Abstract
TiO2 anatase is considered to play a significant importance in energy and environmental research. However, for developing artificial photosynthesis with TiO2, the major drawback is its large bandgap of 3.2 eV. Several non-metals have been used experimentally for extending the TiO2 photo-absorption to the visible region of the spectrum. It’s therefore of paramount importance to provide theoretical guidance to experiment about the kind of defects that are thermodynamically stable at a realistic condition (e.g. Temperature (T), oxygen partial pressure ({{boldsymbol{p}}}_{{{bf{O}}}_{{bf{2}}}}), doping). However, disentangling the relative stability of different types of defects (viz. substitution, interstitial, etc.) as a function of charge state and realistic T, {{boldsymbol{p}}}_{{{bf{O}}}_{{bf{2}}}} is quite challenging. We report here using state-of-the-art first-principles based methodologies, the stability and meta-stability of different non-metal dopants X (X = N, C, S, Se) at various charge states and realistic conditions. The ground state electronic structure is very accurately calculated via density functional theory with hybrid functionals, whereas the finite T and {{boldsymbol{p}}}_{{{bf{O}}}_{{bf{2}}}} effects are captured by ab initio atomistic thermodynamics under harmonic approximations. On comparing the defect formation energies at a given T and {{boldsymbol{p}}}_{{{bf{O}}}_{{bf{2}}}} (relevant to the experiment), we have found that Se interstitial defect (with two hole trapped) is energetically most favored in the p-type region, whereas N substitution (with one electron trapped) is the most abundant defect in the n-type region to provide visible region photo-absorption in TiO2. Our finding validates that the most stable defects in X doped TiO2 are not the neutral defects but the charged defects. The extra stability of {({bf{S}}{bf{e}}{bf{O}})}_{{bf{O}}}^{+{bf{2}}} is carefully analyzed by comparing the individual effect of bond-making/breaking and the charge carrier trapping energies.
Highlights
Doping-mediated modulation of the band gap is one of the most pragmatic approaches adopted in the pursuit to improve photo-absorption of TiO2 in the visible region[11]
We have used state-of-the-art density functional theory (DFT) with hybrid functionals combined with ab initio atomistic thermodynamics approach[38,39]
We have determined the energetically most favorable geometric configurations (X)O, (XO)O and (X2)O defects optimized at various charge states q = −2, −1, 0, +1, +2 for various non-metal dopants (X = N, C, S, Se) in the Ti16O32 framework
Summary
Doping-mediated modulation of the band gap is one of the most pragmatic approaches adopted in the pursuit to improve photo-absorption of TiO2 in the visible region[11]. The energetics of PBE + U to estimate defect formation energy of a single oxygen vacancy at various charge states doesn’t match with respect to advanced hybrid xc-functional HSE06 (see Supplementary Information; Figs S1 and S2).
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