High Reactivity of the ZnO(0001) Polar Surface ... - ACS Publications

Subscriber access provided by UNIV OF TEXAS ARLINGTON

Article

High Reactivity of the ZnO(0001) Polar Surface: The Role of Oxygen Adatoms Yang Liu, Wangping Xu, Yue-Yue Shan, and Hu Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03326 • Publication Date (Web): 06 Jul 2017 Downloaded from http://pubs.acs.org on July 12, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 18

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

The Journal of Physical Chemistry

High Reactivity of the ZnO(0001) Polar Surface: The Role of Oxygen Adatoms Yang Liu, Wangping Xu, Yueyue Shen* and Hu Xu* Department of Physics, Southern University of Science and Technology, Shenzhen 518055, China

ABSTRACT: Understanding the mechanism of water dissociation on metal oxide surfaces is of particular interest in catalytic reactions. In this work, the interaction of water with the ZnO(0001) polar surface is investigated, and the role of oxygen adatoms in water splitting is uncovered. The individual surface energies and electronic properties of ZnO polar surfaces are investigated based on density functional theory calculations. The oxygen adatoms on the ZnO(0001) surface introduce in-gap surface states, resulting in a direct-to-indirect band gap transition. Water strongly interacts with oxygen adatoms to spontaneously form hydroxyl groups, recovering the direct band gap characteristics of ZnO polar surfaces. Furthermore, water prefers to adsorb at step edges of cavities after all the oxygen adatoms are consumed, and the hydrogenbonded network among water molecules triggers the dissociation of water at the edge sites, which is also confirmed by molecular dynamics calculations. Our results provide atomic-scale insight into the interactions of water with the ZnO(0001) surface.

ACS Paragon Plus Environment


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

Page 2 of 18

1. INTRODUCTION Since the first discovery of water splitting on TiO2 in 1972,1 great efforts have been made to study the adsorption behavior of water on metal oxide surfaces.2-5 Prior studies have shown that water adsorbs in the molecular form on some metal oxide surfaces,6-7 while it completely dissociates on some other metal oxide surfaces.7-8 In addition, partial dissociation is also found at high coverage of water due to the intermolecular hydrogen bonding.3,9 Typically, water dissociation is caused by defects such as adsorbates,10 vacancies,2,11 and steps.12-13 However, the dissociation mechanism of water on stoichiometric metal oxide surfaces is the subject of much debate, especially at low coverage of water. It is widely accepted that hydrogen bonds are important in the states of water adsorption on metal oxide surfaces. In addition, the position of the surface O 2p levels relative to the top of the bulk valence bands may play crucial roles in identifying the adsorption behaviors of water on metal oxide surfaces.7 The interaction of water with ZnO surfaces has received extensive attention due to its outstanding catalytic and corrosion characteristics. The adsorption of water on non-polar ZnO(101 0) surfaces has been well established.5,9,14 Prior results showed that water adsorbs molecularly at low coverage on the stoichiometric ZnO(101 0) surface, while intermolecular hydrogen bonds become important as coverage increases, resulting in the half-dissociated structure. This mixed form of water adsorption on the ZnO 1010 surface has been observed in experiments.9,

14

By

contrast, the adsorption states of water on the ZnO(0001) polar surface (Zn-face) are controversial. As early as 1983,15 water was believed to adsorb in molecular form on the Zn-face, as suggested by experiments using ultraviolet photoemission spectroscopy (UPS) and temperature-programmed desorption (TPD). However, this point of view has been questioned by Önsten et al.16 in 2010. They used scanning tunneling microscopy (STM) and photoemission spectroscopy (PES) to study the water adsorption behaviors, and concluded that water adsorbs dissociatively on surface Zn sites next to step edges of the triangular islands and cavities. Unfortunately, the resolution of the STM images in the work by Önsten et al.16 was insufficient in showing the exact adsorption site for water. In addition to these experiments, some theoretical calculations have been carried out to investigate the interaction of water with the Zn-face. Casarin et al.17 performed density functional theory (DFT)


