First-principles study the behavior of oxygen vacancy on the surface of ZrO2 and Zr0.97M0.03O2

Accepted Manuscript First-principles study the behavior of oxygen vacancy on the surface of ZrO2 and Zr0.97M0.03O2 Hongchun Luo, Dong Tian, Chunhua Zeng, Yunchang Fu, Hua Wang PII:

S2352-2143(16)30039-9

DOI:

10.1016/j.cocom.2016.12.001

Reference:

COCOM 62

To appear in:

Computational Condensed Matter

Received Date: 14 June 2016 Revised Date:

26 November 2016

Accepted Date: 5 December 2016

Please cite this article as: H. Luo, D. Tian, C. Zeng, Y. Fu, H. Wang, First-principles study the behavior of oxygen vacancy on the surface of ZrO2 and Zr0.97M0.03O2, Computational Condensed Matter (2017), doi: 10.1016/j.cocom.2016.12.001. 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.

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First-principles study the behavior of oxygen vacancy on the surface of ZrO2 and Zr0.97M0.03O2 Hongchun Luo1, Dong Tian2,3,4 , Chunhua Zeng1,2,3∗, Yunchang Fu1 , Hua Wang2,3,4 , Faculty of Science, Kunming University of Science and Technology,

Kunming 650093, China 2

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State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization,

Kunming University of Science and Technology, Kunming 650093, Yunnan, China 3

Center of Metallurgical Energy Conservation and Emission Reduction,

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Ministry of Education, Kunming University of Science and Technology, Kunming 650093,China

Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology,

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Kunming 650093, Yunnan, China

Abstract: ZrO2 is an important functional material,the structure variation can greatly influence on its natural properties. As a catalytic material, the existence of oxygen vacancy can enhance the catalytic activity. As optical materials, the existence of oxygen vacancy has an important effect on optical performance. Due to the important role of oxygen vacancy, this paper has studied the oxygen vacancy systematically by first principles calculation. This work demonstrates

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that the oxygen vacancy is easier to appear on the ZrO2 (111) surface. The calculated value of Evac for the undoped ZrO2 system is -2.33 eV per vacancy, and the O vacancy formation energies are -2.54 eV and -2.91 eV for the Ti and Pr-doped systems, respectively. This shows that it is easier to reduce in Ti and Pr-doped systems than pure ZrO2 , it suggests that it is possible to increase the oxygen storage capacity by by dissolving Ti and Pr into ZrO2 .

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1 Introduction

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Key words: ZrO2 , First Principles Calculation, Doping,Oxygen Vacancy

ZrO2 material has high hardness[1], high strength[2], high toughness[3], high wear resistance and chemical corrosion resistance [4]. ZrO2 has been widely used in many fields, such as ceramics[5], refractory materials[6] and optical fiber communication[7]. As a ceramic material, Catalyst support[8] and photosensitive material[9] has been widely studied. All ceramic teeth of zrO2 are treated as the root of the crown. Crown is a layer of porcelain transparent support framework, after it is on the outside of the crown on the surface Roast a layer of similar to the color of natural teeth, it is a kind of high-tech dental method which is carried out in recent years[10]. Because the oxidation of zirconium is the only one with acid, alkaline, oxidation and reduction of metal oxides, and it is a p-type semiconductor, which is easy to ∗ e-mail:

[email protected]

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ACCEPTED MANUSCRIPT produce oxygen hole. It also used as a catalyst support, which can produce the interaction with the active component, therefore, it has more excellent properties compared with other catalysts support[11]. Recently, some studies have found that Ce doping ZrO2 is expected to become the third generation of visible light sensitive material[12]. Because its wide application, the basic properties of ZrO2 have been well studied[13, 14, 15]. Shoji Kaneko et al. via experimental study found that P+ or B+ ions into yttria-stabilized ZrO2 (YSZ) lead the change of the oxygen vacancies concentration,

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the concentration of oxygen vacancies has great influence on the photoluminescence[16]. There was an experimental study of oxygen vacancies improve catalytic activity for CO oxidation on ZrO2 supported copper oxide catalyst[17]. Oxygen vacancies have great influence on the properties of ZrO2 , so we are very interested in oxygen vacancies. An increase in the number of oxygen vacancies in CeO2 has been found to enhance its oxygen storage capacity as well as its

