N co-doped anatase TiO2 by first-principles calculations

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Electronic structure of Gd/N co-doped anatase TiO2 by first-principles calculations S.K. Zhenga,b,n, Yi Wua, Mingju Zhanga, Wenming Lia, Xiaobing Yana a

Research Center for Computational Materials & Device Simulations, College of Electronic & Informational Engineering, Hebei University, Baoding 071002, PR China b Department of Analytical Chemistry, Campus Universitario de Rabanales, Universidad de Córdoba, Córdoba 14071, Spain Received 12 June 2015; received in revised form 22 June 2015; accepted 26 June 2015

Abstract The lattice parameters, energy band structures, density of states, and optical absorption spectra of Gd-doped, N-doped, and Gd/N co-doped anatase TiO2 were calculated by the first-principles based on the density functional theory. The calculated results indicate that the three kinds of doping all induce lattice distortion for TiO2, but the structures still keep unchanged. Gd doping introduces an empty energy band in the forbidden band of TiO2, and N doping introduces an impurity energy level above the maximum valence band. Gd/N co-doping forms an impurity energy level and a full-filled energy band in the forbidden band of TiO2, where the impurity energy level lies above the full-filled energy band. The experimental results for the photocatalytic activity enhancement of TiO2 by the Gd/N co-doping can be explained perfectly from the calculation results. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: TiO2; Electronic structure; First-principles

1. Introduction TiO2 has been attracted more and more attention in the research field of environmental protection due to its nontoxicity, long-term stability, anti-oxidation and high photocatalytic activity [1,2]. Under the irradiation of photons with energy no less than the band gap of TiO2, electrons will be excited to the conduction band from the valence band and participate complex chemical reactions, eventually most organic pollutants could be degraded completely into CO2 and H2O, and gain the goal of environmental protection. But the band gap of TiO2 (3.2 eV for anatase) is too large to utilize the visible light of the solar irradiation. To overcome this disadvantage, a strategy called doping is mostly adopted during the photocatalytic process. It is reported that when n Corresponding author at: Research Center for Computational Materials & Device Simulations, College of Electronic & Informational Engineering, Hebei University, Baoding 071002, PR China. Tel.: þ 86 312 5079368. E-mail address: [email protected] (S.K. Zheng).

the Fe ions are doped into the lattice of TiO2, the photocatalytic activity of Fe–TiO2 nanorods exceeds that of Degussa P25 by a factor of more than two times under visible-light irradiation [3], which can be ascribed to the fact that the Fedoping induces the shift of the absorption edge into the visiblelight range with the narrowing of the band gap and reduces the recombination of photo-generated electrons and holes. Li et al. [4] reported N doped 3D TiO2 nanorods architecture with significantly enhanced visible light photoactivity, it is believed that N doping lowered the band gap of TiO2 NRs and effectively activated visible light photoactivity, and N doping also largely improved the incident-photon-to-current-conversion efficiency in the UV range. TiO2 nanoparticles co-doped with C and Y were prepared to deal with methyl orange under visible light illumination [5]. It was found that the photocatalytic activity was enhanced due to the synergistic effects of Y and C, and the absorption spectra of TiO2 showed a red-shift of absorption band edge in the visible light region. Venditti et al. [6] has investigated the removal of phenolic compound

http://dx.doi.org/10.1016/j.ceramint.2015.06.129 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: S.K. Zheng, et al., Electronic structure of Gd/N co-doped anatase TiO2 by first-principles calculations, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.129

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S.K. Zheng et al. / Ceramics International ] (]]]]) ]]]–]]]

Fig. 1. Structural model of Gd/N-TiO2.