Page 3 of 18

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


calculations using cluster models to study the adsorption of water, and their results showed that water prefers to adsorb on the Zn-face in the molecular form. Later DFT calculations18-19 suggested that the dissociative adsorption of water on the perfect (1×1) ZnO(0001) polar surface is the most energetically favorable. The instability of perfect (1×1) ZnO polar surfaces owing to a divergent electrostatic energy is well known. The stable polar surfaces must be auto-charge compensated to avoid the polar catastrophe, and many works have been carried out to study the stabilizing mechanism.20-22 Among these, the electrostatic Madelung energy has been widely cited as the driving force to form triangular cavities or islands to stabilize the Zn-face.20,23-24 Recently, the adatom-cavity (ADC) structure on the Znface and the disordered Y (DY) structure on the O-face driven by the bonding flexibility of under-coordinated Zn ions have been theoretically proposed to be the most stable structures,23 and such calculations have been confirmed by STM and dynamical low energy electron diffraction (LEED) measurements. Many surface defects can be present on even the most stable Zn-face, including oxygen adatoms, single vacancies, and step edges. Regardless of whether the water adsorbs in the molecular or dissociative forms, the interaction of water with the perfect (1×1) Znface does not accurately reflect interactions with real surfaces. Therefore, the understanding of water adsorption on the Zn-face at the atomic level is still lacking. Further study of the interaction of water with the stable and reconstructed Zn-face is necessary to gain greater insight into the surface catalytic reactions under more realistic conditions.

2. CALCULATION METHOD AND DETAILS All DFT calculations are performed using the Vienna Ab initio Simulation Package (VASP).25 The projector augmented wave method26 is used to describe the interaction between valence electrons and ion cores. The Perdew-Wang 1991 (PW91) functional 27

is applied for the exchange-correlation potentials. The energy cutoff of the plane-

wave expansion is set to 400 eV, and the vacuum region between periodic images is larger than 10 Å to avoid interactions between neighboring images. The force convergence criterion on each atom is 0.02 eV/Å. The optimized lattice parameters (a = 3.28 Å and c = 5.27 Å) of bulk ZnO in the wurtzite phase are used to construct the slab models.



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

Page 4 of 18

The ZnO polar surfaces built in this work contain five Zn-O bilayers bound by optimal reconstructed surfaces; i.e., the ADC model was bound at the Zn-face and the DY model at the O-face. To evaluate the interaction between water and the Zn-face, both the (4×4) and (6×6) supercells are considered in this work, and the MonkhorstPack sampling28 with grid spacing less than 0.05×2π Å−1 for structural optimization are used. The reactive pathway and barrier of water dissociation are calculated using the climbing image nudged elastic band (CI-NEB) method.29 To overcome the band gap problems of standard DFT calculations, Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional30 is employed to study the electronic properties of ZnO polar surfaces. Ab initio molecular dynamics (AIMD) simulations are performed to study the interplay between water molecules and step edges of cavities on the Zn-face. The adsorption energy per water molecule (Ead) is an important parameter in determining the adsorption behaviors of water, and it is calculated as:

Ead  ( Etot  Eslab  nEH2O ) / n

(1)

where Etot is the total energy of a slab with the adsorption of water molecules, Eslab is the energy of the reconstructed ZnO polar surface, EH2O is the energy of an isolated water molecule, and n is the number of the adsorbed water molecules.

3. RESULTS AND DISCUSSION

Figure 1. Top (side panels) and side views (middle panel) of the p(4×4) ZnO polar surface with the ADC model at the Zn-face and the DY model at the O-face. The gray and red spheres represent Zn and O atoms, respectively. The O adatom is labeled in the left-most image.