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catalytic activity[18]. Wei Zhao et al. also found that oxygen vacancies are omnipresent on TiO2 surface under ambient conditions, the increases in the number of oxygen vacancies which can react with O2 to form a superoxide species. Oxygen

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vacancies can improve catalytic activity of the TiO2 [19]. The oxidation of carbon monoxide has caused a lot of attention due to its wide applications for indoor air cleaning, CO gas sensors, CO2 lasers, and automotive exhaust treatment[20, 21]. Particularly, it is found that Pr doped CeO2 has a high concentration of oxygen vacancies[22]. The existence of oxygen vacancies essentially change its catalytic activity, optical and electrical properties [23, 24]. Defects control and engineering applications play a fundamental role in catalysis, photocatalysis[25, 26], information technology[27], gas sensors[28] and smart windows[29]. Because of the importance of oxygen vacancies, a lot of work has been contributed

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to research oxygen vacancies in cerium oxide [20, 30, 31, 32, 33],and Zirconia[16, 34, 35]. Zirconium oxide and cerium oxide are used as important catalyst carrier, oxygen vacancies have great influence on their catalytic properties. There are a lot of researchs on the oxygen vacancies in the cerium oxide. However, the formation energy of oxygen vacancies in ZrO2 is still poor. At low pressure the zirconia presents monoclinic, tetragonal and cubic phases with the increase of

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5 (space group temperature. From 0 to 1180 0 C, the stable phase is baddeleyite[36], which has monoclinic symmetry C2h

P21 /C) with 4 formula units per crystalline cell and only 4 symmetry operations in the point group as well as sevenfold

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cation coordination. From 1180 to 2370 0 C, the stable phase has tetragonal symmetry D15 4h (P42 /nmc)[37] with 2 formula units and 16 symmetry operations as well as eightfold cation coordination. The monoclinic phase has a larger volume than the tetragonal phase and a very anisotropic thermal expansion. From 2370 0 C to the melting point 2600 0 C, the stable phase is fluorite (CaF2 ) phase, which has cubic symmetry O5h (Fm3m)[38] with 1 formula unit and 48 symmetry operations as well as eightfold cation coordination. We choose cubic zirconia as the research object in this article. The published works on nonmetal Y doped zirconia can improve the activation of sulfuric acid[39], N doped TiO2 /ZrO2 can increase the photocatalysis ability[40], B, C, N doped CeO2 exhibits very high photocatalytic activity[41]. Meanwhile, metal particles La , Mn, Fe doped zirconia can increase the zirconium oxide catalytic activity[42]. Pr doped Ce-Zr oxides

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ACCEPTED MANUSCRIPT can improve their performance as a promoter for three way catalyst[43]. Ti doping actually increased trap number such as oxygen vacancy[44]. Non-metal and metal doping has important influence on the catalyst performance of zirconia , so , we did research for non-metal and metal doping in the article. Our goal is to systematically understand the effect of oxygen vacancies on the structure of ZrO2 by using density functional theory (DFT), and try to find out doping effect on the formation energy of oxygen vacancies. The follow problems need to be further investigated: (1) the preferred position

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of oxygen vacancies on different surfaces, (2) the effect of oxygen vacancies on the electronic structure and chemical properties of ZrO2 , (3) how to change the oxygen vacancy formation energy of ZrO2 by doping.

2 Calculation methods

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All calculations are carried out by using the generalized gradient approximation (GGA)[45] and pseudopotential approximations[46] to Kohn-Sham density-functional theory (DFT) in the Cambridge Serial Total Energy Package (CASTEP)

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program.[47]. We expand the one-electron wave functions in a plane-wave that the energy cutoff of is 300 eV. All the calculations were carried out by using the Brillouim zone sampled with (4 × 4 × 4)[48] and (4 × 4 × 1) Monkhorst-Pack mesh k-points grid for bulk and surface calculations, together with a Gaussian smearing broadening of 0.01eV. The electron-ionic core interaction is represented by the projector augmented wave (PAW) potentials[49], which are more accurate than the ultra-soft pseudopotentials. To treat electron exchange and correlation, we chose the PerdewBurke-Ernzerhof (PBE) formulation of the generalized gradient approximation and take into account the on-site Coulomb

