Adsorption of water on the KNTN (0 0 1) surface: A density functional theory study

Applied Surface Science 298 (2014) 102–108

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Adsorption of water on the KNTN (0 0 1) surface: A density functional theory study Wenhan Wang a , Yanqing Shen a,b,∗ , Xiaoou Wang a , Zhongxiang Zhou a , Weidong Fei b a b

Department of Physics, Harbin Institute of Technology, Harbin 150001, PR China School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China

a r t i c l e

i n f o

Article history: Received 22 November 2013 Received in revised form 21 January 2014 Accepted 21 January 2014 Available online 1 February 2014 Keywords: First principle Perovskites Water adsorption Bandgap

a b s t r a c t We present a density functional theory study of water adsorption on K1−y Nay Ta1−x Nbx O3 (KNTN) surface terminations. The adsorption configuration and energy are determined and the bond formation between water molecule and KNTN surface are investigated by analysis of difference electron density and partial density of states. Our calculations reveal that the energetically favorable configurations for water monomer adsorption is at the K–Na bridge site on the K(Na)O-termination and the Nb top site on the Ta(Nb)O2 -termination. With the coverage increasing, the water–surface interaction per water molecule decreases because of the formation of interwater hydrogen bonds, whereas the surface geometry becomes more roughness. The variation of bandgap for water adsorbed KNTN surface is also studied. We find that the interaction between water and surface would lead to a bandgap increase of KNTN surface, which is correlated to the electrons density redistribution. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The perovskites are promising for many device applications both in bulk and thin-film form owing to their various interesting properties. Potassium sodium tantalate niobate (K1−y Nay Ta1−x Nbx O3 , KNTN) is a very outstanding lead-free material with perovskite structure. As the A-site substituent for potassium tantalate niobate (KTN), which is intensively studied for many years [1–7], it shows an extremely large Kerr effect and large electrostrictive effect near its phase-transition temperature [8–10]. Because of its unusual piezoelectric, electro-optic, electromechanical, and photorefractive properties, KNTN crystals have been a promising material for electro-optic modulators and electroholography applications [11,12]. For all these applications, the surface structure and the associated surface electronic and physics properties are of primary importance [13]. Investigation of interactions between the considered surface and adsorbates is of fundamental interest for surface research. In recent years, molecular adsorption studies have attracted much attention. Water molecules adsorption would influence the performance of the fine devices. The functionality of devices operating may even depend on the relative humidity in

∗ Corresponding author at: Harbin Institute of Technology, Department of Physics, and School of Materials Science and Engineering, Xidazhi Street No. 92, Harbin, China. Tel.: +86 45186414141; fax: +86 45186414141. E-mail address: [email protected] (Y. Shen). 0169-4332/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2014.01.132

particular environments [14]. Therefore, it is not surprising that the interaction between water molecules and surfaces of various ABO3 perovskite structures has been studied extensively in theoretical and experimental aspects. For example, Kato et al. investigated the reactivity of H2 O molecules on atomically flat SrTiO3 (0 0 1) by observing the microscopic friction contrasts [15]. Using the transition from mirror electron microscopy to low energy electron microscopy, the surface potential contrast between oppositely polarized ferroelectric domains of a BaTiO3 (0 0 1) single crystal under ultraviolet illumination before and after the dissociative adsorption of H2 O is measured [16]. On the theoretical side, H2 O adsorption on cubic SrTiO3 , SrZrO3 , and SrHfO3 perovskites are performed in a single-slab model framework by ab initio calculations [17]. Recently, by means of hybrid Hartree–Fock DFT, Bandura et al. study the atomic structure, preferred sites and adsorption energies for H2 O adsorption at different terminations of the cubic phase of BaHfO3 and BaZrO3 [18]. We early reported the physical properties of KNTN surfaces using the first-principles calculations [19]. However, there is no first-principle study has been published yet about water molecules adsorption on the KNTN surfaces. In this paper, we report the coverage-dependent adsorption of water at the KNTN (0 0 1) surface by means of atomistic simulations based on the density functional theory (DFT) and pseudopotential method. Adsorption energy, position, and configuration are determined. The adsorbing mechanisms of water on these surfaces are also discussed with examination of the partial density of states and the difference electron density. We anticipate that results from the study of water adsorption on the KNTN (0 0 1) surface will allow us

