6. Summary

The properties include the geometrical structure, the ground state energy, the work function, and the band structure. In order to design the more efficient photocatalyst, it is necessary for theoretical worker knows the basic physics prior to performing first principle calculations.

First, the photocatalyst performance depends on the chosen of the TiO2 surface. Asymmetric anatase (101) surface has the most uncoordinated Ti and O atoms, and thus, it is suitable for the molecular adsorption of CO2, thus the better photocatalyst performance (Figure 4). Second, proper energy alignment between TiO2 and the adsorbed molecules is required. This is due to the built-in driving force largely affect the direction of the ET, and a negative driving force should lead to the inefficient ET process, in order to avoid this issue. The TiO2 CB should be higher than the molecular LUMO orbital, whereas the TiO2 VB should be lower than the molecular HOMO orbital. Third, the interface details between TiO2 and adsorbed molecule affect the time scale of interface charge separation. This is because interface charge transfer can be described using the Fermi's Golden rule if the coupling between the electron/hole donor orbital in TiO2 and acceptor orbital in adsorbed molecule is large, then the ET will be fast. The interface interaction depends on the adsorption details of molecular onto TiO2 surfaces.

The last, the work function is defined as the minimum energy required transferring an electron from the highest filled level of a solid to a point in the vacuum outside the solid surface. The work function is a property of the surface, which has been widely used to measure how difficult the charge transfer is. Screen the materials with the proper work function is required.

The (101)-(001) surface heterojunction constructed on polyhedral TiO2 nanocrystals has recently been proposed to be favorable for the efficient electron-hole spatial separation due

Figure 4. Photocatalytic reduction of CO2 on the anatase TiO2 (101) surface (Copyright 2016 American Chemical Society).

5.3. Other main photocatalytic reactions

242 Titanium Dioxide

In this chapter, we aimed to review and summarize of recent important theoretical studies of titanium dioxide for dye-sensitized solar cell and photocatalytic reaction. How to build a suitable TiO2 model to start a theoretical study for fast interfacial electron transfer between the dye and TiO2 in DSSC system, and the mechanism for photocatalytic reactions on the TiO2 surface will be impressive for readers. We need to describe the electronic band structure of excited state or ground state of TiO2, the surface morphology, active energy for reaction as well as the fast interfacial electron transfer between the adsorbed molecule and TiO2 accurately for fine theoretical studies. For some situation, we also need to do molecular dynamics for the surface structure of adsorbed TiO2. May the summary report will help the readership to strengthen the fundamental knowledge or to find more interesting ideas.