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repulsive interaction (GGA+U, U+4eV for Zr)[50]. The force on each atom is less than 0.05eV /Å. The Zr 4s2 4p6 4d2 5s2 and O 2s2 2p4 electrons are treated as valence electrons. In the calculation of surface energy, in order to ensure the accuracy of the calculation results, we considered the effect of atomic layer thickness on the surface energy and vacuum gap on the total energy, respectively. Considering the large amount of calculation, we only consider to test

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the convergence of the surface energy by ZrO2(111) surface. We calculated the surface energy of ZrO2(111) surface by using different atomics layers of three layers, four layers,five layers, six layers, respectively. The sur-

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face energy of ZrO2 (111) surface is 0.020J/m2 for three layers atomics, 0.020J/m2 for four layers atomics, 0.019J/m2 for five layers atomics, 0.018J/m2 for six layers atomics. We can find that the surface energy along with the change of the layer number is not big, but calculations will need more time after adding a layer. In order to reduce the amount of calculation, we adopt three layers of atoms in calculations. We calculated the total energy of ZrO2 (111) at different vacuum layer thickness, and total energy is -25353.632 eV for 6 Å, -25354.682 eV for 8 Å, -25356.409 eV for 10 Å, -25356.406 eV for 12 Å. We can see that the change of the total energy is very small when the vacuum layer thickness is 10 Å. So, we think that adopting a vacuum layer thickness of 10 Å to calculate surface energy is reasonable. In order to find out in which the oxygen vacancy is easier to form for the surface of ZrO2 , we

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ACCEPTED MANUSCRIPT set up a structure model that contains 36 atomic, among them, the bottom atoms are fixed, other atoms are relaxed, as shown in Fig. 1. For the doping system, we adopt a 2 × 2 × 2 supercell[51, 52], contain 96 atoms, we use the

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other atom replace a zirconium atom in the middle of the crystal (Zr0.97 M0.03 O2 ), as shown in Fig. 1. (c) and (d).

Fig.1 The optimized crystal structure of ZrO2 : (a) Unit Cell; (b) Primitive Cell; (c) and (d) is 2 × 2 × 2 supercell, an atom in the center instead by doping atoms (O and Zr atoms are represented by red and light blue balls, respectively).

3 Results and discussions

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3.1 The formation energy of oxygen vacancy and surfaces energy of ZrO2 The calculated results for ZrO2 primitive cell in Table 1 show that the above method gives a good description for ZrO2 primitive pell. Our calculated equilibrium lattice parameter and ZrO2 primitive cell volume are 3.601913Å and

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33.0431Å3, respectively, in agreement with the experimental values of lattice constant 3.6012Å[53] and cell volume 32.723 Å3 [54]. Here are some earlier theoretical calculations which have given values of lattice constant 3.55381Å. Our

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calculated density of states in Fig. 2 shows clearly that ZrO2 is an insulator.The highest occupied valence band exhibits significant Zr 4d character mixing with some contribution from Zr 4p and O 2p states, while the conduction band situated just above the Fermi level is mainly due to Zr 4d states mixing with Zr 4p and O 2p states. The valence band of O 2p is from 0 to -7ev,

The valence band of zirconia is from 0 to -5.9eV in the reference[1]. Our calculated value of valence band width 0 to -7 eV is larger than that of 0 to -5.9eV, but it is in agreement with the earlier DFT- LDA calculations 0 to -6.1 eV and GW approximation 0 to -6.5 eV [60] and LCAO calculations 0 to 5.9 eV [61]. All the results are shown in Table 2.

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1 (E slab − Ebulk ). 2s

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where E slab is the total energy of the slab, Ebulk is the energy of the bulk unit cell containing the same number of atoms in

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TDOS of ZrO

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the slab, and S is the surface area. The lower the surface energy, the more stable the surface.

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Fig.2 Total and partial density of states(DOS) at calculated equilibrium configuration of ZrO2 . The Fermi level is set at zero energy.