Page 5 of 18

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


3.1. Surface Energies and Electronic Properties of Polar Surfaces. The surface

energy is an important parameter in crystal growth and surface reactivity. Therefore, it is very important to know the exact surface energies of ZnO polar surfaces. The surface energies of ZnO polar surfaces have been frequently studied.31-34 However, all these studies focus on the surface energies of the perfect (1×1) ZnO polar surfaces. As mentioned above, perfect (1×1) ZnO polar surfaces do not exist due to the divergent electrostatic energy. The reconstructed ZnO polar surfaces with the ADC model at the Zn-face and the DY model at the O-face are shown in Figure 1. The individual surface energies were calculated using the wedge method35 to separate the surface energies of the Zn-face and O-face. The wedge with the number of atoms on the edge m = 10 is shown in Figure 2. Wedges with the numbers of edge atoms m = 9 and m = 10 are constructed to calculate the surface energies. Pseudo-hydrogen atoms are usually used to passivate surface dangling bonds on the wedges. Each atom on the (001) sidewall of the wedge has two dangling bonds, and two pseudo-hydrogen atoms with 1.5 valence electrons (H-1.5e) are required to saturate the dangling bonds of each surface atom and to maintain charge neutrality. However, small distances between the pseudo-hydrogen atoms cause strong steric repulsion, which can induce larger structural distortions. To eliminate such steric effect, one fluorine (F) atom is introduced to replace two pseudohydrogen atoms. One pseudo-hydrogen atom with 0.5 valence electrons (H-0.5e) is used to passivate the dangling bond of each O atom on the (1 1 1) sidewall. In addition, it requires an extra H-1.5e atom to passivate each Zn atom at the corner of the (001) sidewall. The computational details in the calculations of the surface energy are shown in Supporting Information (SI). The calculated surface energies are 1.23 J/m2 for the ADC model at the Zn-face, and 1.11 J/m2 for the DY model at the O-face. Meanwhile, it should be noted that many different ADC and DY models have the built-in dipole field cancelled to meet the reconstruction requirement. To verify the convergence of surface energies, we also calculated the corresponding surface energies using the p(6×6) ADC-DY supercell. For the reconstructed p(6×6) supercell, the surface energies are also 1.23 J/m2 and 1.11 J/m2 for the Zn-face and O-face, respectively. The calculated results strongly confirm the convergence of surface energies. Because the PW91 functional underestimates the surface energy, we have also calculated surface energies for (1010)



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

and (1120) surfaces for comparison. The calculated surface energies are 0.88 J/m2 and 0.93 J/m2 for the (1010) and (1120) surfaces, respectively. Our results show that the ZnO polar surfaces have higher surface energies than the non-polar surfaces, indicating the possible high reactivity of the polar surfaces. In addition, the surface energy of the Zn-face is slightly higher than that of the O-face, which suggests that the Zn-face is more reactive than the O-face. In other words, the stability order for ZnO surfaces is (1010) > (1120) > (0001) > (0001).

Figure 2. Cross-sectional view of the wedge with the number of edge atoms m = 10. The small (large) white spheres denote H-0.5e (H-1.5e), and the cyan spheres denote F atoms.

The perfect ZnO polar surfaces are usually considered to have metallic characteristics due to the principle of auto-compensation.36-37 However, the electronic properties of the reconstructed ZnO polar surfaces are still not fully understood owing to the lack of stable structures. The band structure and charge density distribution of the highest valence band of the reconstructed ZnO polar surfaces are shown in Figure 3. Interestingly, the reconstructed ZnO polar surfaces have semiconducting characteristics with a band gap of 0.34 eV, which is in sharp contrast with the prior results of the metallic material. It is worth pointing out that the reconstructed ZnO polar surfaces have an indirect band gap. The conduction band minimum (CBM) is at the Γ point, while its valence band maximum (VBM) is at the K point. As shown in


Page 6 of 18

Page 7 of 18

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


Figure 3, there is an almost undispersed occupied gap state in the gap region, which is highlighted using the blue line. As shown in Figure 3(b), the charge density distribution clearly indicates that this gap state mainly originates from the 2p state of the O adatom (Oad-2p) at the Zn-face. The gap states introduced by surface O adatoms locate significantly above the bulk VBM, indicating that the surface O adatoms should be effective in splitting water according to our prior work.7 Standard DFT calculations are well known to dramatically underestimate the gap values of semiconductors. The calculated band gap of wurtzite bulk ZnO is only 0.77 eV, while the experimental value of ZnO is 3.3 eV.38 To further study the electronic properties, we have performed HSE06 calculations. As shown in Figures 3(c) and 3(d), we can find that the energy gap becomes 2.98 eV, and the gap state still comes from the Oad2p state.

Figure 3. (a) The band structure and (b) the charge density distribution of the highest valence band of the reconstructed ZnO polar surface using PW91 calculations. The corresponding results for the HSE06 calculations are shown in (c) and (d), respectively. The isosurface charge density is taken to be 0.02 e/bohr3. The Fermi level is set to 0 eV.

