attachment process with an energy threshold of1.8eV above the Fermi level (or1.4eV above the TiO2conduction band onset), while the lowest unoccupied molecular orbital (LUMO) of the adsorbed CO2is located at2.3eV with respect to the Fermi level. The observed dependence of the dissociation rate on the tunneling current suggests that the reduction of CO2induced by the electron attachment is a single electron process. This practical information can be used as practical guidelines for the design of effective catalysts for CO2photoreduction.In chapter5, photocatalytic dissociations of methanol and water, CH3OH and H2O, are studied by combining light illumination in situ STM method at80K. We observed the dissociation of individually adsorbed CH3OH or H2O molecules at the five-fold coordinated Ti (Ti4+) sites of the reduced TiO2(110)-1×1surface under the irradiation of UV lights with the wavelength shorter than400nm, which accords well with the band gap of3.1eV for the rutile TiO2, and thus suggests a photocatalytic dissociation process. For CH3OH, the dissociation process undergoes via O-H bond cleavage while H transferring to the adjacent bridging oxygen sites. For H2O, the dissociation process has similar H transferring behavior and produces two kinds of hydroxyl species. One is always present at the adjacent bridging oxygen sites, i.e., OHb, and the other either occurs as OHt at Ti4+sites away from the original ones or even desorbs from the surface. In comparison, the tip-induced dissociation of CH3OH and H2O can produce OHt or oxygen adatoms exactly at the original Ti4+sites, but no OHb is present. Such a difference clearly indicates that the photocatalytic dissociations of CH3OH and H2O undergo different processes from the attachment of electrons injected by the tip. Our results suggest that the initial step of the CH3OH and H2O dissociation under the UV light irradiation may not be reduced by the electrons, but most likely oxidized by the holes generated by the photons.In chapter6, I investigate the adsorption behaviors of methanol (CH3OH) at the relatively high coverage on TiO2(110) surface at room temperature. It is found CH3OH can spontaneously dissociate at Ov at a low coverage. However, when all of the Ov are fully filled at a high coverage of CH3OH, it is observed that both the dissociative CH3OH and molecular CH3OH may coexist at Ov-Dynamically, the dissociation and recomposition of CH3OH show a certain equilibrium state. It turns out to be the proton exchange between the diffusive CH3OH along the Ti4+rows and the dissociative CH3OH at Ov-Combined with DFT calculations, this process can be well interpreted.