Structures and Mechanisms of Water Adsorption on ZnO(0001) and

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Structures and Mechanisms of Water Adsorption on ZnO(0001) and GaN(0001) Surface Honggang Ye,*,† Guangde Chen,† Haibo Niu,† Youzhang Zhu,‡ Li Shao,† and Zhijuan Qiao† †

Department of Applied Physics and the MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, and ‡Microelectronic Department, School of Electronics and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China ABSTRACT: The adsorption structures and mechanisms of water adsorption on ZnO(0001) and GaN(0001) surface are investigated by using the first-principles methods. It is found that the stable adsorption structure at full monolayer (ML) coverage is (2 × 1) reconstructed. A (2 × 1) molecular adsorption is definite for ZnO, and a (2 × 1) dissociative adsorption is also possible for GaN. For these structures the hydrogen bonds between adsorbates are significant besides the covalent interaction with substrate. For the coverage below 0.5 ML for GaN and 0.25 ML for ZnO, the individually adsorbed H2O can easily decompose to OH and H. Both covalent and electrostatic attractions contribute to the stability of dissociative adsorption. For the coverage between the above two cases, molecular adsorption is found to be stable in theory, but the real structure may be greatly dependent on the chemical condition. These results give a detailed description of the interaction between the first water adlayer and ZnO(GaN)(0001) surface.

1. INTRODUCTION The surface of semiconductor plays a key role for many applications such as catalysis, gas sensing, solar cells, and microor optoelectronic devices. So it is for ZnO and GaN, which are of great importance in diverse technological areas.1−4 A detailed knowledge of their surface structure and properties is essential for the development and improvement of related devices. Water molecule (H2O) is one of the most considered adsorbates on the semiconductor surface because it is always present at ambient condition and has significant influences on surface stability and electronic properties. The interest in this field has been raised in recent years since (Ga1−xZnx)(N1−xOx) solid solution was proposed as a promising catalyst for photochemical splitting of water.5,6 Investigation of water adsorption is helpful to understand fundamental surface reactions. However, the adsorption of H2O seems to be more intricate than the others such as O2, H2, and CO etc. due to the hydrogen bonding capabilities.7−11 Both ZnO and GaN crystallize in the wurtzite phase. The low-index surfaces include polar (0001), (0001̅) and nonpolar (101̅0), (112̅0). Many discussions have been done on water adsorption on their nonpolar surfaces, and some common views have been obtained. Fully molecular adsorption was reported to be energetically favorable at coverage ⩽0.5 monolayer (ML) on ideal ZnO(101̅0) surface, while the energetically stable adsorption structure at 1 ML was found to be a (2 × 1) superstructure where every second water molecules is cleaved.12 The interesting (2 × 1) mixed adsorption was further observed in experiment,13 and its stabilization mechanism was explored by others.14,15 Comparatively, investigation on the behaviors of water on ZnO © XXXX American Chemical Society

polar surfaces is insufficient, especially for the Zn-terminated (0001) surface. A reason might be that the wurtzite (0001) surface itself is complex, always accompanied by defects, reconstructions, or adsorbates to satisfy the electrostatic convergence.16−20 A few literature results offer contradictory suggestions. In the earlier experimental study from 1983, water was believed to be in its molecular form on ZnO(0001) surface,21 while this result was questioned in later works22,23 and dissociative adsorption was found in a recent experiment of Ö nsten et al.22 By means of density functional theory (DFT) based calculations, Casarin et al.23−25 showed that molecular adsorption of water is favored on the perfect ZnO(0001) surface while Nishidate and Hasegawa26 proposed that molecular adsorption is just a high-energy transition state, and the stable structure may be either desorbed or dissociated. The discrepancy might be because the cluster models used by Casarin et al. are too small to describe the surface perpendicular to the polarization orientation. So far any conclusive result has not been obtained. Maybe that is also the reason why the case of water adsorption on ZnO(0001) surface is exclusively absent in the review article of Wöll27 for ZnO surfaces. The situation of water adsorption on the (101̅0) nonpolar surface of GaN is very different from that of ZnO. Dissociative adsorption is preferred at 1 ML and lower coverage; the energy barrier for the dissociation is negligible (1.0 meV).28 To get a thorough understanding of the photocatalytic property of GaN, the details of the water dissociation process and the Received: December 30, 2012 Revised: July 10, 2013