3.2 Interaction of Water with the Zn-face 3.2.1 Effect of the Oxygen Adatom on the Dissociation of Water. Next, we

investigate the interplay between water molecules and the Zn-face. First, the adsorption of a single water molecule on the Zn-face is studied, and the possible adsorption configurations with the corresponding adsorption energies are shown in Figure 4. Both the molecular and dissociative adsorption of water are considered. A single proton bonding to the surface Zn is not considered in this work due to its relative instability.39 Water adsorption in the molecular form with one hydrogen bond between water and the neighboring (Oad) atom is shown in Figure 4(a). Water



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

adsorption in dissociative forms is shown in Figures 4(b-d), while molecular water adsorption in the cavity is shown in Figures 4(e, f).

Figure 4. Water adsorption on the p(4×4) reconstructed Zn-face. (a) The molecular adsorption of water on the Zn-face near the O adatom. (b-d) Water adsorption in the dissociative forms on the Zn-face. (e, f) Water adsorption in the molecular forms at the cavity edge. The pink and green spheres represent the oxygen and hydrogen atoms of water, respectively.

When water is close to the O adatom, it forms a strong hydrogen bond with a distance of 1.64 Å between the H of water and O adatom, as shown in Figure 4(a), and the adsorption energy in this case is -0.81 eV. In addition, the adsorption structures of water at step edges of the cavity are also considered. The molecular configurations are shown in Figures 4(e) and 4(f), and their corresponding adsorption energies are -1.36 eV and -1.22 eV, respectively. These configurations lead to the formation of two stronger hydrogen bonds between the adsorbed water molecule and the edge atoms (Oed) atoms, which is the reason why these configurations have relatively high stability. By comparison, the adsorption structure shown in Figure 4(f) is 0.14 eV higher in energy than that shown in Figure 4(e). This is because not enough space is present at the corner of the cavity to hold one water molecule comfortably. In Figure 4(e), the angle of H-Ow-H slightly decreases from 104.7˚ (in an isolated water molecule) to 103.9˚, while this angle is markedly compressed to 96.5˚. Furthermore,


Page 8 of 18

Page 9 of 18

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


the dissociative adsorption shown in Figure 4(d) is energetically unfavorable, indicating that Oed atoms are not very reactive.

Figure 5. Potential energy profile for water dissociation from the molecular to dissociative adsorption on the Zn-face.

Surprisingly, the adsorption energy is dramatically reduced to -1.53 eV when the hydrogen of the water molecule is abstracted by the O adatom, as shown in Figure 4(c). We can find that water adsorption in the dissociative form is the most energetically favorable. By contrast, the lowest adsorption energy for water on ZnO(10 1 0) is only around -1.10 eV.5,

11

The results suggest that water interacts

strongly with the Zn-face because the Oad atoms on the Zn-face are very reactive due to their highly occupied gap states. In the dissociative configuration, both the OwH group and the OadH group bond to two surface Zn atoms located at the Zn-Zn bridge sites. The energy barrier and reaction pathway from the molecular adsorption configuration to the dissociative configuration are shown in Figure 5. After dissociation, the energy is greatly reduced by 0.72 eV. The calculated energy barrier is only 0.01 eV, which indicates that the dissociation of water on the Zn-face through the interaction with the O adatom is facile at extremely low temperatures. Hence, our results suggest that Oad atoms will promptly trigger the dissociation of water on the Zn-face.



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

Figure 6. (a) The band structure and (b) the charge density distribution of the VBM of the reconstructed ZnO polar surfaces, with the dissociation of water calculated using PW91. The corresponding results for HSE06 calculations are shown in (c) and (d). The isosurface charge density is taken to be 0.018 e/bohr3. The Fermi level is set to 0 eV.

After the dissociation of water on the Zn-face, the gap state originating from the O adatom at the Zn-face disappears, as shown in Figure 6(a), suggesting that the dangling bond of the Oad atom is saturated by the H atom from the dissociated water. In addition, the energy band of the O adatom is shifted downwards in energy due to the formation of the Oad-H bond. As shown in Figure 6(b), the major contributions to the VBM arise from the 2p states of the edge oxygen atoms. Notably, the direct band gap at the Γ point of the reconstructed ZnO polar surfaces is recovered, and the band gap increases to 0.48 eV. Furthermore, we also performed HSE06 calculations to study the corresponding electronic properties. As shown in Figures 6(c) and 6(d), the charge distribution of VBM is almost the same, while the band gap significantly increases to 3.31eV. Therefore, the adsorption of water can effectively tune the electronic properties of a ZnO polar surface. The question remains whether all Oad atoms react with water to form hydroxyl groups at higher Oad coverage on the Zn-face. In our ADC model in a p(4×4) supercell, only one Oad atom is present and the coverage of Oad is 1/16. In the following discussion, we consider a bigger p(6×6) supercell. As shown in Figure 7, three O adatoms are present in the p(6×6) supercell, and the Oad coverage is 1/12. The adsorption of one to four water molecules is studied to investigate the interplay between the water molecule and the O adatoms. For the molecular configuration shown in Figure 7(a), the edge site of the cavity is also the preferred site for the water molecule, and the corresponding adsorption energy per water molecule is -1.35 eV.


