Effects of oxygen vacancy on 3d transition-metal doped anatase TiO2: First principles calculations

Accepted Manuscript Title: Effects of oxygen vacancy on 3d transition-metal doped anatase TiO2 : First principles calculations Author: Ya Fei Zhao Can Li Song Lu Li Jin Yan Yin Yan Gong Leng Yuan Niu Xin Juan Liu PII: DOI: Reference:

S0009-2614(16)00053-1 http://dx.doi.org/doi:10.1016/j.cplett.2016.01.040 CPLETT 33574

To appear in: Received date: Revised date: Accepted date:

16-12-2015 13-1-2016 15-1-2016

Please cite this article as: Y.F. Zhao, C. Li, S. Lu, L.J. Yan, Y.Y. Gong, L.Y. Niu, X.J. Liu, Effects of oxygen vacancy on 3d transition-metal doped anatase TiO2 : First principles calculations, Chem. Phys. Lett. (2016), http://dx.doi.org/10.1016/j.cplett.2016.01.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effects of oxygen vacancy on 3d transition-metal doped anatase TiO2: First principles calculations Xin. Juan. Liu

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Ya. Fei. Zhao, Can. Li*, Song. Lu, Li. Jin. Yan, Yin. Yan. Gong, Leng. Yuan. Niu and

Engineering, China Jiliang University, China E-mail: [email protected]

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Abstract

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Institute of Coordination Bond Metrology and Engineering, School of Materials Science and

In this work, systematic study of the formation energy, crystalline and electronic

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structures of 3d transition metal (Sc, V, Cr, Mn, Fe, Co and Ni) doped anatase TiO2 specimens with and without oxygen vacancy has been carried out by the first principles calculations. The impurity states located at the band gaps enhance the

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visible light absorption, and the oxygen vacancy result in the EF move into the CB for some doped systems, which induce the Ti3+ ions and promote the separation of

d

photogenerated carriers. Doping and oxygen vacancy can change the hybrid strength

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gap.

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and MP value of TM–O bonding which has the approximately linearly with the band

Keywords: TiO2 based photocatalysts; formation energies; Fermi level; electronic and

photonic properties; bond formation and relaxation.

I. Introduction

Titanium dioxide (TiO2) has attracted much interest as a promising photocatalyst

in recent decades [1-3]. However, its low catalytic efficiency remains a main challenge preventing a widely practical application. TiO2 has the wide intrinsic band gap (Eg = 3.00 eV for the rutile phase and 3.20 eV for the anatase phase), and thus can only be excited by ultra-violet (UV) light radiation, which covers about ~ 5% energy of the solar spectrum [4, 5]. Therefore, it is of great importance to efficient utilization of visible-light that accounts for 45% energy of the solar spectrum. For this purpose,

Page 1 of 22

the band gap of photocatalyst need be reduced for efficient utilization of solar spectrum [6-8]. Several theoretical and experimental reports have indicated that dopants can improve the photocatalytic activity of TiO2 by introducing the impurity level or

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narrowing its band gap [9-14]. Up to now, various transition metals (TM) and/or

nonmetals doped and co-doped TiO2 specimens have investigated. TM dopants, such

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as Fe, V , Ni, Co and Cu, promote the absorption edge of redshift and decrease the

recombination rate of photogenerated carriers [10, 15-17]. Nonmetal dopants, such as

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C, N and S, shift the top of the valence band to higher energies, which thus reduce the band gap and make the specimens to response visible-light [4, 11, 18, 19]. Moreover,

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the codoped specimens may combine the features of each dopant based on the synergistic effect and exhibit enhanced optical properties and photocatalytic

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efficiency compared with the monodoped photocatalysts [20-23]. Meanwhile, oxygen vacancy is one of the most important in metal oxides. Pan et

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al. report that the defect of oxygen vacancies related properties of TiO2 including structural, electronic, optical, dissociative adsorption and reductive properties, which

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are intimately related to the photocatalytic performance of TiO2 [24]; Wang et al.