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dx.doi.org/10.1021/jp312847r | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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intermediate steps for the water oxidation at the GaN(1010̅ )− water interface are investigated by ab initio molecular dynamics simulations.29,30 For the polar GaN (0001) surface, experiments on water adsorption have been carried out.31 In this experiment dissociative adsorption (OH and H) with a saturation coverage of 0.46 ML was found at room temperature; the adsorbed OH can be further decomposed after annealing at ∼200 °C. In the later DFT calculations, the details of the splitting process of isolated water molecule on GaN(0001) were studied by Hu et al.32 and Chen et al.,33 using a (3 × 3) and (2 × 2) surface cell, respectively. Their calculations support dissociative adsorption at low coverage (⩽0.25 ML); the energy barrier to break the first O−H bond of H2O is moderate (0.1 eV) while the barrier to break the second O−H bond is very high (1.42 eV). The remaining problem is that the details of adsorption at high coverage are still unknown; the saturation coverage of 0.46 ML found in experiment is still expecting a reasonable explanation. It can be predicted that the water−water interaction must have great influence on the adsorption morphology and reaction barrier at high coverage. In this paper, a systematical investigation is done for water adsorption on ZnO(0001) and GaN(0001) surface by using first-principles calculation methods. For simplicity, only the perfect (0001) surface is considered. We try to assess whether dissociative or molecular adsorption is favored at different coverages and to explore the detailed adsorption structures and mechanisms. The first adlayer is mainly concerned since it determines much of the physical and chemical processes at the surface. The coexistence of GaN and ZnO here is due to their similarity in crystalline structure and cooperating application in photochemical splitting of water. The comparison between them is also helpful to explore and verify some intrinsic mechanisms.

module in the software of Materials Studio is used to produce the stick-ball models in some figures. The adsorption energy per H2O is defined by the equation of Eads = −1/N(Etotal − Eref − NEH2O), where Etotal and Eref are the total energy of the adsorbed model and the corresponding relaxed clean surface model, EH2O the energy of a free H2O molecule, and N the number of water molecules. This definition implies that an exothermic process corresponds to a positive adsorption energy. To further validate our results, calculations are performed for water adsorption on ZnO(101̅0) surface, which has been well studied. It is reconfirmed that the (2 × 1) mixed adsorption is more stable than the (1 × 1) fully molecular adsorption with an energy difference of 0.086 eV/ H2O, well consistent with the result in the literature.12,14

3. RESULTS AND DISCUSSION 3.1. Full Monolayer Coverage. The work begins from full monolayer coverage, which is defined as each surface primitive cell possessing one H2O. To seek the stable adsorption configurations with (1 × 1) symmetry, geometric optimization calculations are performed for several initial structures including both molecular and dissociative H2O. Finally only molecular adsorption is obtained. All the dissociated species, OH and H, recombine to H2O after relaxation. GaN and ZnO show very high similarity in adsorption geometry. Two specific configurations are found for both of them. As shown in Figure 1, the

2. COMPUTATIONAL DETAILS AND MODELS The first-principles calculations are based on the density functional theory, using the Vienna ab initio simulation package (VASP) code.34,35 The PW91 generalized gradient approximation (GGA) is used for exchange correlation.36 The interaction between core and valence electrons are treated with the projector augmented wave method.37 The 3d electrons of Ga and Zn are included in valence electrons. The energy cutoff for the plane wave basis function is 500 eV. We employ the Monkhorst−Pack38 sampling scheme with a k-point mesh density of 9 × 9 × 1 for the (1 × 1) surface cell. The slab models are built containing six ZnO(GaN) bilayers with 12 Å vacuum space separating the slabs. The dangling bond of the O(N)-terminated side of the slab is saturated by 0.5(0.75)e charged pseudohydrogen atoms. The top two bilayers of the slabs and any adsorbates are allowed to relax freely by minimizing the quantum mechanical force on each ion to be less than 0.01 eV/Å. The others are fixed in the optimized bulk configuration. The effect induced by the net dipole moment perpendicular to the slab is corrected in our calculations.39 The lattice parameters used in building the slab models are derived by optimizing the corresponding bulk primitive cell with the same accuracy, which are a = 3.214 Å, c = 5.239 Å, u = 0.376 for GaN and a = 3.288 Å, c = 5.267 Å, u = 0.382 for ZnO. The discrepancies with experiment values are within 1%. The transition state from one to another adsorption configuration is probed by the nudged elastic band method.40 The Visualizer

Figure 1. (Color Online) Top views of the two (1 × 1) adsorption structures. One side view of them is given to account for the definition of the tilt angle θ of adsorbed H2O.