and caffeic acid by photodegradation using C-doped TiO2 particles under visible light. It was found that the whole photodegradation process is governed by a synergic mechanism in which adsorption and photodegradation are involved. Myilsamy et al. [7] synthesized In and Ce co-doped mesoporous TiO2 nanocomposites by sol–gel route. The results indicate that the light absorption band-edge position of In and Ce codoped TiO2 shifts to the visible region and the TiO2 shows high surface area with large pore diameter. Sotelo-Vazquez et al. [8] synthesized multifunctional P-doped TiO2 thin films by atmospheric pressure chemical vapor deposition and found that the P ions in TiO2 existed as P5 þ and P3  states. Compared to un-doped TiO2 films, the photocatalytic performance of the doped films was vastly inferior one. As a supplementation, theoretical calculations for the electronic structures of materials are always performed to further understand the origin for the photocatalytic activity improvement of TiO2. A first-principles calculation indicates that P doping into anatase TiO2 will enhance the photocatalytic activity of TiO2 [9], especially the interstitial P plays an important role in the photocatalytic process under both UV and visible light. To understand the photocatalytic activity of Sand P-doped TiO2, Yang et al. [10] investigated the effects of S- and P-doping on the electronic structures of TiO2 by firstprinciples calculations based on the density functional theory. It was found that the band gap has few changes but some mixing states are localized in the band gap, which result in the reduction of photon absorption energy. Zhu et al. [11] reported the electronic and optical properties of Mo/C co-doped anatase TiO2 using the first principles calculations. The results indicate that the effective band gap is narrowed about 0.9 eV and the optical absorption edges of the Mo-C co-doped TiO2 shift towards the visible light region. Recently, there have been several literatures reported that Gd/N co-doping can enhance the photocatalytic activity of TiO2. Yang et al. [12,13] synthesized a series of Gd/N codoped nano-TiO2 photocatalysts using sol–gel method. The photocatalytic activity of the samples was evaluated by the degradation of Humic Acids under ultraviolet light. The results showed that the removal of Humic Acids could be as high as

88%. In 2013, Wang et al. [14] measured the photocatalytic activity of Gd/N co-doped TiO2 under both ultraviolet and visible lights irradiation. It was found that the Gd/N co-doped TiO2 shows a higher photocatalytic performance compared to the single Gd or N doped TiO2, especially under the visible light irradiation. Zhou et al. [15] reported that Gd/N co-doped TiO2 could degrade 85% Humic Acids under solar light irradiation. This is due to the synergistic effect of Gd and N doping into TiO2. Trititanate nanotubes were prepared using hydrothermal method and then co-doped with Gd3 þ and N through ion-exchanging with H þ [16]. When using Rhodamine B as the model pollutant, it was found that the photocatalytic activities of TiO2 were enhanced significantly under visible light irradiation by the Gd/N co-doping. As mentioned above, the literatures only did some experimental researches on the Gd/N co-doped TiO2 and without any further theoretical calculations. To understand well the photocatalytic activity enhancement of TiO2 by Gd/N co-doping, we have calculated the electronic structures of Gd, N and Gd/N doped TiO2 and discussed the results, aiming at providing some profound theoretical illustrations for the related experimental researches on Gd/N co-doped TiO2. 2. Calculation details The literatures reported that the doped and un-doped TiO2 were in the form of anatase phase [12–16]. So a 108 atoms anatase model was selected to perform the calculations. To build the Gd/N co-doped TiO2 (Gd/N-TiO2) model, one Ti atom was substituted by one Gd atom, and one O atom was substituted by one N atom, as shown in Fig. 1. If only Ti was substituted by Gd or O was substituted by N, then we can obtain Gd-doped TiO2 (Gd–TiO2) or N-doped TiO2 (N-TiO2). All calculations were carried out by the CASTEP [17] software. The Perdew–Burke–Ernzerhof is selected as the exchange-correlation function. The interaction between valence electrons and the ionic core is described by ultrasoft pseudopotential, which is used with Ti: 3s23p63d24s2, O: 2s22p4, Gd: 4f75s25p65d16s2 and N: 2s22p3 as the valence electrons configuration. The cut-off energy of plane wave basis is set as 300 eV. The special points sampling integration over the Brillouin zone is carried out using the Monkhorst–Pack method with a 2  2  2 special-point mesh. The convergence threshold for the maximum energy change, maximum force, maximum stress and displacement tolerances are set as 2.0  10  5 eV/atom, 0.05 eV/Å, 0.1 GPa and 0.002 Å, respectively. 3. Results and discussions 3.1. Structural parameters Firstly, the structural parameters were calculated for the doped systems of Gd–TiO2, N-TiO2 and Gd/N-TiO2. The calculated results are listed in Table 1. From Table 1 it can be seen that the three kinds of doping all lead to lattice distortion of TiO2, especially Gd/N co-doping results in the biggest

Please cite this article as: S.K. Zheng, et al., Electronic structure of Gd/N co-doped anatase TiO2 by first-principles calculations, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.129

S.K. Zheng et al. / Ceramics International ] (]]]]) ]]]–]]] Table 1 Structural parameters for different TiO2 systems.