Page 10 of 18

Page 11 of 18

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


Similar to the case of the p(4×4) supercell, the single water molecule still prefers to dissociate on the Zn-face (see Figure 7(b)), and the adsorption energy is -1.48 eV. For two water molecules that adsorb on the Zn-face, dissociative adsorption is the most energetically favorable configuration with an adsorption energy per water molecule of-1.54 eV (see Figure 7(d)); the molecular configurations of water in the cavity have an adsorption energy per water molecule of -1.37 eV (see Figure 7(c)). The mixed configuration with both the molecular and dissociative adsorptions in the p(6×6) supercell is slightly energetically unfavorable due to the relative instability of the molecular adsorption. Even though three water molecules are present in the p(6×6) supercell, the O adatoms still can trigger the dissociation of water as shown in Figure 7(f). Our results strongly show that the O adatoms on the Zn-face have sufficiently high reactivity to split water molecules. When all of the O adatoms are consumed by the interactions with water molecules, the subsequent water molecule prefers to adsorb molecularly at the edge site in the cavity, as shown in Figure 7(g).

Figure 7. The molecular and dissociative adsorption of water on the p(6×6) Zn-face with three O adatoms.

3.2.2 Effect of a Hydrogen-bonded Network among Water Molecules. Many

studies have reported that a hydrogen-bonded network formed among water molecules will trigger the dissociation of water molecules on the surface of metal oxides such as MgO (100),3 ZnO (10 1 0),9 and rutile TiO2 (110).40 Although an isolated water molecule at the edge step of a cavity is not dissociative on the Zn-face, we expected that water molecules may undergo dissociative adsorption in wetter conditions based on the observed phenomena by Önsten et al.16 To consider the effect



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

of hydrogen bonding on the adsorption configuration, a larger triangular cavity with the side length of 19.6 Å in the p(6×6) ADC model is used. The configurations with different water molecule adsorption at the edge of a cavity are shown in Figure 8. Figure 8(a) shows that the adsorption energy per water molecule is -1.49 eV, while the adsorption energy for the partial dissociation of the water molecule is -1.51 eV (see Figure 8(b)); this indicates that partial dissociative adsorption is more energetically favorable. In addition, the dissociative adsorption of water is unstable, and it will transform to partial dissociation of water after structural relaxation. In the partial adsorption configuration, the molecular water prefers to point to the dissociated water to form a hydrogen bond with a distance of 1.80 Å, which stabilizes the water dimer. Three and four water molecules are also considered to adsorb at the edge of a cavity. As shown in Figures 4(c)-(f), the mixed adsorption of water is energetically favorable on the Zn-face. These results further confirm that hydrogen bonds between water molecules play a key role in the dissociation of water at the edges of cavities.

Figure 8. The molecular and dissociative adsorption of water in a p(6×6) Zn-face.

To further explore the effects of the hydrogen-bonded network on the dynamic behaviors of water dissociation at the edges of the cavity, more water molecules were considered at the step edges in the p(6×6) unit cell. As shown in Figure 9, ten water molecules are present in the p(6×6) ADC model used to perform the AIMD calculations. Each water molecule was adsorbed on a subsurface Zn atom in the large cavity. The water density in this cavity is 0.06/Å2, and the initial state of water is


Page 12 of 18

Page 13 of 18

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


obtained by structural relaxation. Figure 9(a) shows the atomic structure at the moment of 5 ps in the 7 ps AIMD calculations at 200 K. It is obvious that water molecules denoted by W-1, W-2, and W-3 in Figure 9(a) are dissociated and the other water molecules are intact; this result depends on the hydrogen bonding with the neighboring water molecules.