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indicated the higher the single-electron-trapped oxygen vacancy concentration in N doped anatase TiO2 is, the better the visible light photocatalytic activity will be [25]; Janotti et al. report that the formation energy of oxygen vacancy (2+) is relatively low

in n-type TiO2 under O-poor conditions but it rapidly increases with the oxygen chemical potential [26]. Thus, oxygen vacancy plays a significant role in the photocatalytic activity of TiO2.

The doping and oxygen vacancy can both affects the crystalline and electronic

structures of system by the chemical bond relaxation [27], the effects of doping on the chemical bond relaxation have been studied by pervious works [28, 29], however, detailed studies about the effects of oxygen vacancy are ambiguous, which contributes to further understand the fundamental mechanism of chemical bonds in doped TiO2. In this work, systematic studies of the formation energy, crystalline and electronic structures of 3d TM (Sc, V, Cr, Mn, Fe, Co, and Ni) doped anatase TiO2 specimens

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with and without oxygen vacancy have been carried out by the first principles calculations. The effects of dopant on electronic structures and the chemical bond of TM–O bonding were analyzed.

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II. Computational methods All considered TiO2 based specimens with space group I41/amd were built. The

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formation energy, crystal structure, band structure, partial density of states (PDOS)

and Mulliken bond population (MP) were calculated based on the density functional

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theory with the Perdew-Burke-Ernzerhof functional of the generalized gradient approximation (GGA) [30] and norm-conserving pseudopotentials [31], as implemented in the CASTEP code. Three kinds of supercells (TMTi15O32,

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TMTi15O1O31 and TMTi15O2O31) were built, where the TM atom replaces the Ti atom and the superscript 1st and 2nd indicate the oxygen vacancy positions (see Fig. 1). The

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cutoff kinetic energy with 750 eV and 443 k-point sampling set ware sufficiently large for the specimens considered [32]. The convergence tolerance of energy,

d

maximum force and maximum displacement were 5.010-6 eV/atom, 0.01 eV/Å and

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5.010-4 Å, respectively. The MP was calculated according to the formalism described by Segall et al [33]. The GGA+U approach was used to account the strongly

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correlated interactions of the d electrons [9, 34, 35], and the calculated band gap of pure anatase with U= 5.0 eV for Ti elements is 2.79 eV. The formation energies Ef of dopants were calculated to estimate the thermal

properties:

Ef = E(doped) – E(pure) + μ(Ti) – μ(TM)+ nμ(O)

(1)

where E(doped) is the total energies of all specimens, E(Ti) and E(TM) are the

chemical potential of Ti and TM atoms, n = 0 or 1 denotes the number of oxygen vacancy. Considered the growth of TiO2 being a dynamic process, so the Ef is not fixed which depend on the growth condition and can be changed from O-rich to Ti-rich. The chemical potentials of O atom and Ti atom satisfy the equations: μ(Ti) + 2μ(O) = μ(TiO2)

(2)

Under O-rich conditions, μ(O) is the chemical potential of the ground-state energy of

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the O2 molecule, while the chemical potential of Ti is obtained by the equation (2). Under Ti-rich conditions, the μ(Ti) is the chemical potential of Ti from bulk atoms and the chemical potential of O atoms is calculated by the equation (2).

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III. Results and discussion Lattice constant and formation energy

In Fig. 1b, the TM atom is bonded to six adjacent O atoms forming a

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body-centered octahedron for TMTi15O32 systems, but bonded to five adjacent O

atoms in the TMTi15O1O31 and TMTi15O2O31 systems. The supercell lattice constants

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for all doped TiO2 were calculated and listed in Table 1. For pure TiO2, the supercell lattice constants a and c are 7.564 and 9.564 Å, respectively, while these are reduced

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in TMTi15O32 systems except the ScTi15O32. The supercell lattice parameters a and b of TMTi15O1O31 systems increased except the NiTi15O1O31, while the supercell lattice

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parameters a and b of some TMTi15O2O31 decreased and the others increased, the supercell lattice parameters c of all specimens with oxygen vacancy reduced. The

d

changes of supercell lattice constants in doped systems are caused by the changes of the bond length. In pure TiO2, the lengths of Ti–O are 1.933 and 1.990 Å in xy plane

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and z axis, respectively. However, the TM-O bonds in the doped TiO2 are larger than

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the corresponding Ti–O bonds in pure TiO2 except ScTi15O32, ScTi15O1O31, ScTi15O2O31, owing to the smaller binding energies of Sc-O bonds. Consequently, both doping atoms and their oxygen vacancy positions influence the lattice structure of doped TiO2 together.

