The Effect of Mechanical Strain and Band Alignment at the Anatase-Rutile Interface on the Photocatalytic Activity of TiO2

TiO2 is considered as one of the most important photocatalysts to date primarily due to many of its superior physical and chemical properties. Among its various polymorphs, rutile and anatase TiO2 are the two most important phases and have been widely used in many practical applications. However, due to its relatively large band gap, only a small portion of the solar spectrum in the ultraviolet light region can be absorbed to excite electrons to generate photocurrents for use in photocatalytic reactions. To further improve its performance, many research efforts have been made to increase its photocatalytic efficiency under sunlight, including chemical doping, mechanical strain and formation of heterojunction structures. Nevertheless, the progress of current research in photocatalysis for water splitting reaction is still very slow, and many fundamental details and underlying mechanisms remain unraveled. In this study, we employed first-principles density functional theory calculations to investigate the electronic property changes of the strained anatase TiO2 as well as the band lineup at the rutile-anatase interface. There are two main focuses in this thesis:
In the first part of the thesis, we investigated the effect of mechanical strains on the electronic property changes of anatase TiO2, which include the variations of electronic bang gaps, energy levels of VBM and CBM, and the effective masses of charge carriers. In our strained models, biaxial and uniaxial strains were imposed along the directions parallel to the (101), (100), and (001) surfaces, respectively, to mimic the lattice deformations arising from the lattice mismatch with the underlying substrates. Our calculated results show that the band gap of anatase TiO2 can be effectively reduced when [1 ¯01] uniaxial compressive strain is in the (101) surface, [001] uniaxial compressive strain is in the (100) surface, and [100]&[010] biaxial tensile strains is in the (001) surface, respectively. Our calculations also show that it is possible to make the energy level of CBM go upward while the band gap is reduced in the meanwhile when the (001) surface is under biaxial tensile stress. Furthermore, for all the strained structures that can cause band gap reduction, the variations of the effective masses for electrons and holes do not show negative impact on charge carrier separation. These results indicate that the photocatalytic activity of anatase TiO2 can be fine-tuned by applying mechanical strain along certain direction on this material system.
In the second part of the thesis, we studied the intrinsic band alignment at the rutile-anatase interface to understand the origins of the synergistic effect observed in the mixed phase TiO2 system. This synergic effect to enhance the separation of photo-excited charge carrier is generally believed to be attributed to a staggered band offset between the two phases. Nevertheless, the explicit direction of charge flow remains controversial and is still under intensive debate. To clarify this controversy, we have constructed two interface models, anatase(112)/rutile(100) and anatase(110)/rutile(011), respectively, to calculate their band alignments using first-principles density functional theory calculations. Our calculated results show that there is indeed a staggered band lineup at the rutile/anatase interface and both VBM and CBM of rutile lie higher in energy than those of anatase phase. The offset values of VBM/CBM were found to be 0.468 ±0.12eV/0.268 ±0.12eV for rutile(100)/anatase(112) interface, and 0.467±0.07eV/0.267±0.07eV for rutile(011)/ anatase(110) interface, respectively. Based on this result, the photo-excited electrons would majorly transport from rutile to anatase while the hole would favor in the opposite direction, which can help enhance the charge carrier separation resulting in better photocatalytic activity of the mixed phase TiO2. On the other hand, we also employed the vacuum level alignment method to study the band lineup at the rutile/anatase interface without acquiring detailed knowledge of the interface structures. Basically, the band alignments obtained using this method are consistent with those predicted based on the realistic interface structure models, providing a convenient way to acquire the preliminary guess for the band lineup of the heterojunction material systems.