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Table 1: Calculated properties of ZrO2 Primitive Cell, optimized lattice constants a, b, c, cell volume, band gap and the bond lengths of Zr-O. Parameters present work DFT Experiment Bond Length(Zr-O)(Å) 2.22171 2.1763a,2.19537b 2.205d 3 a Cell Volume(Å ) 33.0431 31.8803 32.723e f c band gap(ev) 3.46 3.51 ,3.318 4.2g ,5.7h a a=b=c (Å) 3.601913 3.55381 3.6012e a From Ref.[53] b From Ref.[55] c From Ref. [1] d From Ref.[56] e From Ref.[54] f From Ref.[57] g From Ref.[58] h From Ref.[59] As shown in Fig. 3 the surfaces (111), (110) and (100) of ZrO2 are modeled as periodically (in z). A vacuum layer of 10Å was introduced to separate the films for the above surface. To study the oxygen vacancy of the relaxed surfaces, we also calculated the differences between the consecutive inter planar distances in the relaxed slabs and those in bulk ZrO2 for (111), (110) and (100), respectively. The results of surface energy and the vacancy formation energy are shown 5

ACCEPTED MANUSCRIPT in the Table 3. Our current GGA+U calculations [for (111), (110) and (100), respectively] give the following results. The (111) surface has the smallest surface energy among the low-indes surfaces studied, 0.02J/m2. The (100) surface has the highest surface energy of 0.16J/m2. The (110) surface is secondary stability, with a surface energy of 0.14J/m2 . Our results are very close to the earlier theoretical calculation, 0.1193J/m2, 0.3058J/m2, and 0.2288J/m2 for (111), (110) and (100) surface[62]. Our DFT investigations of the surfaces energy of stoichiometric ZrO2 found that the sequence of

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stability of the studied surfaces is (111) > (110) > (100). The results are in good agreement with the earlier theoretical

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calculation [62].

Fig.3 Slab models used for the studies of reduced sufaces: (a), (b), (c) is the (111) of ZrO2 , (d), (e), (f) is the (110) of ZrO2 , (g), (h), (i) is the (100) of ZrO2 , (b), (e), (h) represent (s-1) surface layer have a oxygen vacancy, (c), (f), (i) represent (s-2) subsurface layer have a oxygen vacancy

The oxygen vacancy formation energy, Evac , is defined as: 1 Evac = E slab − EO2 − E slabv . 2

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where E slab is the total energy of the (undoped or doped) zirconia slab without a vacancy, E slabv is the total energy of the (undoped or doped) zirconia slab with a vacancy and E(O2 ) is the calculated total energy for an O2 molecule, respectively. A negative value of Evac means that the vacancy is easier to creat.

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Table 2: Main peak scope of the orbit, all units are (eV), VB is valence band, CB is conduction band. Main peak scope of the orbit present work DFT LCAO O-2p(VB) 0 to -7 (o to -7)a (0 to -7)b O-2s(VB) -2.5 to -7 Zr-4d(CB) 3.4 to 5.2 (3.5-5.5)a Zr-4p(CB) 6.5 to 8 Zr-5s(VB) -2.1 to -6.5 Valence bandwidth 6.7 6.5a a From Ref.[60] b From Ref.[61] The total DOS of the unreduced and reduced surfaces for ZrO2 (111), (110) and (100) surfaces are shown in Fig. 4.

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Fig.4 Density of state (DOS) of ZrO2, (S 1 ) surface layer have a oxygen vacancy, (S 2 ) subsurface layer have a oxygen vacancy, (a), (b) and (c) is the DOS of (110), (100) and (111) unreduced surfaces, (d), (e) and (f) is the DOS of (110), (100) and (111) surface layer reduced surfaces, (g), (h) and (i) is the DOS of (110), (100) and (111) subsurface layer reduced surfaces, The Fermi level is set at zero energy.