Meanwhile, the Table 1 also exhibits the formation energies of all doped systems

under the Ti-rich and O-rich growth conditions. It can be clearly seen that Ti-rich conditions favor the formation of TMTi15O1O31 and TMTi15O1O31 systems (except

VTi15O1O31, VTi15O2O31 and CrTi15O2O31 systems). However, O-rich conditions easily promote the growth of TMTi15O32. This confirms that the impurity atoms prefer to replace the Ti atom under O-rich growth conditions [36]. Especially, the ScTi15O32 system is the most stable doped system due to its lowest formation energy (-8.66 eV) under O-rich growth conditions. Considering the relationship between Ef and growth conditions different TM-doped anatase TiO2 can be easily prepared by controlling the

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pressure of oxygen. Band structure and PDOS The band structures of TMTi15O32, TMTi15O1O31 and TMTi15O2O31 specimens were shown in Fig. 2 and the band gap values and the band edges values (EVBM and

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ECBM) were listed in Table 1, where the VBM and CBM denote the valence band

maximum and conduction band minimum. Compared with pure TiO2, the band gap

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values of all doped specimens were reduced by 0.51 ~ 0.92 eV, which reduces the energy gap of electron transition and is comparable with the previously reported

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values obtained using first principles calculations [37-39]. Meanwhile, several impurity levels (ILs) appear between the VB and CB of Mn-, Fe-, Co- and Ni-doped

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specimens and reduce the energy gap of electron transition further. Meanwhile, compare with the band gap values of TMTi15O32 specimens, for the TMTi15O1O31 and

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TMTi15O2O31 specimens, the VBM (and CBM) of Sc-, V-, and Fe- doped specimens move down (and upper), so the band gap values of them increase about 0.06 ~ 0.24 eV;

d

the VBM and CBM of Cr- doped specimens move upper about 0.02 ~ 0.07 eV and 0.21 ~ 0.26 eV, respectively, so the band gap values of them reduce about 0.14 ~ 0.24

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eV; the VBM and CBM of Mn- doped specimens move down about 0.12 ~ 0.16 eV

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and 0.30 ~ 0.34 eV, respectively, so the band gap values of them reduce about 0.14 ~ 0.22 eV; the VBM and CBM of Co- doped specimens move down about 0.02 ~ 0.10 eV and 0.13 ~ 0.25 eV, respectively, so the band gap values of them increase about 0.11 ~ 0.15 eV; the VBM and CBM of Ni-doped specimens move down synchronously, so the band gap values of them basically keep constant. For pure TiO2, the Fermi level (EF) is just above the top of the VB and its CB is

not occupied by the electron. While the EF in some doped systems move into the CB and some Ti 3d states at the bottom of the CB are occupied by the electron, it results in the Ti4+ ions are reduced to the Ti3+ ions. Many previous studies also indicate that

the Ti3+ ion denotes the highly efficient separation center of photogenerated electrons and holes and enhance photocatalytic activity [40, 41]. The EF still stay at the top of VB for ScTi15O32, CoTi15O32 and NiTi15O32, move into the band gaps for MnTi15O32 and FeTi15O32 and are located the bottom of CB for VTi15O32 and CrTi15O32. However,

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the EF enters the CB for TMTi15O1O31 and TMTi15O2O31 specimens except the EF of FeTi15O1O31, CoTi15O2O31, NiTi15O1O31 and NiTi15O2O31 systems. Several TM dopants and the oxygen vacancy lead to the extra electrons and can push the EF into the bottom of the CB, the extra electrons easily transfer to the Ti atoms.