Figure 9. Snapshots of the atomic structures of water adsorption in the large cavity of a p(6×6) Zn-face in AIMD simulations: (a) t = 5 ps at 200 K, (b) t = 2.5 ps at 300 K, and (c) t = 5 ps at 300 K. (d) H-OH distances of W-2 and W-4 as a function of time. OH denotes the oxygen atom of an OH group.

Due to the relatively low temperature at 200 K, hydrogen bonds are locked in the AIMD calculations. Typically, experiments on water adsorption are carried out at room temperature. Therefore, we performed AIMD calculations at 300K. Another two water molecules denoted by W-0 and W-4 are shown in Figures 9(b) and 9(c). The time evolution of distances between the H and O atom in the molecular or dissociated water molecules for W-2 and W-4 in 5 ps is shown in Figure 9(d). Distances of approximately 1 Å indicate the molecular state of water. Snapshots of the atomic structures at 2.5 ps and 5 ps are shown in Figures 9(b) and (c), respectively. It is clear that the states of W-2 and W-4 undergo dynamic dissociation and association in the process of AIMD simulations, as shown in Figures 9(b)-(d). The AIMD simulations are divided into three areas labeled I, II, and III (see Figure 9(d)). In area II, the dissociative adsorption is dominated by W-2, and W-4 is in the molecular form. In



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

area III, the adsorption states for W-2 and W-4 are reversed compared with area II. The changes are attributed to hydrogen bonds between W-0 and W-2 (or W-4). The above results also further confirm that the hydrogen-bonded network is responsible for the dissociation of water.

4. CONCLUSIONS In summary, DFT calculations were performed to study the surface energies and electronic structures of ZnO polar surfaces. Using the wedge method, the individual surface energies for the ADC model at the Zn-face and the DY model at the O-face are 1.23 and 1.11 J/m2, respectively. The Oad atoms contribute to the surface in-gap states, which is responsible for the indirect band gap of the ZnO polar surfaces. Moreover, surface gap states originated from Oad atoms indicate that the surface Oad atoms are reactive in the surface reactions. The Oad atoms interact strongly with water and spontaneously trigger the dissociation of water molecules, forming hydroxyl groups on the Zn-face. Water adsorption on the Zn-face has a remarkable effect on the electronic properties of ZnO polar surfaces, again recovering the direct band gap feature. Water molecules behave differently as the water coverage increases. At low coverage, water prefers to adsorb dissociatively, while the edge sites of cavities are preferred for the water molecules when all the Oad atoms are consumed. A hydrogenbonded network among water molecules is responsible for the dissociation of water at the cavity edges. The phenomenon of dissociation-association of water molecules at the edge sites is confirmed by AIMD calculations. Our findings are expected to contribute to the understanding of water molecule behaviors on the Zn-face.

■ ASSOCIATED CONTENT Supporting Information The details of the computational procedures for surface energies, and every single structure in Figures 7 and 8.


Page 14 of 18

Page 15 of 18

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


■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS The work is supported by the National Natural Science Foundation of China (NSFC, Grant Nos. 11674148 and11334003) and the Basic Research Program of Science, Technology and Innovation Commission of Shenzhen Municipality (Grant No. JCYJ20160531190054083).

■ REFERENCES (1)

Fujishima, A.; Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode.

Nature 1972, 238, 37-38. (2)

Schaub, R.; Thostrup, P.; Lopez, N.; Laegsgaard, E.; Stensgaard, I.; Norskov, J. K.;

Besenbacher, F., Oxygen Vacancies as Active Sites for Water Dissociation on Rutile TiO2 (110). Phys. Rev. Lett. 2001, 87, 266104. (3)

Giordano, L.; Goniakowski, J.; Suzanne, J., Partial Dissociation of Water Molecules in the (3

X 2) Water Monolayer Deposited on the MgO(100) Surface. Phys. Rev. Lett. 1998, 81, 1271-1273. (4)

Mulakaluri, N.; Pentcheva, R.; Scheffler, M., Coverage-Dependent Adsorption Mode of Water

on Fe3O4(001): Insights from First Principles Calculations. J. Phys. Chem. C 2010, 114, 11148-11156. (5)

Meyer, B.; Marx, D.; Dulub, O.; Diebold, U.; Kunat, M.; Langenberg, D.; Wöll, C., Partial

Dissociation of Water Leads to Stable Superstructures on the Surface of Zinc Oxide. Angew. Chem., Int. Ed. 2004, 43, 6641-6645. (6)

Harris, L. A.; Quong, A. A., Molecular Chemisorption as the Theoretically Preferred Pathway

for Water Adsorption on Ideal Rutile TiO2(110). Phys. Rev. Lett. 2004, 93, 086105. (7)

Xu, H.; Zhang, R. Q.; Ng, A. M. C.; Djurišić, A. B.; Chan, H. T.; Chan, W. K.; Tong, S. Y.,

Splitting Water on Metal Oxide Surfaces. J. Phys. Chem. C 2011, 115, 19710-19715.