H2O presents “H-up” geometry with O atom binding with the surface Ga(Zn) and the H atoms locating in an outer plane parallel to the surface. The difference between the two configurations lies in the arrangement of H atoms relative to the two-dimensional surface lattice. The detailed geometric parameters are listed in Table 1, including the length of the bond between O of H2O and surface Ga(Zn) (dO−sub), the tilt angle (θ) defined in Figure 1, and the intramolecular bond length (dO−H) and bond angle (ϕH−O−H) of H2O after adsorption. It can be seen that the difference between GaN and ZnO lies in dO−sub and θ; the adsorption distance is shorter and the tilt angle is larger for GaN. The adsorption energies, Eads’s, of the two structures for GaN and ZnO are also listed in Table 1. For either GaN or ZnO, the two structures have approximately equivalent Eads, which is about 0.28 eV/H2O for ZnO and 0.08 eV/H2O for GaN. The Eads for ZnO is much larger than that for GaN, even though the Ga−O bond is shorter. To explore the origin of this difference, the adsorption energy is decomposed by the equation of Eads = Ew−sub + Ew−w + Ew + Esub. Here Ew−sub is the binding energy with H2O and substrate in their final adsorbed geometries B



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Table 1. Geometric Parameters and Adsorption Energies (in Units of eV per H2O) of the (1 × 1) Adsorption Structures of H2O on ZnO(0001) and GaN(0001) Surface ZnO GaN

dO−sub/Å

θ/°

dO−H/Å

ϕH−O−H/°

Eads

Ew−sub

Ew−w

Ew

Esub

2.326 2.315 2.115 2.131

25.0 24.3 44.9 43.2

0.992 0.991 1.012 1.013

101.0 100.9 103.4 102.9

0.283 0.288 0.084 0.086

0.223 0.208 0.271 0.240

0.091 0.106 −0.058 −0.043

−0.024 −0.024 −0.073 −0.052

−0.006 −0.003 −0.055 .

before putting them together, Ew−w is the energy contributed by water−water interaction in adlayer, Ew is the energy due to the deformation of H2O relative to free H2O, and Esub is the energy required to distort the substrate surface to the final adsorbed geometry from fully relaxed clean surface. The values of the four items are listed together with Eads in Table 1. The data in Table 1 reveal that the Eads for ZnO is mainly contributed by the first item Ew−sub, the covalent bond between surface Zn and O of H2O. The other three items are relatively small. An unexpected result is that the values of the first item for GaN are no less than those for ZnO. The small Eads for GaN is from the counteractions of the other three items rather than weak Ga−O bond. Difference in the second item Ew−w is apparent, which is positive for ZnO but negative for GaN, indicating attractive and repulsive interaction, respectively, between water molecules. This item is calculated by removing the substrate from the models and maintaining the water adlayer in adsorbed geometry, so it is determined only by the geometry of adlayer. After exclusion of the divergence in lattice constant, the difference is attributed to the tilt angle θ of adsorbed H2O, which is ∼24° for ZnO and ∼45° for GaN. It is found that this angle determines whether the hydrogen bonding interaction between water molecules or the electrostatic repulsion interaction between neighbor H atoms is stronger. According to the valence shell electronic pair repulsion (VSEPR) model,41,42 the difference in tilt angle can be further attributed to the charge transfer between substrate and H2O. The calculated Bader charge of the O of H2O on ZnO and GaN surface is 8.16 and 8.27, respectively (it is 8.0 for the free H2O). It indicates that more charge is transferred form GaN to O than from ZnO to O. There is thus more local positive charge on the surface of GaN than on ZnO. Interaction of such positive charges with the partial positive charges on H results in repulsion, leading to the distinct values of the tilt angle. The values of the third and fourth items, Ew and Esub, can be understand legitimately. The greater charge transfer induces more significant disturbance to semiconductor surface and H2O, resulting in more negative Ew and Esub. Therefore, the distinct adsorption energy between ZnO and GaN intrinsically originates from the magnitude of charges transferred from substrate to H2O. Possible reconstructions under 1 ML coverage are further considered. We clarify that the reconstruction here is referred to the reorganization of adsorbates and not the substrate surface. By using the (2 × 1) models two special adsorption geometries are obtained, a molecular and a dissociative configuration. The schematic representations of them are displayed in Figure 2. In the molecular adsorption one H2O is bonded chemically with the surface Ga(Zn) (dO−sub = 2.053 Å for GaN and 2.190 Å for ZnO, shorter than those of (1 × 1) symmetry), and the other one is trapped by hydrogen bonds (marked by dashed lines in cyan). The average Eads is 0.902 eV/ H2O for GaN, increased approximately by an order of

Figure 2. (Color Online) Top and side views of the (2 × 1) reconstructions of (a) molecular and (b) dissociative adsorption.