Pure TiO2 [9] Gd–TiO2 N-TiO2 Gd/N-TiO2

a (Å)

b (Å)

c (Å)

Volume (Å3)

3.8073 3.8093 3.7900 3.8321

3.8073 3.8094 3.8087 3.8045

9.8232 9.8913 9.8535 9.9713

1281.55 1291.80 1280.12 1308.35

Gd/N-TiO2

N-TiO2

Gd/TiO2

TiO2 20

30

40

50

60

70

80

2θ (deg.)

Fig. 2. XRD patterns of TiO2, Gd–TiO2, N-TiO2 and Gd/N-TiO2.

lattice distortion. After Gd, N doping and Gd/N co-doping, the volumes of TiO2 are 1291.80 Å3, 1280.12 Å3, and 1308 Å3, respectively. The reason is that the ion radii of Gd3 þ (0.094 nm) and N3  (0.171 nm) are different to those of Ti4 þ (0.068 nm) and O2  (0.132 nm) [14]. Because of the synergistic effect of Gd/N co-doping, the Gd/N-TiO2 has the biggest lattice distortion. The lattice distortion caused by the doping is in agreement with the experimental results where different dopings can cause different particle sizes [12–15]. Although the doping results in lattice distortion of TiO2, the crystal phase keep unchanged, as shown in Fig. 2. The calculated X-ray powder diffraction diagrams (XRD) indicate that only some diffraction peak positions for the doping samples shift a little compared to the un-doped TiO2. All the structures are anatase phase. From Fig. 2 it can be seen that the peak shifts for Gd/N-TiO2 are very clear, which is corresponding to the biggest lattice distortion. The calculated XRD results are consistent with the experimental researches that doping does not change the crystal phase of TiO2. As Yang reported [12,13], Gd-, N-doped TiO2 and Gd/N co-doped TiO2 were all in anatase phase. Wang [14] and Zhou [15] also obtained the same results that Gd, N single doping or co-doping did not affect the structure of anatase TiO2. 3.2. Band structures The band structures of TiO2 [9], Gd–TiO2, N-TiO2 and Gd/ N-TiO2 were calculated using first-principles method, as

3

shown in Fig. 3(a–d). During the calculations, the highest energy level that electron occupied is set as the energy zero point. The calculated band gap of TiO2 is showed in Fig. 3(a). After Gd doping, the band gap of TiO2 decreases to 2.04 eV, as shown in Fig. 3(b). An empty energy band is formed above the maximum valence band. This empty energy band can accept electrons that excited from the valence band, and then the electrons will have a certain possibility to transit to the conduction band and participate into chemical reactions. Therefore, this empty energy band will decrease the energy for an electron transiting from the valence band to the conduction band, which will result in a higher photocatalytic activity in Gd–TiO2. Actually, the experimental researches have shown that Gd-Tiwed O2 has a higher photocatalytic activity than pure TiO2 [12–15]. N-TiO2 has a band gap of 2.09 eV, as shown in Fig. 3(c). An impurity energy level appears near the maximum valence band. Once the N doping concentration arrive a certain value in TiO2 matrix, this impurity energy level will overlap with the valence band and decrease the band gap of TiO2, which will reduce the transiting energy for the electrons form the valence band to the conduction band, eventually improve the photocatalytic activity of TiO2. For Gd/N-TiO2, the calculated band gap is 2.45 eV which is greater than un-doped TiO2, as shown in Fig. 3(d). There is an electron full-filled band formed in the middle of the forbidden band of TiO2. At the same time, an impurity energy level appears about 0.51 eV above the full-filled band. The electrons in the full-filled band and the impurity energy level can easily transit to the conduction band of TiO2 under suitable energy photon irradiation and thereby provide more electrons to participate into the chemical reactions occurred on the surface of TiO2, which will eventually improve the photocatalytic activity of TiO2. The experimental researches have shown that the photocatalytic activities are Gd/N-TiO2 4Gd–TiO2 4 N-TiO2 4 TiO2 [12–15]. From the experimental results and the calculated results we can deduce that during the photocatalytic process, the impurity band maybe play a more important role than the impurity energy level for the photocatalytic activity when TiO2 is single elemental doped. When TiO2 is co-doped with Gd and N, it is possible that the Gd and N have a synergistic effect on the photocatalytic activity of TiO2, and Gd/N-TiO2 shows the highest photocatalytic activity. 3.3. Partial density of states (DOS) In order to know the components of the band structures, we calculated the partial density of states (PDOS) for the systems. It is well known that the conduction band of TiO2 is composed of Ti 3p orbital, and the valence band is composed of Ti 3p and O 2p orbital, as shown in Fig. 4(a) [9]. After Gd doping, it is very clear that the empty band appeared in the middle of the forbidden band of TiO2 is mainly contributed by Gd 4f orbital, as shown in Fig. 4(b). Gd 4f orbital also provides contribution to the Fermi energy level (energy zero point). Gd 5d orbital