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

(8)

Hass, K. C.; Schneider, W. F.; Curioni, A.; Andreoni, W., First-Principles Molecular

Dynamics Simulations of H2O on α-Al2O3(0001). J. Phys. Chem. B 2000, 104, 5527-5540. (9)

Dulub, O.; Meyer, B.; Diebold, U., Observation of the Dynamical Change in a Water

Monolayer Adsorbed on a Zno Surface. Phys. Rev. Lett. 2005, 95, 136101. (10)

Rodriguez, J. A.; Evans, J.; Graciani, J.; Park, J. B.; Liu, P.; Hrbek, J.; Sanz, J. F., High

Water-Gas Shift Activity in TiO2(110) Supported Cu and Au Nanoparticles: Role of the Oxide and Metal Particle Size. J. Phys. Chem. C 2009, 113, 7364-7370. (11)

Yan, Y.; Al-Jassim, M. M., Structure and Energetics of Water Absorbed on the ZnO(1010)

Surface. Phys. Rev. B 2005, 72, 235406. (12)

Zheng, T.; Wu, C. Y.; Chen, M. J.; Zhang, Y.; Cummings, P. T., A Dft Study of Water

Adsorption on Rutile TiO2(110) Surface: The Effects of Surface Steps. J. Chem. Phys. 2016, 145, 044702. (13)

Raymand, D.; van Duin, A. C. T.; Spångberg, D.; Goddard, W. A., III; Hermansson, K.,

Water Adsorption on Stepped Zno Surfaces from Md Simulation. Surf. Sci. 2010, 604, 741-752. (14)

Wang, Y.; Muhler, M.; Woll, C., Spectroscopic Evidence for the Partial Dissociation of H2o

on Zno(1010). Phys. Chem. Chem. Phys. 2006, 8, 1521-1524. (15)

Zwicker, G.; Jacobi, K., Site-Specific Interaction of H2O with Zno Single-Crystal Surfaces

Studied by Thermal-Desorption and Uv Photoelectron-Spectroscopy. Surf. Sci. 1983, 131, 179-194. (16)

Önsten, A.; Stoltz, D.; Palmgren, P.; Yu, S.; Gothelid, M.; Karlsson, U. O., Water Adsorption

on ZnO(0001): Transition from Triangular Surface Structures to a Disordered Hydroxyl Terminated Phase. J. Phys. Chem. C 2010, 114, 11157-11161. (17)

Casarin, M.; Maccato, C.; Vittadini, A., An Lcao-Ldf Study of the Chemisorption of H2o and

H2S on ZnO(0001) and Zno(1010). Surf. Sci. 1997, 377, 587-591. (18)

Ye, H. G.; Chen, G. D.; Niu, H. B.; Zhu, Y. Z.; Shao, L.; Qiao, Z. J., Structures and

Mechanisms of Water Adsorption on ZnO(0001) and GaN(0001) Surface. J. Phys. Chem. C 2013, 117, 15976-15983. (19)

Viñes, F.; Iglesias-Juez, A.; Illas, F.; Fernández-García, M., Hydroxyl Identification on ZnO

by Infrared Spectroscopies: Theory and Experiments. J. Phys. Chem. C 2014, 118, 1492-1505. (20)

Dulub, O.; Diebold, U.; Kresse, G., Novel Stabilization Mechanism on Polar Surfaces:

ZnO(0001)-Zn. Phys. Rev. Lett. 2003, 90, 016102. (21)

Freeman, C. L.; Claeyssens, F.; Allan, N. L.; Harding, J. H., Graphitic Nanofilms as

Precursors to Wurtzite Films: Theory. Phys. Rev. Lett. 2006, 96, 066102. (22)

