Theoretical Studies Of Vacancy And Molecule Adsorption On Titanium Dioxide Surface

With rapid progress in related theories and development of computer technology, thefirst-principles calculations based on the density functional theory (DFT) have beenwidely used in condensed matter physics, quantum chemistry, and material science.Nowadays, due to its broad applications on photocatalysis, heterogeneous catalysis,and solar energy conversion, TiO2has been paid a great attention. In particular, theresearches on rutile TiO₂(110) surface have attracted a lot of interests due to itsspecific structure and property. As an important tool, scanning tunneling microscope(STM) has the powerful ability in the research of TiO₂surface. Employing thefirst-principles calculations in the research of STM can help people understand theexperimental results from the basic physic concepts vividly. In this dissertation, thefirst-principles calculations have been employed to explore the origin of variousexperimental results in physics and make predictions on the further researches.In chapter1, we introduced the catalysis and applications of TiO₂, and analyzed thestructure and characteristic of TiO₂(110) surface in details. Then the history and basicconcepts of DFT and STM have been introduced. At last, some theoretical methodsoften used in STM related problems and the density functional packages used in theworks presented in this dissertation have been also introduced briefly.In chapter2, we carried out the first-principles calculations on the theoretical study ofa new type defect–oxygen vacancy pair (OVP) produced using STM tip. We'vesimulated the STM images by the Tersoff-Hamann formula and they are in goodagreement with the experimental ones. The OVP not only modulate the O_vac'sdiffusion, but also have higher catalysis activity. The calculated transition states isvery helpful for us to understand the surface atoms, molecular diffusion and reactionmechanism, and is also very important to comprehend the catalysis reactivity.In chapter3, we reported systematic density functional theory calculations of Tiinterstitials in the near-surface region of rutile TiO₂(110). Our calculated results pointout the most stable site for interstitial Ti atoms. We also simulate the images of thesurface when the Ti interstitials exist in different sites near surface. The simulated images have been compared with oxygen vacancy (OV) and OH images simulatedunder the same parameters. We calculate water dissociation reaction on the surfacewith interstitial Ti existing in the near-surface region, and interstitial Ti atoms wouldhelp water molecules on the surface to dissociate. It is very reasonable that theinterstitial Ti has a lot of associations with the chemistry activity of TiO₂(110) surface.In chapter4, a theoretical study on the adsorption of CO molecules on rutile TiO₂(110)surface has been carried out. Our results show that CO molecules preferentiallyadsorb at the next nearest-neighbor Ti5catoms close to the Obrvacancy, whichdisagrees with the idea that the OV acts as an adsorption site for CO. Then wecalculate the diffusion barrier of adsorbed CO molecules along Ti5crows. At last,first-principles calculations have been employed to investigate the experimentalobservation that a CO₂molecule could dissociate into a CO molecule on the OVunder the effect of proper voltage pulses. We have shown the energy barrier for thedissociation reaction and made an explanation in good agreement with experiments.In chapter5, we reported the investigation on the adsorption behaviors of COmolecules on the rutile TiO₂(110) surface with pre-adsorbed O adatoms using DFTcalculations joint with STM. Our results show that the diffusive CO molecules can betrapped by the pre-adsorbed O adatoms on Ti⁴⁺rows of TiO₂surface, forming CO-Oand CO-O-CO complexes. These complexes are quite stable. The activation energybarrier for CO oxidation through the CO-O complex to produce CO₂has beencalculated. Our results indicate that the dissociative O₂, i.e., the O adatoms on Ti⁴⁺,may not be directly responsible for the catalytic oxidation at low temperature.In chapter6, we presented first-principles calculations on the study of thehydroxylated TiO₂(110) surface. We have introduced a systematic study on thestructural, energetic and electronic properties of OH. We have calculated the energybarrier for the dehydrogenation of OH and analyzed its reversion in STM images. OurDFT calculations successfully reproduce the STM experiment observations. Theseresults afford us a good method to distinguish the OHs and OV.