Please cite this article as: S.K. Zheng, et al., Electronic structure of Gd/N co-doped anatase TiO2 by first-principles calculations, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.129

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4

5.0

5.0

pure TiO

Gd-TiO

2.5

2.5

Energy (eV)

Energy (eV)

0.0

-2.5

-5.0

-7.5

0.80eV

0.0

-2.5

-5.0

F

G

5.0

Q

-7.5

G

Z

G

F

5.0

N-TiO

2.5

Q

G

Z

Gd/N-TiO

2.5

1.71eV

2.09eV 0.0

Energy (eV)

Energy (eV)

0.93eV

2.04eV

2.20eV

-2.5

0.68eV

0.0

0.51eV

2.45eV

1.05eV

-2.5

-5.0

-5.0

-7.5

-7.5

G

F

Q

Z

G

G

F

Q

Z

G

Fig. 3. Band structures of (a) TiO2 [9], (b) Gd–TiO2, (c) N-TiO2 and (d) Gd/N-TiO2.

only provides a little contribution to the valence band. At the same time, it is obviously observed that Gd 5d, O 2p and Ti 3p orbital are hybridized above the minimum valence band. Gd 4f and O 2p orbital are hybridized below the maximum valence band. The orbital hybridization of Ti 3d, O 2p and N 2p forms the valence band of N-TiO2, as shown in Fig. 4(c). Because the N 2p orbital is lying very near to the maximum conduction band of TiO2, it decreases the band gap of TiO2 and improves the photocatalytic activity. Fig. 4(d) shows the PDOS for Gd/N-TiO2. From Fig. 4(d) it can be seen that the impurity energy level in the forbidden band is contributed by Gd 4f orbital. The hybridization of N 2p and Gd 4f forms the full-filled impurity band in the forbidden band of TiO2. Gd 4f, N 2p and Ti 3d orbital hybridized at about  2.0 eV. In Fig. 3(d), there is an impurity energy level below the minimum valence band, compared with the PDOS in Fig. 4(d), it is the contribution of N 2p orbital. From Fig. 4(d) we can also know that the Gd 4f orbital almost has no contribution to the valence band. The valence band is mainly composed of Ti 3d, O 2p and N 2p orbital. The conduction band is still composed of Ti 3d orbital.

3.4. Charge populations The chemical environment where the doping element exists could be known from the charge populations. Table 2 shows the calculated charge populations of Gd, N in doped anatase TiO2. From Table 2 it can be seen that Gd loses electrons and N obtains electrons. Compared with Gd/N-TiO2, the net charge of Gd in Gd–TiO2 is less than that in Gd/N-TiO2, and N obtains more electrons in N-TiO2 than that in Gd/N-TiO2. These results mean that Gd and N in Gd/N-TiO2 form the interaction with each other, Gd and N maybe have a synergistic effect on the electronic structure of TiO2, and then affect the photocatalytic activity of TiO2. 3.5. Absorption spectra The absorption spectra of Gd–TiO2, N-TiO2 and Gd/N-TiO2 were calculated and are shown in Fig. 5. Because there is an underestimation of the band gap in the calculated un-doped TiO2, we used a 1.00 eV “scissor operation” to move the absorption edge to the experimental value. Although Hubbard

Please cite this article as: S.K. Zheng, et al., Electronic structure of Gd/N co-doped anatase TiO2 by first-principles calculations, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.129

S.K. Zheng et al. / Ceramics International ] (]]]]) ]]]–]]]

3

12

Full Ti 3d O 2p

2

1

-5.0

-2.5

0.0

2.5

Full Ti 3d O 2p Gd 5d Gd 4f

6

3

0 -7.5

0 -7.5

Gd-TiO 2

9

PDOS (electrons/eV)

PDOS (electrons/eV)

TiO2

5

5.0

-5.0

-2.5

0.0

2.5

5.0

Energy (eV)

Energy (eV)

3

12

N-TiO2

2

1

0 -7.5

Gd/N-TiO2

Full Ti 3d O 2p Gd 5d Gd 4f N 2p

9

PDOS (electrons/eV)

PDOS (electrons/eV)

Full Ti 3d O 2p N 2p

6

3

0 -5.0

-2.5

0.0

2.5

5.0

-7.5

-5.0

-2.5

0.0

2.5

5.0

Energy (eV)

Energy (eV)

Fig. 4. PDOS (a) TiO2 [9], (b) Gd–TiO2, (c) N-TiO2 and (d) Gd/N-TiO2.