On the ZrO2 (111) surface, the formation energy of an O vacancy on the topmost O-atomic layer (first layer) is -3.41eV, in the second O-atomic layer Evac =-3.13eV, our calculations show that it is easier to create an O vacancy in the first layer. On the ZrO2 (110) surface, we obtain a lower Evac value in the first layer compared with the subsurface layer, -1.57eV and -0.43eV, respectively. On the ZrO2 (100) surface, we obtain a Evac value 6.7eV in the first layer, 5.3eV in the subsurface layer. A positive value for Evac means that energy is difficult to create an O vacancy, the higher the value, the more difficult to create a vacancy. By comparing the formation energies of an O vacancy on the (111), (110) and (100) surfaces, it can be concluded that formation of an O vacancy seems easier on the ZrO2 (111) surface than on other surfaces. And the result are summarised in the Table 3. 7

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Table 3: The surface energy of (111),(110),(100) of cZrO2, (s-1) surface layer,(s-2) subsurface layer, EVac is oxygen vacancy formation energy. Surface energy(J/m2) EVac (eV) T 0.02,0.1193,a 111 s-1 -3.41 s-2 -3.12 T 0.14,0.2288a 110 s-1 -1.57 s-2 -0.43 T 0.16,0.3058a,0.116b 100 s-1 6.7 s-2 5.3 a From Ref.[62] b From Ref.[63]

1 Evac = E(Cell) − EO2 − E(Cellvac ) 2

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The oxygen vacancy formation energy, Evac , is defined as:

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3.2 Structure and oxygen vacancy formation energy of (Zr0.97 M0.03 O2 )

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where E(cell) is the total energy of the (undoped or doped) zirconia supercell without a vacancy, E(cellvac ) is the total energy of the (undoped or doped) zirconia supercell with a vacancy and E(O2 ) is the calculated total energy for an O2 molecule, respectively.

The formation energy of an O vacancy within the bulk was studied by using a cubic 96-atom unit cell(Zr0.97 M0.03 O2 )

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with one of the O atoms removed. We use an atom to replace a centre atom of Zr0.97 M0.03 O2 (M=Al, Cu, Ti, Co, Ca, Sr, Pr, P, Y, N, Be, B ). Instead of the local atomic, Atom caused a structure disturbanc, we can clearly see that the changes of neighboring atoms bond length and structure from Fig. 5. Table 4 gives the values of bond length, band gap Egap and

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oxygen vacancy formation energy Evac . By comparing the Table 4 with the Table 5, we found that the nearby four oxygen atoms are equivalent, they have the same charge, and the distance to central atom is still same after doping. When no

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doping, the Zr-O is 2.222Å, O atom total charge is -0.75 e; Al doping is 2.167Å, charge is -0.77 e; Ti doping is 2.278Å, charge is -0.68 e; Co doping is 2.246Å, charge is -0.68 e; Ca doping is 2.297Å, charge is -0.71 e; Sr doping is 2.323Å, charge is -0.70 e; Pr doping is 2.277Å, charge is -0.71 e; P doping is 2.187Å, charge is -0.74 e; Y doping is 2.272Å, charge is -0.72 e; N doping is 2.175Å, charge is -0.73 e; Be doping is 2.177Å, charge is -0.69 e; B doping is 2.141Å, charge is -0.67. By calculating values we found that the local structure of doping atoms don’t cause dramatic changes. The bond length of O2 -M, O3 -M and O4 -M causes great changes when the oxygen vacancy exists, and O atom total charge of O2 , O3 and O4 are different. The shorter bond lengths are indicative of a stronger bond, this suggest that such interstitial locations of oxygen ions are stable, while the longer bond length suggests that the oxygen ions are more unstable. These results suggests that the presence of oxygen vacancy will cause nearby atoms violent disturbance. Bond length of O2 -M 8

ACCEPTED MANUSCRIPT and O3 -M become longer, it means that the oxygen ions O2 and O3 - are more unstable. But O4 becomes more stable, all

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of the bond length become shorter when oxygen vacancy exists.

Fig.5 The optimized structure of unreduced (Zr0.97 M0.03 O2 ) and reduced (Zr0.97 M0.03 O2 ).