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The PDOS of all specimens were shown in Fig. 3 with the EF set zero for comparing, where only the average PDOS of per O, Ti and TM atoms is shown. It is

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found that the VB and CB are mainly made up of O-2p, Ti-3d and TM-3d orbitals for

all doped specimens. For TMTi15O32 doped specimens, the O-2p orbitals interact with

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the Ti-3d and TM-3d orbitals in both VB and CB, and the ILs are the interaction states caused by O-2p and TM-3d orbitals. After the oxygen vacancy being introduced into

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the TMTi15O32 specimens, the Ti-3d and TM-3d orbitals in CB (or in VB) increase (or reduce) and the O-2p orbitals in CB and VB keep constant, it means that the amount

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of the filled electrons reduces for Ti-3d and TM-3d orbitals but is basically invariable for O-2p orbitals. Along with the increasing empty Ti-3d and TM-3d orbitals, the

d

energy of ILs increases and the systems become less stable. Thus, the oxygen vacancy could influence the electronic properties, band gaps and ILs of doped specimens.

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Further, Figs. 2 and 3 indicates that the CB and VB of all doped TiO2 move down to

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the low energy region compared with pure TiO2, which enhances the oxidizability of photogenerated holes at the top of VB and reduces the reducibility of photogenerated electrons at the bottom of CB. Mulliken bond population

Pervious works have reported that the band gap value of semiconductor is related

with the hybrid strength of chemical bond, which can be confirmed by the MP values of it. To find relationship between the band gap and MP values of the doped specimens, the MP values were calculated and shown in Table 1. It is found that the MP values of Ti-O bonds in all specimens are basically invisible (about 0.38), while the MP values of TM–O bonding are different for the doped specimens, which are related with the type of TM and the oxygen vacancy. It is an approximate linear relationship between the band gap and MP values for all doped specimens except for CoTi15O1O31 and CoTi15O2O31 in Fig. 4. The deviation relationship for CoTi15O1O31

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and CoTi15O2O31 may be caused the serious lattice deformation after the oxygen vacancy being introduced into and the cobalt oxide tendency to appear in Co-doped TiO2 [42], which destroys the symmetry of system. Although the band gap value is influenced by the type of TM and the oxygen vacancy, it is decided directly by the

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TM–O bonding. Thus, we can adjust the band gap of semiconductor by changing the

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hybrid strength of TM–O bonding, such as: doping and manufacturing vacancy.

Conclusions

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In summary, formation energy, crystalline and electronic structures of TMTi15O32, TMTi15O1O31 and TMTi15O2O31 were studied by first principles calculations. It is

an

found that, the stabilities of the doped TiO2 depend on its growth conditions (Ti-rich or O-rich), and the ScTi15O32 system is the most stable due to its lowest formation

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energy (-8.66eV) under O-rich growth conditions. The band gaps of doped systems were reduced by 0.51 ~ 0.96 eV compared with pure TiO2. The impurity states located at the band gaps enhance the absorption of visible light, the oxygen vacancy result in

d

the EF move into the CB for some doped systems, which induces the Ti3+ ions and

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promote the separation of photogenerated electrons and holes. Doping and oxygen

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vacancy are effective means to changing the hybrid strength and MP value of TM–O bonding, and the band gap increases approximately linearly with the MP value of TM–O bonding.

ACKNOWLEDGEMENTS

This work was supported in part by the National Natural Science Foundation of

China (Nos. 51402274, 21401180 and 51402277). Computational resources were provided by the Jilin University.

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ip t

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ip t

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cr

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te

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te

d

M

an

us

199.

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Captions: Figure 1. (Color online) (a) Anatase TiO2 doped by TM, where red, gray, and purple

the oxygen vacancy. (b) The band structure of pure anatase TiO2.

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denotes oxygen, titanium, and TM atoms, respectively. 1st and 2nd denotes the site of

Figure 2. (Color online) The band structures of TMTi15O32, TMTi15O1O31 and

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TMTi15O2O31 doped specimens.

Figure 3. (Color online) The PDOS of TMTi15O32, TMTi15O1O31 and TMTi15O2O31

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doped specimens.

Figure 4. (Color online) The relationship between MP values and Eg of all doped

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te

d

M

an

specimens. Eg increases approximately linearly with MP values.