W. Wang et al. / Applied Surface Science 298 (2014) 102–108

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to understand in detail the nature variation of the KNTN surfaces in humid environment. 2. Computational model and method All calculations were carried out by periodic density functional theory (DFT) with the plane–wave pseudopotential method, as implemented in the CASTEP [20]. The exchange-correlation energy and potential are described by Wu–Cohen generalized gradient approximation (GGA-WC) [21], which yields more accurate lattice constants than the most popular Perdew–Burke–Ernzerhof (PBE) GGA. The electron–ion interactions are described by ultrasoft pseudopotential (USP) and norm-conserving pseudo-potentials (NCP) method. The KNTN crystal is cubic and has a Pm3m space group at room temperature and crystal lattice parameters of a = b = c = 3.981 A˚ [9]. XRD results indicate that the crystal is paraelectric at room temperature, demonstrating that its specific Curie temperature is less than room temperature [19]. We choose cubic K0.5 Na0.5 Ta0.5 Nb0.5 O3 (0 0 1) surface as an example suggested by previous studies [19]. The KNTN (0 0 1) surface was modeled by slab of four atomic layers in the z-direction, containing a total of 40 atm. For 3D periodic boundary conditions, the surfaces comprising the atomic layers were separated by a vacuum space equivalent of 10 A˚ in the direction perpendicular at the built surface. KNTN (0 0 1) surface has two termination surfaces, namely K(Na)O-terminated and Ta(Nb)O2 -terminated. We name them for AO-terminated and BO2 -terminated, respectively. In the geometric optimization, all layers are allowed to be relaxed. The dipole correction has been used to correct the artificial forces generated by the slab images. To get converged results, a plane–wave kinetic energy cutoff of 410 eV was used for geometric optimization, and a higher cutoff of 750 eV was used for energy and electronic structure calculation. A 2 × 2 × 1 k-point grid was chosen to sample the Brillouin zone in the self-consistent calculations and 3 × 3 × 2 k-point grid was used for post-processing calculations of the partial density of states (PDOS). The valence electron configurations are K (3s2 3p6 4s1 ), Na (2s2 2p6 3s1 ), Ta (5d3 6s2 ), Nb (4s2 4p6 4d4 5s1 ), O (2s2 2p4 ) and H (1s1 ). In the self-consistent calculation, the convergence threshold for energy was set to 5 × 10−7 eV. Geometry optimization was carried out using the Broyden Fletcher Goldfarb Shannon (BFGS) scheme. All atomic coordinates were fully relaxed until the absolute value of force acting on each atom is less than 0.01 eV/Å. The adsorption energy is a criterion to determine the stability of the adsorption. It is calculated by the following formula:

Fig. 1. Typical adsorption sites on KNTN (0 0 1) surface, including K top (a), O top (b), Na top (c), K–Na bridge (d), Na–O bridge (e), K–O bridge (f) and K–K bridge (g) on AO-termination, and Ta top (h), Nb top (i), O top (j) and hollow (k) on BO-termination.

Eads =



1 E n slab

 nH O  2 KNTN

− Eslab (KNTN) − nEgas (H2 O)

 (1)

where n is the number of H2 O molecules adsorbed. Eslab (nH2 O/KNTN) is the total energy of the adsorption system in the equilibrium state, Eslab (KNTN) and Egas (H2 O) are the energies of KNTN substrate and a gas-phase H2 O molecule, respectively. With this definition, a negative Eads corresponds to a stable adsorbate–substrate system. 3. Results and discussion 3.1. Isolated water molecule adsorption on the KNTN (0 0 1) surface We start the discussion with a single H2 O molecule on KNTN (0 0 1) per unit cell. To obtain the most stable adsorption configuration, the high symmetry sites of the KNTN (0 0 1) surface were chosen as the initial positions for the water molecule to adsorb, as shown in Fig. 1. The water molecule was initially oriented with its surface parallel or vertical to the substrate. Therefore, 14 different starting configurations on AO-terminated surface and 8 different starting configurations on BO2 -terminated surface were selected. After geometric optimization, all different starting configurations converge to similar final configurations. The typical final configurations chosen are shown in Table 1. From Table 1, it can be seen clearly that the configuration DP is the most stable configuration on AO-termination with the adsorption energy of −0.889 eV, and IP is the energetically favorable one on BO2 -termination with the adsorption energy of −1.038 eV. These show that the K–Na

Fig. 2. The most stable H2 O adsorption geometries on the AO-terminated (a) and BO2 -terminated (b) facets of KNTN (0 0 1).

