Importance of Surfaces and Many-Body Absorption Spectra for C-Doped TiO2 Photocatalysts

Heterogeneous photocatalytic processes, in which a photocatalyst absorbs light to produce redox-active electron-hole pairs, have strong prospects of application in light energy conversion and environmental remediation. Although considerable efforts have been dedicated to the development of new photocatalysts, nanoparticulated anatase TiO2 continues to be the reference system. Its limited light absorption properties have been addressed in different ways, and among these, doping with heteroatoms (e.g., C or N) is a simple and efficient (although poorly understood) strategy. In the case of C-doping, although a significant number of computational works have described its major features, there are still conflicting reports on the local coordination of C, on the thermodynamic feasibility of the doping process, and on the optical properties of the doped material. Here, by considering surfaces instead of bulk anatase, we demonstrate that the C-doped structures are stabilized by up to 5 eV, indicating a spontaneous doping process across a broad range of oxygen pressures. Furthermore, we show that the calculated absorption spectrum for the most stable configuration, Ti-by-C substitution, is strongly dependent on the theory level: while semiempirical calculations indicate a red shift with respect to pristine TiO2, calculations at the highest achievable level (qpGW(0)-BSE [Bethe-Salpeter equation] without the Tamm-Dancoff approximation [TDA]) show no visible light absorption. In addition, we find that the commonly used TDA introduces a significant shift to the calculated spectra of the doped (but not the pristine) material. These results have two important implications that can be generalized to other systems: (i) a correct estimation of changes in optical properties upon doping may require theory levels higher than G(0)W(0)-BSE(TDA) and (ii) thermodynamic parameters determined from doping bulk structures may significantly deviate when considering surfaces, of relevance for nanoscaled materials.