Electronic structures and optical properties of Ce-doped anatase TiO<sub>2</sub> with oxygen vacancy

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Abstract

The crystal structures, defect formation energy, electronic structures and optical properties of oxygen vacancy and/or Ce-(co)doped anatase TiO ₂are investigated by using density functional theory plus Ucalculations. The calculated results indicate that lattice distortion induces the enhanced octahedral dipole moment in Ce doped TiO ₂crystal when introducing oxygen vacancy into the lattice of the TiO ₂crystal, which is effective for separating the photo-excited electron-hole pairs; meanwhile, compared with the valence band of pure TiO ₂and TiO ₂mono-doped separately with Ce and oxygen vacancy, the valence band of TiO ₂co-doped with Ce and oxygen vacancy broadens drastically, which is mainly contributed from the electronic states of Ce 5d, Ti 4s and O 2p in the valence band shifting toward the lower energy direction. As a result, Ce doped TiO ₂with oxygen vacancy is beneficial to the mobility of photo-generated carriers in TiO ₂. Similarly, the anti-bonding states also move toward the lower band energy direction, which are formed by the mixture of Ce 4f, Ce 5d, Ti 3d, and O 2p orbits in the conduction band. Due to these shifts, the energy gap of Ce and oxygen vacancy codoped TiO ₂is narrowed to 2.67 eV with the emerge of the occupied impurity energy levels near Fermi level. Because of the above-mentioned excellence features, the absorption spectra for doped systems exhibit remarkable red-shift, especially, the intensity of optical absorption of TiO ₂co-doped with Ce and oxygen vacancy in the visible region and the infra-red region are obviously stronger than those of the Ce mono-doped TiO ₂. When introducing oxygen vacancy into the Ce-doped system, the calculated conduction band energy edge position changes from −0.27 eV to −0.32 eV, which implies that the reducing power of the conduction band edge of TiO ₂is remarkably enhanced. More fascinatingly, the calculated band energy edges for the Ce and oxygen vacancy codoped TiO ₂can satisfy the basic requirement for water splitting under visible light irradiation. In conclusion, Ce and oxygen vacancy co-doped system can effectively strengthen the photo-catalytic activity of TiO ₂and improve the utilization of the solar light; and our calculated results provide a powerful theoretical basis for the applications of the Ce and oxygen vacancy co-doped anatase TiO ₂in visible-light-driven water splitting in the future research.

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Authors and contacts

Corresponding author:Zhou Shi-Wen,swzhou@hainu.edu.cn; Peng Ping,ppeng@hnu.edu.cn;

Funds:Project supported by the Natural Science Foundation of Hainan Province, China (Grant Nos. 117085, 20165193).

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参量	GGA	GGA +U	LDA	LDA +U	Experiment	Theory	Error%
a/nm	0.3791	0.3897	0.3745	0.3806	0.3785^[26]	0.3831^[29]	0.5548
b/nm	0.3791	0.3897	0.3745	0.3806	0.3785^[26]	0.3831^[29]	0.5548
c/nm	0.9805	0.9925	0.9447	0.9674	0.9514^[26]	0.9631^[29]	1.6817
c/a	2.5860	2.5470	2.5230	2.5410	2.5140^[26]	2.5140^[29]	1.0739
u	0.2050	0.2050	0.2080	0.2070	—	0.2050^[29]	—
d_ap/nm	0.2011	0.2031	0.1966	0.2005	0.1980^[27]	0.1973^[29]	1.2626
d_eq/nm	0.1946	0.2000	0.1914	0.1948	0.1934^[28]	0.1930^[29]	0.7238
E_g/eV	2.18	3.18	2.16	3.12	3.32^[24]	3.23^[29]	—
注:d_ap,d_eq分别表示Ti—O长键和短键的键长; 误差指LDA +U方法相对于实验值的误差.

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参量	Ce-TiO₂			O_V-TiO₂	Ce/O_V-TiO₂
参量	This work	Experiment	Other theory	This work	This work
a/nm	0.3822	0.3789^[25]	0.3980^[31]	0.3789	0.3827
b/nm	0.3833	—	0.3980^[31]	0.3793	0.3798
c/nm	0.9956	0.9509^[25]	0.9757^[31]	0.9824	1.0090
d_ap/nm	0.2225	—	0.2273^[32]	0.2012	0.2233
d_eq/nm	0.2183	—	0.2119^[32]	0.1946	0.2252
注:d_ap,d_eq分别表示Ce-TiO₂, Ce/O_V-TiO₂中Ce—O长键和短键的键长, 对于O_V-TiO₂则表示Ti—O长键和短键的平均键长.

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Model	E_f/eV		q/e			p/Debye
Model	Ti-rich	O-rich	O	Ti	Ce	p/Debye
Pure TiO₂	—	—	−0.720	1.440	—	0.0000
Ce-TiO₂	10.42	1.16	−0.719	1.405	1.920	0.0168
O_V-TiO₂	1.02	5.65	−0.727	1.412	—	0.0668
Ce/O_V-TiO₂	−15.84	−20.48	−0.725	1.373	1.920	0.0251

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Vol. 68, Issue 3 - 2019-02-05

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