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

Julio César González-Torres,Enrique Poulain,Raúl García-Cruz,Víctor Domínguez-Soria,Oscar Olvera-Neria

doi:10.1155/2018/7506151

Julio César González-Torres, Enrique Poulain + Show 3 more

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https://doi.org/10.1155/2018/7506151

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Abstract

Anatase TiO2 presents a large bandgap of 3.2 eV, which inhibits the use of visible light radiation (λ > 387 nm) for generating charge carriers. We studied the activation of TiO2 (101) anatase with visible light by doping with C, N, S, and F atoms. For this purpose, density functional theory and the Hubbard U approach are used. We identify two ways for activating the TiO2 with visible light. The first mechanism is broadening the valence or conduction band; for example, in the S-doped TiO2 (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 TiO2 (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 TiO2 (101) presents a “cleaner” gap than the bulk TiO2, 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 TiO2 (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 TiO2 (101) follows the order: C > N > F > S dopant.

Highlights

In the late ’70s, photocatalysis took a turn when researchers discovered the TiO2 ability to degrade stable organic compounds as they were studying the water photoelectrolysis [1,2,3]
For the generalized gradient approximation (GGA)/GGAU and GGAU methodologies, the energy bandgap (Eg) broadens to 2.81 and 2.76 eV, respectively, and there are no significant changes in the electronic structure for the bulk system (see Figure S1(a) in the Supplementary Materials)
For the TiO2 (101) surface, the Eg narrows to 1.85, 2.31, and 2.25 eV for each used methodology, respectively. We attribute this narrowness to the bond unsaturation at the material surface, corresponding to undercoordinated Ti species located at the beginning of the conduction band (CB), which promotes a bandgap reduction

Summary

Introduction

In the late ’70s, photocatalysis took a turn when researchers discovered the TiO2 ability to degrade stable organic compounds as they were studying the water photoelectrolysis [1,2,3]. There has been a great interest to improve the degradation efficiency of organic pollutants using TiO2 [4,5,6,7]. Anatase TiO2 has an energy bandgap of 3.2 eV, that is, UV radiation is mandatory to promote electrons from the valence band (VB) to the conduction band (CB). The primary challenge has been to generate charge carriers using visible light rather than UV radiation. Charge carriers act as oxidation and reduction centers, which enable the creation of reactive species such as hydroxyl radicals, peroxides, or acid compounds. These species promote the degradation of pollutants [27,28,29]

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