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Table 1 Adsorption energies, optimized structural parameters and bandgap for single H2 O molecule adsorbed on KNTN (0 0 1) surface. Configuration AO-termination BP BV DP BO2 -termination IP KP HP

Eads (eV)

∠HOH

dXO (Å)

dOH1 (Å)

−0.830 −0.818 −0.889

108.1 107.8 108.4

2.332 (O–Na) 2.701 (O–K) 2.291 (O–Na)

−1.038 −0.458 −0.968

102.0 100.7 102.3

2.349 (O–Nb) 2.382 (O–Ta)

dOH2 (Å)

Bandgap (eV)

1.476 1.485 1.667

1.671

1.087 1.148 0.936

1.894 1.895 1.853

1.922 1.938 1.885

1.183 1.250 1.216

dXO is the distance from the oxygen atom of H2 O molecule to the different ion underneath of KNTN (0 0 1) surface. The P, V is short for parallel and vertical. The calculated bandgap of clean KNTN surface is 0.815 eV in this work.

bridge and the Nb top site are the preferential adsorption sites for KNTN (1 0 0) surface. In the favored configuration of the AOtermination, the water molecule adsorbs between one Na and two oxygen of the surface, as shown in Fig. 2(a). The atomic distances ˚ between the water molecule and the surface are d(O–Na) = 2.291 A, ˚ The adsorption pulls the d(H1 –O) = 1.667 A˚ and d(H2 –O) = 1.671 A. surface Na out of its relaxed surface position apparently, which indicates that the electrostatic interactions between the water molecule and substrate can lead to structure changes of the KNTN surface. The difference electron density maps resulting from the adsorption of water on KNTN (0 0 1) surface is shown in Fig. 3(a). It clearly exhibits an increase of electron density between two water hydrogen and the neighboring surface oxygen atoms. Meanwhile, the negative charge transfer from Na of the surface into the adsorbed oxygen of the water molecule. Such structural changes and charge distribution illustrate that a O–Na ionic bond and two hydrogen bonds are formed between the water molecules and the AOterminated surface. For the BO2 -termination, as shown in Fig. 2(b), the calculated equilibrium distance between adsorbed water molecule and the ˚ which is close enough to form a Nb–O bond. surface Nb is 2.349 A, The H–O distances between the water molecule and the neighbor˚ The surface ing surface oxygen are found to be 1.894 A˚ and 1.922 A. geometry is not affected apparently by the water adsorption. From Fig. 3(b), the electron density increases substantially between O atom of the water and the surface Nb, whereas accumulation of electron density is visible near surface oxygen atoms which the water hydrogen point to. The results are compatible with a O–Nb bond and two hydrogen bonds. To shed more light on the interaction between single water molecule and the KNTN (0 0 1) surface, we have investigated the partial density of states (PDOS) of the most stable configurations. For the AO-termination, the PDOS of H 1s state of water molecule and O 2s, 2p states of KNTN surface and the PDOS of the surface oxygen before and after adsorption are displayed in Fig. 4(a). There are three main resonance peaks at energy of −7.8 eV and −19.5 eV and a range of −5.8 eV to −2.7 eV between H 1s state of water molecule and O 2p, 2s states of substrate, which reveal that the interaction between H atom of water molecule and the surface oxygen is existed. Because of the interaction just mentioned, the PDOS of surface oxygen shows a significant change integrally and shift towards lower energy lightly, which can be attributed to the hydrogen bonds’ formation. From Fig. 4(b), the resonance peaks between Na 2p states and O 2p states of water molecule are at energy of −19.5 eV and in the range of −5.3 eV to −0.8 eV. The overlap between Na 2p states and O 2p states is small, which is similar with the water adsorption on NaNO3 [22]. The results also illustrate the existence of the interaction between the O atom of water molecule and the Na atom. After adsorption, the shape of surface Na PDOS around the Fermi level has a tiny change, indicating

the O–Na interaction’s contribution to the variation of bandgap is small. For the BO2 -termination, the PDOS of H 1s state of water molecule and O 2s, 2p states of KNTN surface is plotted in Fig. 5(a). Similar to the case of Fig. 4(a), the resonance peaks are at energy of −7.9 eV, −20.2 eV and a range of −5.9 eV to −3.1 eV, suggesting the existence of hydrogen bonds. After adsorption, the conduction band of surface oxygen PDOS distinctly exhibits a shift towards higher energy, which results in an increase of the surface bandgap.

