The stability, electronic structure, and optical property of TiO2 polymorphs (1312.2297v1)

Abstract: Phonon density of states calculation shows that a new TiO2 polymorph with tridymite structure is mechanically stable. Enthalpies of 9 TiO2 polymorphs under different pressure are presented to study the relative stability of the TiO2 polymorphs. Band structures for the TiO2 polymorphs are calculated by density functional theory with generalized gradient approximation and the band energies at high symmetry k-points are corrected using the GW method to accurately determine the band gap. The differences between direct band gap energies and indirect band gap energies are very small for rutile, columbite and baddeleyite TiO2, indicating a quasi-direct band gap character. The band gap energies of baddeleyite (quasi-direct) and brookite (direct) TiO2 are close to that of anatase (indirect) TiO2. The band gap of the newly predicted tridymite-structured TiO2 is wider than the other 8 polymorphs. For optical response calculations, two-particle effects have been included by solving the Bethe-Salpeter equation for Coulomb correlated electron-hole pairs. TiO2 with cotunnite, pyrite, and fluorite structures have optical transitions in the visible light region.

• The paper demonstrates that the new tridymite TiO₂ polymorph is mechanically stable and exhibits a noteworthy direct band gap of 5.67 eV.
• Advanced computational techniques, including DFT, GGA, GW, and BSE, were employed to accurately map phase transitions and refine electronic structures.
• The optical analysis shows that certain TiO₂ polymorphs absorb visible light, highlighting their potential for photocatalytic and optoelectronic applications.

Analysis of the Stability, Electronic Structure, and Optical Property of TiO Polymorphs

The paper conducted by researchers Tong Zhu and Shang-Peng Gao from Fudan University offers a comprehensive analysis of the mechanical stability, electronic structure, and optical properties of several TiO₂ polymorphs. The research employs advanced computational methodologies, including density functional theory (DFT), generalized gradient approximation (GGA), the GW method for band structure corrections, and the Bethe-Salpeter equation (BSE) for optical properties, to present a detailed profile of nine TiO₂ polymorphs.

Stability and Structural Characteristics

The paper introduces a novel TiO₂ polymorph with the tridymite structure, assessing its mechanical stability through phonon density of states analysis. Results indicate positive real phonon modes, suggesting the tridymite-structured polymorph is mechanically stable. Enthalpic calculations across different pressures reveal a complex stability landscape for TiO₂ polymorphs. Particularly low enthalpies for columbite structure suggest it may be more stable than rutile but less stable than anatase and brookite at standard conditions.

High-pressure phase transitions are explored, with transformations delineated from anatase to brookite and subsequently to columbite and baddeleyite structures. These transformations occur under specific pressure ranges, concordant with experimental observations. Such insights into pressure-induced phase transitions are crucial for strategizing synthetic approaches that could yield these structures under varied conditions.

Electronic Band Structure

Using the GW method, energy band gaps were meticulously corrected to provide accurate electronic profiles for each polymorph. Rutile and anatase demonstrate band gaps of 3.18 eV and 3.71 eV, respectively, aligning with existing experimental measures. The paper notably highlights that the newly proposed tridymite TiO₂ exhibits a notably large direct band gap of 5.67 eV, presaging unique electronic properties suitable for specific applications.

The characterization of quasi-direct band gap behaviors in rutile, columbite, and baddeleyite phases emphasizes the nuanced interplay between direct and indirect electronic transitions. Such characteristics can significantly affect the photochemical and photovoltaic performances of these materials.

Optical Properties

Optical absorption properties, influenced by the dielectric function, were computed via the Bethe-Salpeter equation. The paper reveals that polymorphs such as cotunnite, pyrite, and fluorite possess transitions within the visible region, rendering them promising for photocatalytic applications and solar energy harvesting. The stark contrast in absorption profiles of the tridymite phase indicates potential for applications that exploit its unique optical attributes.

The assessments are anchored in the computed macroscopic dielectric constants, which provide a solid theoretical basis for understanding light-material interactions in TiO₂ polymorphs. The ability to model such optical phenomena underscores the potential of these polymorphs in advanced optoelectronic devices.

Implications and Future Directions

This systematic examination of TiO₂ polymorphs not only contributes to the academic understanding of these materials but also delineates potential avenues for practical applications. The insights gained about phase stability and band structure could inform the design of materials for specific catalytic and photo-electronic applications. Furthermore, the tridymite polymorph’s large band gap paves the way for future research on its synthesis and functional deployment.

Future exploration could include experimental synthesis and characterization of the tridymite polymorph and investigation of other possible high-pressure phases. Additionally, developments in theoretical methodologies could refine these predictions and aid in identifying more polymorphs, revealing the broader potential of TiO₂ in technological applications.