C-, N-, S-, and F-Doped Anatase TiO₂ (101) with Oxygen Vacancies: Photocatalysts Active in the Visible Region

Abstract

Anatase TiO 2 presents a large bandgap of 3.2 eV, which inhibits the use of visible light radiation (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M1"><mml:mi>λ</mml:mi></mml:math> &gt; 387 nm) for generating charge carriers. We studied the activation of TiO 2 (101) anatase with visible light by doping with C, N, S, and F atoms. For this purpose, density functional theory and the Hubbard <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M2"><mml:mi>U</mml:mi></mml:math> approach are used. We identify two ways for activating the TiO 2 with visible light. The first mechanism is broadening the valence or conduction band; for example, in the S-doped TiO 2 (101) system, the valence band is broadened. A similar process can occur in the conduction band when the undercoordinated Ti atoms are exposed on the TiO 2 (101) surface. The second mechanism, and more efficient for activating the anatase, is to generate localized states in the gap: N-doping creates localized empty states in the bandgap. For C-doping, the surface TiO 2 (101) presents a “cleaner” gap than the bulk TiO 2 , resulting in fewer recombination centers. The dopant valence electrons determine the number and position of the localized states in the bandgap. The formation of charge carriers with visible light is highly favored by the oxygen vacancies on TiO 2 (101). The catalytic activity of C-doping using visible radiation can be explained by its high absorption intensity generated by oxygen vacancies on the surface. The intensity of the visible absorption spectrum of doped TiO 2 (101) follows the order: C &gt; N &gt; F &gt; S dopant.