magnitude. That for ZnO is 0.532 eV/H2O, increased considerably as well. The great increase is from two factors. First the strength of the Ga−O (Zn−O) bond is enhanced (the reason will be discussed later), and second the hydrogen bonds are optimized by the release of symmetry constrain. The (2 × 1) dissociative adsorption configuration consists of a hydroxyl adlayer and nearly free H2 molecules. The average Eads for GaN is 0.967 eV/H2O, larger than that of the (2 × 1) molecular adsorption. The formation of this structure needs to break two H−O bonds, and the calculated energy barrier for the transition from the molecular to dissociative (2 × 1) reconstruction is 0.23 eV, which is about two times the barrier to break the first H−O bond for each H2O (∼0.14 eV, shown in Figure 3b). If the two H−O bonds can break individually in experiment, the real energy barrier might be ⩽0.14 eV. The low energy barrier indicates that the (2 × 1) dissociative configuration is very possible for GaN. When the H2 molecules deviate, a hydroxyl monolayer is left. It maintains (2 × 1) symmetry due to hydrogen bonding interaction between them. However, the (2 × 1) dissociative adsorption structure is impossible for ZnO because a negative adsorption energy is obtained, implicating an endothermic process. It seems that the adsorption can be significantly enhanced by reconstruction, so reconstructions in larger scale such as (3 × 1), (2 × 2), and (2 × 3) are examined but no valuable structure is obtained. A temporal conclusion is that at 1 ML coverage the stable adsorption structures of H2O on GaN and ZnO (0001) surface are (2 × 1) reconstructed. The one for ZnO is definitely a molecular adsorption while a dissociative adsorption is also possible for GaN. 3.2. Low Coverages. Supercells such as (2 × 1), (2 × 2), (2 × 3), and (3 × 3) are used to investigate the situations with coverage lower than 1 ML. For each coverage possible adsorption morphologies including molecular, half-dissociative (H2O → OH + H), and fully dissociative (H2O → O + 2H) and possible locations such as the hollow (H3), tetrahedral (T4), and top sites are considered. The adsorption energies of C



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Figure 3. (Color Online) (a) Adsorption energy of molecular (black square), half-dissociative (red circle), and fully dissociative (blue trilateral) H2O on GaN(0001) surface with respect to coverage. The background color is utilized to distinguish the coverage with different stable adsorption morphology. (b) Energy profile of the dissociation processes marked by green arrows in panel (a). The insets display some representative adsorption geometries and structures near the transition state.

The trend of the black curve in Figure 3a and Figure 4a shows that molecular adsorption of H2O on GaN(0001) and ZnO(0001) surface is possible at any coverage. Although the situation with coverage lower than 1/4 ML is not calculated in this work, the existence of molecular adsorption at 1/9 ML on GaN (0001) surface has been reported32 (the empty square in Figure 3a). The result is distinct from the prediction given by Nishidate and Hasegawa26 that molecular adsorption of H2O is not stable on ZnO(0001) surface. The reason may be that the surface stoichiometry in their work is different, where a sublayer O atom is used to form the H2O. The Eads’s at low coverage are obviously larger than that at 1 ML coverage, but the variation is weak when the coverage is lower than 5/8 ML for GaN (3/4 ML for ZnO). The adsorption geometries are similar to those shown in Figure 1 for 1 ML coverage. The geometric parameters at 0.5 ML are dO−sub = 2.136 Å, θ = 24.5° for GaN and dO−sub = 2.233 Å, θ = 21.2° for ZnO. The main

the energetically favorable structures with respect to coverage are revealed in Figure 3a for GaN and Figure 4a for ZnO. The curves with square, circle, and trilateral symbols correspond to the three adsorption morphologies, respectively. A whole feature is that the adsorption morphology is greatly dependent on the coverage and dissociative adsorption is preferred in energy at low coverage. The ranges of coverage with different stable adsorption morphology are distinguished by the background colors, which are light pink for O + 2H, pale green for OH + H, and cyan for molecular adsorption. Comparing to the case of nonpolar (101̅0) surface, this result exhibits some different features. Fully molecular adsorption is the lowest energy structure for H2O on ZnO(101̅0) surface at coverage ⩽0.5 ML,12 which is just opposite to the polar (0001) surface. Water is found to adsorb dissociatively on GaN (101̅0) surface at 1 and 1/4 ML,28 but molecular adsorption is stable for (0001) surface at coverage >0.5 ML. The details for polar (0001) surface are discussed below. D



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Figure 4. (Color Online) (a) Adsorption energy of molecular (black square), half-dissociative (red circle), and fully dissociative (blue trilateral) H2O on ZnO(0001) surface with respect to coverage. The background color is utilized to distinguish the coverage with different stable adsorption morphology. (b) Energy profile of the dissociation process marked by the green arrow in panel (a). The insets display some representative adsorption structures and the key images of the dissociation reaction process.

