In general, anatase TiO2 exhibits higher photocatalytic activities than rutile TiO2. However, the reasons for the differences in photocatalytic activity between anatase and rutile are still being debated. In this work, the band structure, density of states, and effective mass of photogenerated charge carriers for anatase, rutile and brookite TiO2 are investigated by the first-principle density functional theory calculation. The results indicate that anatase appears to be an indirect band gap semiconductor, while rutile and brookite belong to the direct band gap semiconductor category. Indirect band gap anatase exhibits a longer lifetime of photoexcited electrons and holes than direct band gap rutile and brookite because the direct transitions of photogenerated electrons from the conduction band (CB) to valence band (VB) of anatase TiO2 is impossible. Furthermore, anatase has the lightest average effective mass of photogenerated electrons and holes as compared to rutile and brookite. The lightest effective mass suggests the fastest migration of photogenerated electrons and holes from the interior to surface of anatase TiO2 particle, thus resulting in the lowest recombination rate of photogenerated charge carriers within anatase TiO2. Therefore, it is not surprising that anatase usually shows a higher photocatalytic activity than rutile and brookite. This investigation will provide some new insight into understanding the difference of photocatalytic activity among anatase, rutile and brookite.

Constructing a TiO based heterostructure is a very effective strategy for enhancing photocatalytic performance. The details of the electronic structure, interfacial interaction, and photogenerated carrier separation are important for explaining the photocatalytic properties of a heterostructure. Herein, the density of states, charge distribution, and the band offset of the monolayer g-CN/TiO heterojunction are systematically investigated through the hybrid DFT method. Results indicated that the valence band offset and the conduction band offset of the g-CN/TiO heterostructure were 0.40 and 0.18 eV, respectively. Interfacial interaction made the TiO surface with negative charge, whereas the g-CN surface with positive charge, which led to the formation of a built-in electric field at the interface. Under illumination, the built-in electric field accelerates the transfer of photoexcited electrons in the CB of TiO into the VB of g-CN, thus resulting in the photoexcited electrons and holes naturally accumulating in the CB of g-CN and the VB of TiO, respectively. The photoexcited electrons and holes gathering in different surface regions efficiently prolonged the lifetime of photogenerated carriers. Meanwhile, electrons in the CB of g-CN and holes in the VB of TiO had a stronger redox ability. Therefore, g-CN/TiO is a direct Z-scheme photocatalyst, and the Z-scheme heterostructure mechanism can well explain the improved photocatalytic activity of the g-CN/TiO heterostructure.

Graphite-like carbon nitride (g-C 3 N 4 )-based heterostructures have attracted much attention because of their prominent photocatalytic performance. However, theoretical understanding on the relationship of the interface and enhanced photocatalytic activity is still lacking. In this study, we systematically calculated energy band structure and charge transfer of the g-C 3 N 4 /CdS heterojunction using the hybrid density functional approach. The interaction between g-C 3 N 4 and the CdS (110) surface was explored. Results indicated that g-C 3 N 4 and CdS were in contact and formed a van der Waals heterojunction. The valence and conduction band edge positions of g-C 3 N 4 and CdS changed with the Fermi level and formed a standard type-II heterostructure. Furthermore, density of states, Bader charge, and charge density difference indicated that the internal electric field facilitated the separation of electron−hole pair in the g-C 3 N 4 /CdS interface and restrained carrier recombination. These results demonstrated that the band structure of the g-C 3 N 4 /CdS heterojunction had significant advantages to improve photocatalytic efficiency under visible-light irradiation. Moreover, our work may be used as a basis for the design of other highly active heterostructures.