The four oxygen atoms near doping position are equivalent when no there is oxygen vacancy. We can see four oxygen atoms with the same charge from the table 5 of our article. For example, four oxygen atoms of the charge is -0.75 e in perfect ZrO2 crystal cell. Four oxygen atoms of charge has a little change after doping, but they are always equal. In the doped system, the charge of oxygen is -0.68 e for Cu doping, -0.72 e for Ti doping, -0.68 e for Co doping, -0.71 e for Ca doping, -0.70 e for Sr doping ,-0.71 e for Pr doping, -0.74 e for P doping, -0.72 e for Y doping, -0.73 e for B doing, -0.69 e for Be doping and -0.67 e for B doping. The increasing of the charge of four oxygen atoms in the doping system 9

ACCEPTED MANUSCRIPT suggest that some charge of the four oxygen atoms ran to doping atomic evenly. Only Al doping the charge of oxygen atoms change to -0.77 e, which means that oxygen atoms got charge from AL atom. For the reduction system, the lack of oxygen atoms leave the charge to nearby atoms, which makes the neighboring atoms reduced. For example, the charge of O2 changes from -0.77 e to -0.90 e for AL doping, changes from -0.68 e to -0.72 e for Cu doping, changes from -0.68 e to -0.72 e for Co doping, changes from -0.71 e to -0.77 e for Ca doping, changes from -0.70 e to -0.76 e for Sr doping,

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changes from -0.71 e to -0.73 e for Pr doping, changes from -0.72 e to -0.75 e for Y doping, changes from -0.69 e to -0.72 e for Be doping, changes from -0.67 e to -0.71 e for B doping. This suggests that the oxygen got the charge in these systems. The charge of alternative location atoms changes from 1.51 e to 1.30 e for undoped system, changes from 0.71 e to 0.61 e for Cu doping, changes from 1.29 e to 0.97 e for Ti doping, changes from 1.36 e to 1.21 e for Pr doping, changes

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from 1.35 e to 1.03 e for P doping, changes from 0.86 e to 0.71 e for Be doping, which suggests that alternative location atoms lost charge.

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The Oxygen vacancy formation energy was -2.33eV for the undopded system, 2.07eV for Al doping, 1.29 eV for Cu doping, -2.45eV for Ti doping, 0.37eV for Co doping, 2.35eV for Ca doping, 2.82eV for Sr doping, -2.91eV for Pr

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Table 4: The optimized bond length of unreduced (Zr0.97 M0.03 O2 ) and reduced (Zr0.97 M0.03 O2 ), Evac is oxygen vacancy formation energy, Egap is the band gap of no oxygen vacancy, bond length unit is (Å). unreduced reduced O1 -M O2 -M O3 -M O4 -M Egap (eV) O2 -M O3 -M O4 -M Evac (eV) undoped 2.222 2.222 2.222 2.222 3.46 2.298 2.295 2.066 -2.33 undoped 5.7a AL 2.167 2.167 2.167 2.167 2.89 3.081 3.081 1.743 2.07 Cu 2.251 2.021 2.279 2.250 2.87 2.843 1.950 1.676 1.29 Cu 3.3a Ti 2.278 2.278 2.278 2.278 1.82 2.400 2.399 2.112 -2.54 Co 2.246 2.246 2.246 2.246 2.73 2.275 2.293 1.954 0.37 Co 3.7a Ca 2.297 2.297 2.297 2.297 3.16 2.342 2.340 2.255 2.352 Sr 2.323 2.323 2.323 2.323 3.41 2.375 2.375 2.301 2.815 Pr 2.277 2.278 2.278 2.277 1.41 2.337 2.352 2.212 -2.91 P 2.187 2.187 2.187 2.187 2.48 2.828 2.797 1.662 2.76 Y 2.272 2.272 2.272 2.272 3.19 2.305 2.307 2.236 1.07 Y 2.281b 2.281b 2.281b 2.281b N 2.175 2.175 2.175 2.175 2.98 2.919 2.944 2.515 4.54 Be 2.177 2.177 2.177 2.177 3.42 3.142 3.098 1.557 4.54 B 2.141 2.141 2.141 2.141 2.89 3.317 3.279 1.494 5.01 b From Ref.[64] b From Ref.[65]

doping, 2.76eV for p doping, 2.07eV for Y doping, 4.54eV for N doping, 4.54eV for Be doping, 5.01eV for Sr doping. In all the doped system, only the Ti and Pr atoms doping can improve the formation of oxygen vacancies. By comparing the oxygen vacancy formation, we can find that the oxygen storage capacity sequence is Zr0.97 Ti0.03 O2 >Zr0.97 Pr0.03 O2 >Zr O2 . This may explain the experimental observation that the combination of Pr with ZrO2 makes the TWC highly active of CO 10