Page 11 of 22

Table 1. The lattice constants (a, b and c) and bond length (d) of calculated results with unit Å. Ef (Ti-rich and O-rich), Eg, EVBM and ECBM are the formation energy, band gap, the energy of VBM and CBM, respectively, with unit eV; MP values of all

TMTi15O2O31 systems, respectively.

d

7.595

7.564

7.562

7.554

7.556

7.554

7.554

7.551

O1

7.604

/

7.565

7.567

7.576

7.589

7.572

7.554

O2

7.590

/

7.562

7.574

7.574

7.565

7.549

7.535

Non

7.566

7.554

7.554

7.551

7.564

7.562

7.563

7.594

/

7.583

7.590

O2

7.614

/

7.541

7.535

Non

9.556

O

9.566

9.564

9.533

O

1

9.541

/

9.461

O

2

9.521

/

9.490

Non

2.022/ 2.135

/

1.900/ 1.933

O1

1.978/ 2.066

/

O2

1.984/ 2.014

Non

1.61

TM-O

O

1

O2 Non

7.564

7.561

7.560

7.572

7.576

9.550

9.542

9.464

9.462

9.452

9.441

9.477

9.489

9.480

9.468

9.461

9.454

9.450

1.907/ 1.966

1.886/ 1.909

1.912/ 1.913

1.880/ 1.885

1.874/ 1.892

1.865/ 1.959

1.859/ 1.946

1.870 /1.938

1.876/ 1.930

1.910/ 1.912

1. 878/ 1.996

/

1.873/ 1.952

1.880/ 1.882

1.866/ 1.890

1.878/ 1.899

1.901/ 1.939

1.887/ 2.227

/

2.57

4.41

7.44

6.33

8.02

8.82

-0.41

/

2.65

4.29

6.62

4.71

5.10

5.10

-0.21

/

2.75

4.59

7.12

5.41

5.60

5.60

/

-7.70

-5.86

-2.83

-3.94

-2.25

-1.45

O

-5.54

/

-2.48

-0.84

1.49

-0.42

-0.03

-0.03

O

2

-5.34

/

-2.38

-0.54

1.99

0.28

0.47

0.47

Non

4.16

5.42

4.02

4.01

4.22

4.23

4.27

4.67

O1

4.26

/

4.15

4.08

3.92

4.28

4.17

4.17

2

4.24

/

4.03

4.03

3.88

4.29

4.25

4.26

Non

2.09

2.63

2.15

1.81

2.17

2.13

2.20

2.48

O

1

1.98

/

2.05

2.02

2.01

2.09

1.95

1.95

O

2

2.01

/

2.10

2.07

2.05

2.05

2.07

2.05

2.07

2.79

1.87

2.20

2.05

2.10

2.07

2.19

O

1

2.28

/

2.10

2.06

1.91

2.19

2.22

2.22

O

2

2.23

/

1.93

1.96

1.83

2.24

2.18

2.21

Non

0.28

0.38

0.21

0.33

0.28

0.30

0.29

0.32

O1

0.35

/

0.30

0.28

0.23

0.32

0.27

0.33

O2

0.33

/

0.23

0.25

0.2

0.34

0.26

0.33

Non

MP

7.572

7.547

-8.66

O

Eg

Ni

1

O-rich

EVBM

Co

9.458

Ac ce p

Ef

Fe

7.583

an

7.595

1

Mn

cr

Non

Ti-rich

ECBM

Cr

M

c

V

d

b

Ti

te

a

Sc

us

Dopant

ip t

considered specimens. Non, O1 and O2 indicate the TMTi15O32, TMTi15O1O31 and

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*Highlights (for review)

1. The impurity level states are mostly decided by TM-O bonding and band gaps of doped systems were reduced by 0.51 ~ 0.96 eV compared with pure TiO2, which enhance the visible light absorption. 2. The impurity states located at the band gaps enhance the absorption of visible

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light, the oxygen vacancy result in the EF move into the CB for some doped systems, which induces the Ti3+ ions and promote the separation of

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photogenerated electrons and holes.

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3. Doping and oxygen vacancy are effective means to changing the hybrid strength and MP value of TM–O bonding, and the band gap increases approximately

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linearly with the MP value of TM–O bonding.

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*Graphical Abstract (pictogram) (for review)

The relationship between MP values and Eg of all doped specimens. Eg increases

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approximately linearly with MP values.

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Figure 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

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Figure 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

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Figure 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

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Figure 4

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

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Figure

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