Fig. 3. Difference electron density maps highlighting the electron charge density redistribution due to the H2 O adsorption for the (a) AO-, (b) BO2 -terminated KNTN (0 0 1) surfaces. Yellow and blue colors represent depletion and accumulation of charge, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

H1s O2p O2s

1.5 1.0 0.5

Density of States(electrons/eV)

0.0

-20

-15

-10 -5 Energy(eV)

0

2.0

5

After Before

O 1.5 1.0 0.5 0.0

-8

-6

-4

-2 0 Energy(eV)

2

4

Density of States(electrons/eV)

2.0

Density of States(electrons/eV)

Density of States(electrons/eV)

W. Wang et al. / Applied Surface Science 298 (2014) 102–108

105

2.0

O2p Na2p

1.5 1.0 0.5 0.0 -25

-20

-15

-10 -5 Energy(eV)

0

2.0

5

After Before

Na 1.5 1.0 0.5 0.0

-8

-6

(a)

-4

-2 0 Energy(eV)

2

4

(b)

Fig. 4. Partial density of states (PDOS) of H2 O molecule and O, Na atoms of AO-termination (a and b). The zero point of the energy axis corresponds to the Fermi level.

1.5 1.0 0.5 0.0

-20

-15

-10 -5 Energy(eV)

0

5

After Before

O

1.5 1.0 0.5 0.0

-8

-6

-4

-2 0 Energy(eV)

(a)

2

4

To study the water adsorption at different coverage, we systematically changed the number of water molecules from 0.5 to 4 per surface unit cell on AO-terminations. The computational adsorption energies, bond length and bandgap with increase of water coverage are summarized in Table 2. With the inclusion of a second water molecule, the water–water interaction sets in besides the water–surface interaction [23].A variety of models of two adsorbed water molecules have been considered and the most stable ones are presented in Fig. 6. In

Density of States(electrons/eV)

2.0

H1s O2p O2s

3.2. Water clusters adsorption on the KNTN (0 0 1) surface

Density of States(electrons/eV)

Density of States(electrons/eV)

2.0

Density of States(electrons/eV)

From Fig. 5(b), the resonance peaks between O 2p states of water molecule and Nb 4p, 4d states of KNTN surface are at energy of −8.0 eV and in the range of −6.0 eV to −0.5 eV. The large hybridization of Nb 4p, 4d states and O 2p states demonstrates a strong interaction between the O atom of water molecule and the Nb atom of KNTN surface. In addition, attributing to the strong interaction, the DOS of Nb have a comparatively large variation, namely that the portion of Nb PDOS above 0 eV are obviously upshifted. With this we can explain the reason that the bandgap of BO2 -temination adsorbed by water molecule have a bigger variation than the case on AO-temination, as shown in Table 1.

2.0 1.5

O2p Nb4d Nb4p

1.0 0.5 0.0

-20

2.5

-15

After Before

2.0

-10 -5 Energy(eV)

0

5

Nb

1.5 1.0 0.5 0.0

-8

-6

-4

-2 0 Energy(eV)

2

4

(b)

Fig. 5. PDOS of H2 O molecule and O, Nb atoms of BO2 -ternination (a and b). The zero point of the energy axis corresponds to the Fermi level.

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W. Wang et al. / Applied Surface Science 298 (2014) 102–108

Table 2 Adsorption energies, optimized structural parameters and bandgap for H2 O adsorbed on KNTN (0 0 1) surface with different coverage. Coverage (ML) 0.5 2

3 4

Configuration

Eads (eV)

dOH–S (Å)

2A 2B 2C 3A 3B 4A 4B

−0.951 −0.767 −0.671 −0.757 −0.657 −0.628 −0.644 −0.566

1.635/1.656 1.631/1.650/1.672 1.461/1.464 1.533/1.646/1.656 1.520/1.556 1.588/1.605/1.697/1.873 1.492/1.509/1.719 1.569/1.570/1.656/1.663

dOH–W (Å)