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Table 5: The atomic total charge distribution of unreduced (Zr0.97 M0.03 O2 ) and reduced (Zr0.97 M0.03 O2 ), M represents the dopant atoms, all of the unit is e. unreduced reduced O1 O2 O3 O4 M O2 O3 O4 M undoped -0.75 -0.75 -0.75 -0.75 1.51 -0.75 -0.75 -0.75 1.30 AL -0.77 -0.77 -0.77 -0.77 1.57 -0.90 -0.90 -0.78 1.66 Cu -0.68 -0.68 -0.68 -0.68 0.71 -0.72 -0.75 -0.70 0.61 Ti -0.72 -0.72 -0.72 -0.72 1.23 -0.71 -0.71 -0.70 0.97 Co -0.68 -0.68 -0.68 -0.68 0.89 -0.72 -0.74 -0.70 0.67 Ca -0.71 -0.71 -0.71 -0.71 1.25 -0.77 -0.77 -0.78 1.29 Sr -0.70 -0.70 -0.70 -0.70 1.14 -0.76 -0.76 -0.76 1.26 Pr -0.71 -0.71 -0.71 -0.71 1.36 -0.73 -0.72 -0.71 1.21 P -0.74 -0.74 -0.74 -0.74 1.35 -0.70 -0.70 -0.70 1.03 Y -0.72 -0.72 -0.72 -0.72 1.00 -0.75 -0.75 -0.72 1.05 N -0.73 -0.73 -0.73 -0.73 0.02 -0.70 -0.70 -0.68 0.04 Be -0.69 -0.69 -0.69 -0.69 0.86 -0.72 -0.72 -0.76 0.71 B -0.67 -0.67 -0.67 -0.67 0.68 -0.71 -0.71 -0.71 0.74 hydrogenation to methanol[66].

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The formation of an oxygen vacancies left two electrons in the system. A few atoms around oxygen vacancy will share the two electrons, and additional electronic will reduce from T i4+ to T i3+ . Ti atoms have a obvious characteristics, the atomic total charge from 1.23 e to 0.97 e, when oxygen vacancy present, which shows that the Ti atoms are restored. The total charge of Zr from 1.51e to 1.30e, Zr4+ to Zr3+ , Pr from 1.36 e to 1.21 e, Pr only has Pr3+ , it could not be restored, extra two electrons in system will increase the reducing power. As is shown above, the differences in the electronic

these mixed oxides. 250 (a) 200 150

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Fig.6 The TDOS (total density of states) and PDOS (partial density of states) for the structure of (a) unreduced Zr O2 and (b) reduced Zr O2 , respectively.

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Fig.7 The TDOS (total density of states) and PDOS (partial density of states) for the structure of (a) unreduced Zr0.97 Ti0.03 O2 and (b) reduced Zr0.97 Ti0.03 O2 , respectively.

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Fig.8 Difference charge density , in which the yellow isosurfaces represent gain charge, blue isosurfaces represent the loss of charge, (a) Zr O2 , (b) Zr0.97 Ti0.03 O2 , (c) Zr0.97 Pr0.03 O2 , The isosurface value used is 0.01 eÅ−3 .

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Fig.9 The TDOS (total density of states) and PDOS (partial density of states) for the structure of (a) unreduced Zr0.97 Pr0.03 O2 and (b) reduced Zr0.97 Pr0.03 O2 , respectively.

orbital hybridization. From Fig .9 (a), we can clearly see that Pr atoms doping caused the density of state shifts to the left, This is the result of orbital hybridization, Pr-d with Zr-4d and O-2p, Pr-f with Zr-4d.