Wander, A.; Harrison, N. M., The Stability of Polar Oxide Surfaces: The Interaction of H2o

with ZnO(0001) and ZnO(0001). J. Chem. Phys. 2001, 115, 2312-2316. (23)

Xu, H.; Dong, L.; Shi, X. Q.; Van Hove, M. A.; Ho, W. K.; Lin, N.; Wu, H. S.; Tong, S. Y.,

Stabilizing Forces Acting on ZnO Polar Surfaces: Stm, Leed, and Dft. Phys. Rev. B 2014, 89, 235403. (24)

Lai, J. H.; Su, S. H.; Chen, H.-H.; Huang, J. C. A.; Wu, C.-L., Stabilization of Zno Polar Plane

with Charged Surface Nanodefects. Phys. Rev. B 2010, 82, 155406. (25)

Kresse, G.; Furthmuller, J., Efficiency of Ab-Initio Total Energy Calculations for Metals and

Semiconductors Using a Plane-Wave Basis Set. Comp. Mater. Sci. 1996, 6, 15-50.


Page 16 of 18

Page 17 of 18

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


(26)

Blochl, P. E., Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979.

(27)

Wang, Y.; Perdew, J. P., Correlation Hole of the Spin-Polarized Electron Gas, with Exact

Small-Wave-Vector and High-Density Scaling. Phys. Rev. B 1991, 44, 13298-13307. (28)

Monkhorst, H. J.; Pack, J. D., Special Points for Brillouin-Zone Integrations. Phys. Rev. B

1976, 13, 5188-5192. (29)

Henkelman, G.; Uberuaga, B. P.; Jónsson, H., A Climbing Image Nudged Elastic Band

Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901-9904. (30)

Krukau, A. V.; Vydrov, O. A.; Izmaylov, A. F.; Scuseria, G. E., Influence of the Exchange

Screening Parameter on the Performance of Screened Hybrid Functionals. J. Chem. Phys. 2006, 125, 224106. (31)

Tang, C.; Spencer, M. J. S.; Barnard, A. S., Activity of ZnO Polar Surfaces: An Insight from

Surface Energies. Phys. Chem. Chem. Phys. 2014, 16, 22139-22144. (32)

Wilson, H. F.; Tang, C.; Barnard, A. S., Morphology of Zinc Oxide Nanoparticles and

Nanowires: Role of Surface and Edge Energies. J. Phys. Chem. C 2016, 120, 9498-9505. (33)

Zhan, J.; Dong, H.; Sun, S.; Ren, X.; Liu, J.; Chen, Z.; Lienau, C.; Zhang, L., Surface-Energy-

Driven Growth of Zno Hexagonal Microtube Optical Resonators. Adv. Opt. Mater. 2016, 4, 126-134. (34)

Zhang, J.; Zhang, Y.; Tse, K.; Deng, B.; Xu, H.; Zhu, J., New Approaches for Calculating

Absolute Surface Energies of Wurtzite (0001)/(000 1): A Study of ZnO and GaN. J. Appl. Phys. 2016, 119, 205302. (35)

Zhang, S. B.; Wei, S.-H., Surface Energy and the Common Dangling Bond Rule for

Semiconductors. Phys. Rev. Lett. 2004, 92, 086102. (36)

Meyer, B.; Marx, D., Density-Functional Study of the Structure and Stability of ZnO Surfaces.

Phys. Rev. B 2003, 67, 035403. (37)

Wander, A.; Schedin, F.; Steadman, P.; Norris, A.; McGrath, R.; Turner, T. S.; Thornton, G.;

Harrison, N. M., Stability of Polar Oxide Surfaces. Phys. Rev. Lett. 2001, 86, 3811-3814. (38)

Srikant, V.; Clarke, D. R., On the Optical Band Gap of Zinc Oxide. J. Appl. Phys. 1998, 83,

5447-5451. (39)

Kresse, G.; Dulub, O.; Diebold, U., Competing Stabilization Mechanism for the Polar

Zno(0001)-Zn Surface. Phys. Rev. B 2003, 68, 245409. (40)

Lindan, P. J. D.; Harrison, N. M.; Gillan, M. J., Mixed Dissociative and Molecular Adsorption

of Water on the Rutile (110) Surface. Phys. Rev. Lett. 1998, 80, 762-765.



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

TOC Graphic


Page 18 of 18

High Reactivity of the ZnO(0001) Polar Surface ... - ACS Publications

Recommend Documents