Enhanced Optical Absorption Due to Symmetry Breaking in TiO2(1−x)S2x Alloys

Titania (TiO2) is frequently used in photovoltaic and photocatalytic applications, despite the fact that its main optical absorption occurs only at ∼4 eV. Absorption across the band gap of 3 eV is dipole-forbidden in rutile TiO2. By means of first-principles theoretical spectroscopy calculations, we demonstrate that alloying with TiS2 introduces an absorption band into the fundamental gap of TiO2. In addition, band-edge transitions contribute to optical absorption because the S incorporation breaks the symmetry of the TiO2 lattice. Both effects lead to pronounced absorption of visible light for S concentrations as low as 1.5%. ■ INTRODUCTION Titanium dioxide (TiO2) has been studied for several decades due to its versatility for a diverse range of applications. In the context of photovoltaics or photocatalysis, it is used in dye-sensitized solar cells and has great potential for water splitting or the degradation of hazardous substances. Furthermore, TiO2 is robust, thermally stable, nontoxic, as well as inexpensive. The large fundamental band gap (Eg = 3.03 eV for the rutile polymorph, see, e.g., refs 2 and 6) is the reason why TiO2 is transparent across the entire visible spectral range. Moreover, direct optical transitions from the valence-band maximum (VBM) to the conduction-band minimum are dipole forbidden, and the main onset of optical absorption occurs only at ∼4 eV. Whereas this is beneficial for applications as a transparent conducting oxide, it constitutes a problem for the photoactivity because only photons in the ultraviolet range or higher are absorbed. To utilize a larger part of the solar spectrum for photochemical applications, the optical absorption of TiO2 has to be extended into the visible part of the spectrum. A number of experimental and theoretical studies have focused on dopants such as N or F and C, Se, P, or S, but also codoping, as well as self-doping. For S, some of us recently reported that the band gap is drastically reduced as soon as a small amount of S is added to TiO2. 17 Here we extend this study toward optical properties and show by means of state-of-the-art theoretical spectroscopy techniques that sulfur incorporation not only introduces an alloy band in the fundamental gap of TiO2 but also breaks the rutile symmetry, which makes direct band-edge transitions dipole allowed. These two factors combined lead to a dramatic increase in absorption in the visible range for S concentrations as low as 1.5%. Our calculations also explain why experimental studies of S incorporation observed a corresponding red shift of the absorption onset and not the emergence of a defect-related peak in the band gap region. ■ METHODOLOGY We model TiO2(1−x)S2x alloys of different compositions x ranging from 0 to 0.25 by replacing one O atom with one S atom in rutile supercells of 12 to 96 atoms. The geometries were fully relaxed in density functional theory. Our study focuses on rutile TiO2 (space-group: P42/mnm or D4h 14 (SG136)), because we want to study the optical-absorption properties for the band structures reported in ref 17 to investigate the impact of the dipole-selection rules. Nevertheless, the symmetry-breaking and lattice-distortion mechanism discussed below is expected to also apply to the anatase phase because it is driven by the different radii of the oxygen and the sulfur atoms. Optical spectra are calculated using manybody perturbation theory by solving the Bethe−Salpeter equation (BSE). In a first step, the Kohn−Sham (KS) eigenvalues and eigenstates are calculated using the local-density approximation (LDA) to exchange and correlation. The projectoraugmented wave method as implemented in the Vienna ab initio simulation package is used to describe the electron− ion interaction. Pseudopotentials are the same as those used in Received: October 29, 2012 Revised: January 18, 2013 Published: January 28, 2013 Article