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ACCEPTED MANUSCRIPT To further elucidate the nature of the above phenomenon, we calculate the total density of states and partial density of states of the optimized Zr O2 , Zr0.97 Ti0.03 O2 and Zr0.97 Pr0.03 O2 unreduced and reduced structure. In the Fig .6, it is shown the calculated TDOS and PDOS for unreduced and reduced Zr O2 . In Fig .6 (a), we can see that the position on the top of valence band in the zero energy for unreduced Zr O2 , -2.4eV for reduced Zr O2 , in accordance with the Fig .4 (b), which shows that the energy is decreased after reduction. Fig .6 shows that the TDOS of valence band mainly comes from the

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contribution of O-2p, and the O-2p band of reduced Zr O2 shifted toward lower energy, compared with the unreduced Zr O2 . The Fig .7 and Fig .9 have similar results, the other articles also have similar results[52], which suggests that it is a common phenomenon that the presence of the oxygen vacancy reduces the energy of the system. Fig .7 shows the calculated TDOS and PDOS for unreduced and reduced Zr0.97 Ti0.03 O2 . The highest occupied valence band contribution

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from Ti-d, Zr-4d and O-2p, the conduction band exhibits significant Ti-d mixing with Zr-4d, this suggests that Ti-doping can caused

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From Fig.10 (a), the energy band structure of Zr O2 has four evident band gaps, the first band gap is the gap between the conduction band (from 3.4 to 5 eV) and the valence band (from -6.1 to 0.06 eV), the band gap is 3.46 eV, in agreement with the theoretical calculations values of 3.15 eV[57] and 3.318 eV[1]. The second band gap is the gap between the conduction band (from 6.25 to 19.95 eV) and the conduction band (from 3.4 to 5 eV). The third band gap is the gap between the valence band (from -6.1 to 0.06 eV) and the valence band (from -16.1 to -17.8 eV). The fourth band gap is the gap between the valence band (from -16.1 to -17.8 eV) and the valence band (from -26.5 to -27.3 eV). The values of these

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band gaps are all in excellent agreement with the previous calculations[1]. From Fig.10 (b), the enerty band structure of Zr0.97 Ti0.03 O2 has significant changes compared with undopd Zr O2 . The values of the first band gap energy becomes narrower, because the Ti atom doping introduced the impurity band gap, the band gap of Zr0.97 Ti0.03 O2 is 1.82 eV. From Fig.10 (c), the energy band structure of Zr0.97 Pr0.03 O2 , we can see clearly that four band gaps becomes narrower, the band

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4 Conclusion

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gap is 1.41 eV. This suggests that Pr doping will cause the great changes in material charge nature.

In this paper, the atomic and electronic structure of stoichmetric and O-deficent (111), (110), and (100) surfaces of Zr O2 were studied by using projector-augmented-wave method based on density-functional theory within generalized gradient approximation. According to surface energy calculation, we found that the (111) surface is the most stable surface in the considered surfaces, Which is in good agreement with experimental results[62]. The study of formation energy of an oxygen vacancy on the (111), (110) and (100) shows that the oxygen vacancy is easier to appear on the Zr O2 (111) surface (first layer). Atoms doping has a huge impact on the structure, electronic distribution and reduce nature of Zr O2 . It is found that M dopants give rise to large perturbation of the Zr O2 structure and induce MIGS at the Fermi level suitable

14

ACCEPTED MANUSCRIPT for accommodating extra electrons left on oxygen vacancy formation, and therefore reduce the reducing power of Zr O2 . The O anions in the vicinity of a M doping center display larger charge than O anions in undoped Zr O2 . Doping can change the band gap of Zr O2 , the band gap become narrow after doping, which suggests that the electron of impurities fill the empty band. Ti and Pr ion doping can promote the formation of oxygen vacancies and increase catalytic activity of Zr O2 . The main reason is that the outer electrons have a special structure, structure distortion is also one of the most

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important factor. Ti and Pr ion doping have similar bond length of 2.278Å and 2.278Å. It is also found that Ti and Pr display differences with dopants in Zr O2 , Pr doping has a lower band gap 1.41 eV than Ti doping 1.82 eV, The O anions in the vicinity of a Pr doping center display larger charge than O anions in Ti doping. The O anion relaxation in the vicinity of the Pr dopant is higher than that in the vicinity of the Ti dopant and Pr has a greater impact in lowering the reduction

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energy of Zr O2 . Acknowledgments

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This work was supported by the National Natural Science Foundation of China (Grant No. 11665014), the introduction of talent capital group fund project of Kunming University of Science and Technology(under KKZ3201407030), the Candidate Talents Training Fund of Yunnan Province (Project Nos. 2015HB025).

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