Bandgap (eV) 0.859 1.046 1.116 1.067 1.062 1.028 1.088 1.051

2.083 2.578 1.597/1.679 1.951/1.980 1.610/1.671/1.717 1.753/1.758

dOH–S is the distance from the hydrogen atom of H2 O to the surface oxygen. dOH–W is the distance O–H between H2 O molecules above the surface.

the most favorable configuration (2A), the hydrogen atoms of one water molecule point alternatingly to a surface oxygen and to the oxygen atom of another water molecule, as well as the hydrogen atoms of another water overall point to the surface oxygen. There are four hydrogen bonds formed, one of which is formed between two water molecules, the others is formed between the hydrogen atom of water molecule and the surface oxygen. The hydrogen bond lengths are shown in Table 2. Additionally, the distance between oxygen atom of water molecules and the neighboring surface Na ˚ suggesting strong attraction between and K are 2.308 A˚ and 2.669 A, them [24,25]. The stable configurations for 3 ML and 4 ML water adsorption on KNTN (0 0 1) surface are shown in Fig. 6. For 3 ML, the most favorable adsorption mode is configuration 3A with adsorption

energy of −0.657 eV. Two hydrogen bonds are formed between the water hydrogen atom and the surface oxygen, and two hydrogen bonds are between three water molecules. The hydrogen bond lengths are shown in Table 2. Compare to the two-molecule case, there are more hydrogen bond formed between water molecules and the hydrogen bond lengths are shorter. The water oxygen atoms are coordinated to the surface sodium ion at distances of ˚ 2.443 A˚ and the distance of O–K is 2.712 A. ˚ The surface 2.423 A, Na is slightly pulled up and the surface K is pulled to neighboring O atom of water molecule, which makes the surface more roughness. In the case of 4 ML, the energetically favorable configuration is 4A. There are three hydrogen bonds formed between the water hydrogen atom and the surface oxygen, and three hydrogen bonds between water molecules. The distance of O–Na is 2.348 A˚ and the ˚ The presence of the water distances of O–K are 2.571 A˚ and 2.611 A. induces some relaxation of the surface atoms. It clearly shows that the formation of a O–K bond between the H2 O molecule and the neighboring surface K leads to a strong lateral shift of the surface K atom from the on-top position toward the neighboring water. A ring-like chain is formed by four alternating hydrogen-bonded water molecules above the substrate. To obtain a full picture of the water interaction with KNTN (0 0 1), the energetic trends as a function of water coverage is investigated. The adsorption energies for different coverage are plotted in Fig. 7(a). The adsorption energy slightly increases with coverage due to the hydrogen bond formation between water molecules. The adsorption of a second water molecule results in an

1.2

(b) Bandgap(eV)

1.1 1.0 0.9

Eads(eV)

0.8 -0.6

(a)

-0.7 -0.8 -0.9 -1.0

0

1

2

3

4

Coverage(ML) Fig. 6. Top views of different configurations of H2 O adsorbed on the KNTN (0 0 1) surface with different coverage. The gray lines reveal the adsorption energies of the configurations.

Fig. 7. The adsorption energy (a) and bandgap (b) as a function of H2 O coverage on KNTN (0 0 1) surface.

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Table 3 Mulliken population analysis of sub-surface Ta and Nb of water adsorbed KNTN surface with different coverage.

Density of States(electrons/eV)

6 4

Clean KNTN 1H2O/KNTN 2H2O/KNTN 3H2O/KNTN 4H2O/KNTN

Total

2 0 -3

6 4

-2

-1

0 1 Energy(eV)

Clean KNTN 1H2O/KNTN 2H2O/KNTN 3H2O/KNTN 4H2O/KNTN

2

3

4

Ta

2 0 -3

-2

-1

0 1 Energy(eV)

2

3

4

Ta2 (e)

Nb1 (e)

Nb2 (e)