“U” [18,19] can be used in the calculations to obtain a calculated band gap of 3.2 eV for pristine TiO2, while compared with the calculated results of this study without “U” parameters, the “U” value only affects the relative position of the energy levels [20]. For qualitative explanation of the photocatalytic activity enhancement, with or without “U” parameter calculations will not affect the conclusions. From Fig. 5 it can be seen that the calculated spectra are in good agreement with the calculated band structures. The absorption edges of Gd–TiO2 and N-TiO2 both show a red-shift compared to the un-doped TiO2, while the absorption edge of Gd/N-TiO2 has an obviously blue-shift. As the calculated band gaps of Gd–TiO2, N-TiO2 and Gd/N-TiO2 are 2.04, 2.09 and 2.45 eV, respectively, it is naturally for the absorption spectra show such kind of properties. Above all, we can explain the enhancement of photocatalytic activities in Gd–TiO2, N-TiO2 and Gd/N-TiO2. For Gd– TiO2, the Gd 4f orbital forms un-filled energy band in the forbidden band of TiO2. During the photocatalytic process, this empty band can accept electrons transiting from the valence

Table 2 Charge populations of Gd, N in anatase TiO2. Species Gd–TiO2 N-TiO2 Gd/N-TiO2

Gd N Gd N

s

p

d

f

Total

Charge (e)

2.14 1.77 2.12 1.75

5.84 3.83 5.94 3.79

1.25 0.00 1.05 0.00

7.31 0.00 7.41 0.00

16.54 5.60 16.52 5.54

1.46  0.60 1.48  0.54

band, which is a supplemental transition for electrons except those electrons transit from valance band directly to conduction band. The electrons in the empty band can also transit to the conduction band and participate into chemical reactions, therefore enhance the photocatalytic activity of TiO2. For N-TiO2, N 2p orbital hybridized with O 2p and decreases the band gap of TiO2, then broadened the light response and more photons can be utilized to pump electrons transiting from the valence band to the conduction band, so higher photocatalytic activity could be obtained in N-TiO2. As for Gd/N-TiO2, although the impurity energy band in the forbidden band is

Please cite this article as: S.K. Zheng, et al., Electronic structure of Gd/N co-doped anatase TiO2 by first-principles calculations, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.129

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References

Absorption (a.u.)

TiO2 Gd-TiO2 N-TiO2 Gd/N-TiO2

100

200

300

400

500

600

700

800

wavelength (nm)

Fig. 5. Absorption spectra of un-doped, Gd-, N-doped and Gd/N co-doped TiO2.

full-filled with electrons under the basic state, the electrons can be easily excited to the conduction band and leave holes, and then the electrons in the valence band can be excited to the impurity band or the conduction band. The widened band gap is detrimental to the utilization of visible light, but the impurity full-filled band with an impurity energy level above it defense the detrimental widened band gap, so the photocatalytic activity of Gd/N-TiO2 is enhanced by the synergistic effect of Gd/N co-doping. 4. Conclusions For understanding the photocatalytic activity enhancement of Gd/N co-doped TiO2 better, the lattice parameters, band structures, density of states and absorption spectra of Gd-, Ndoped and Gd/N co-doped TiO2 were calculated using the firstprinciples based on the density functional theory. The doping may result in lattice distortion of TiO2, but the crystal phase maintains unchanged. Gd doping forms an empty energy band in the forbidden band of TiO2; N doping forms an impurity energy level above the maximum valence band and reduces the band gap of TiO2; Gd/N co-doping forms a full-filled band and an impurity energy level in the forbidden band of TiO2. The calculated absorption spectra are consistent with the calculated band structures of un-doped and doped TiO2. The experimental results for the photocatalytic activity enhancement of TiO2 could be explained with the theoretical calculated results. Acknowledgment The authors would like to express highly appreciation to Professor Baoting Liu at Hebei University for providing the CASTEP software and discussing the calculated results. This work is supported by the China Scholarship Council ([2014]3012) and the National Natural Foundation of China (No. 61306098).

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Please cite this article as: S.K. Zheng, et al., Electronic structure of Gd/N co-doped anatase TiO2 by first-principles calculations, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.129