0.76 0.80 0.81 0.81 0.85

1.00 0.99 1.01 1.01 1.03

0.77 0.76 0.74 0.77 0.81

1.00 1.03 1.02 1.04 1.07

sub-surface layer of AO-termination and the HOMO is localized on the surface layer of BO2 -termination. Thus, for water adsorbed on AO-termination, we can infer that the change of bandgap mainly attributing to the LUMO variation on sub-surface layer. The total DOS and the PDOS of sub-surface Nb, Ta and O of KNTN surface with different water coverage are presented in Fig. 9. It is found that the conduction band bottom consisting of Nb 4d and Ta 5d states on sub-surface layer, which shift towards higher energy with the water coverage increasing. The amount of movement is consistent with the change of bandgap. To obtain charge transfer of sub-surface Nb, Ta quantitatively, Mulliken population analysis is performed in Table 3. The Mulliken charges of sub-surface Ta and Nb increase with coverage, namely the electron density at the subsurface Nb and Ta positions is depleted gradually. As the water coverage increasing, the more sub-surface Ta 4d and Nb 5d electrons transfer to the surface and the bonds between Ta, Nb and neighboring O atoms become increasingly weak, which raises the conduction band bottom and results in a band gap widening. The result is in agreement with the investigation of the semiconductor bandgap change by mean of adsorption and doping [28–31]. 4. Conclusion In summary, we studied the adsorption of water on the KNTN (0 0 1) surface with different terminations and coverage via the first-principles DFT approach. The most favorable single water molecule adsorption sites are found to be the K–Na bridge site on the AO-terminated surface and the Nb top site on the BO2 -terminated KNTN (0 0 1) surface, with the adsorption energy of −0.889 eV and −1.038 eV, respectively. The negative adsorption energy indicates that the water adsorption has stable

Density of States(electrons/eV)

Density of States(electrons/eV)

intermolecular bonding and a decrease of water–surface interaction per water molecule, which is consistent with the case of oxygen adsorption on the transition metal surfaces [26]. Water is bound more strongly on oxide surfaces than transition and noble metal surfaces in which adsorption energies are typically in the range of −0.1 eV to −0.5 eV [22]. Although the adsorption energy increasing with coverage, the water molecules still have a strong interaction with the surface. Due to water adsorption would affect the localization of electronic states, and the band gap is generally proportional to the charge transfer density [27], the band gap change of the KNTN surfaces are investigated. The calculated bandgap of clean KNTN surface is 0.815 eV, and the bandgaps of KNTN surface adsorbed with water molecules are displayed in Table 2. As shown in Fig. 7(b), the band gap of the surface slightly increases with water coverage. The molecular orbital picture is feasible for elucidation of bandgap change, the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) of water adsorbed KNTN surface is displayed in Fig. 8. The LUMO is localized on the

Ta1 (e)

0 1 2 3 4

Density of States(electrons/eV)

Fig. 8. Frontier orbitals of H2 O adsorbed KNTN (1 0 0) surface (green: HOMO; red: LUMO). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Coverage (ML)

6 4

Clean KNTN 1H2O/KNTN 2H2O/KNTN 3H2O/KNTN 4H2O/KNTN

Nb

2 0 -3

-2

-1

6

0 1 Energy(eV)

2

3

4

Clean KNTN 1H2O/KNTN 2H2O/KNTN 3H2O/KNTN 4H2O/KNTN

O

4 2 0 -3

-2

-1

0 1 Energy(eV)

2

3

4

Fig. 9. Total and partial density of states of sub-surface Ta and Nb of water adsorbed KNTN surface with different coverage.

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W. Wang et al. / Applied Surface Science 298 (2014) 102–108

configurations on the KNTN surfaces. The surface geometry, charge transfer and partial density of states of the adsorption configuration illustrate that a Na–O ionic bond and two hydrogen bond formed on the AO-termination and a Nb–O bond and two hydrogen bond formed on the BO2 -termination, which make a significant contribution to the stability of the adsorption configurations. With the water coverage increasing, the water–surface interaction per water molecule decrease due to the enhancement of the interwater H bonding, but the surface geometry becomes more roughness owing to the adsorbed water. The bandgap change of KNTN surface after water adsorption is also investigated. We find that the band gap of the surface slightly increases with water molecule coverage, and is correlated to the charge transfer of the sub-surface Nb and Ta. The adsorption of water would cause sub-surface Ta 4d and Nb 5d electrons density decreasing, which result in a bandgap increase of KNTN surface. The calculations provide a qualitative evidence for the physical properties of the KNTN surface in a humid environment. Acknowledgments The research is supported by National Natural Science Foundation of China (No. 11204053 and No. 11074059) and the China Postdoctoral Science Foundation (Grant No. 2013M531028). We would like to thank High Performance Computing Center of Harbin Institute of Technology for the help of the calculation. References [1] J.E. Geusic, S.K. Kurtz, L.G. Van Uitert, S.H. Wemple, Appl. Phys. Lett. 4 (1964) 141.

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