change is that the tilt angle θ becomes smaller at lower coverage. The intrinsic mechanism of molecular adsorption is the coupling between the 1b1 orbital (the lone-pair orbital) of H2O and the surface orbital of Ga(Zn). A schematic representation for the process, by taking 0.5 ML H2O on GaN surface as an example, is shown in Figure 5. The 1b1 orbital is fully occupied and the surface orbital is partially filled by 0.75e(Ga) or 0.5e(Zn); the antibonding state of them hence is nonempty at 1 ML coverage, inducing weak adsorption. At low coverage the electrons at antibonding state can shift downward to the surface state of bare surface cation. The adsorption energy is increased consequently at low coverage. In Figure 5 the charge shift from Ga−O antibonding state to the surface state of bare surface Ga is marked by the arrow in cyan. That is also the reason why the strength of the covalent bond in the 1 ML (2 × 1) molecular adsorption structure discussed before it is enhanced. According to this, the Eads of molecular adsorption should saturate at the

Figure 5. (Color Online) Schematic energy diagram for the interaction between H2O molecule and GaN(0001) surface at 0.5 ML coverage. The location of the antibonding state (yellow) of Ga−O bond and the surface orbital of bare Ga (green) are also exhibited with isovalue of 0.03e/Å3. The direction of charge shift is denoted by the arrow in cyan.

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adsorption was also specified to this phenomenon.22 A key problem here is the amount of energy barrier of the surface reorganization. It has been studied carefully by Nishidate and Hasegawa,26 and a value of 1.50 eV was obtained. However, our calculation shows that this process is nearly barrier free (Figure 4b). The peak in this curve corresponds to the release of the first H from H2O, and the wide shoulder of the curve corresponds to H-induced surface reorganization. The discrepancy may be because the released H atom from H2O in our work first adsorbs atop nearby Zn and then contacts with the sublayer O atom, while it is always above the T4 site in ref 26. In addition, the existence of the nearby OH from H2O might be helpful to the atomic transport. The low energy barrier is also supported by the general observation of this phenomenon in experiments.22,44 As shown by the blue curve (trilateral) in Figure 3a, the OH on GaN(0001) surface can further decompose to O and H when the coverage is ⩽1/6 ML. The isolated O atom locates at the H3 site and the H atoms at top sites. The adsorption energy is larger than that of OH + H, but the probability of this adsorption morphology is also decided by the reaction barrier. The energy barrier to release the second H atom we obtained is ∼1.47 eV (blue curve in Figure 3b), consistent with the result reported by Chen et al..33 This is a considerable amount of energy and much larger than that to detach the first H atom, implicating that the reaction is hard to occur. However, the decomposition of OH on GaN(0001) surface was found in experiment by annealing the sample at ∼200 °C,31 which indicates that the activation barrier is not so high as the value given by theoretical calculation. This discrepancy is thought to be because the reaction in experiment occurs in a complex chemical condition, and the barrier may be reduced by other species in atmosphere, while these factors are not considered in calculation. The fully dissociative adsorption of O + 2H, therefore, is possible for GaN under some conditions. The interaction mechanism should be similar to these discussed for OH + H, including both covalent and electrostatic interaction. For ZnO the adsorption energy of O + 2H is smaller than that of OH + H at 1/9 ML coverage, so it must be impossible, at least in the coverages with which we are concerned. In practice, no matter how much water finally adsorbed on the semiconductor surface, the adsorption process begins from low coverage. The partial pressure of vapor in vacuum space or air atmosphere is very small in general. The water molecules are isolated, and they approach the surface one by one. Based on above results, under this condition the most possible phenomenon is that the adsorbed H2O decomposes to OH and H and the highest coverage is 0.5 ML for GaN and 0.25 ML for ZnO. For higher coverage the stable structure is molecular adsorption, but the recombination of OH and H is dependent on the chemical condition. At low temperature the recombination is hard to occur; further deposition of H2O on GaN surface only can form a second layer interacting with the first layer by hydrogen bonds. That may be the case of the experiment31 by which a saturated coverage of 0.46 ML was obtained, implying that the recombination reaction cannot occur on GaN(0001) surface at room temperature. The recombination might be harder or entirely impossible for ZnO because the surface has been significantly disturbed. Additional water molecules will first stick to the bare surface Zn resulting in mixed adsorption morphology. If the recombination occurs at high temperature (it is also affected by the gas atmosphere), with further deposition the (2 × 1) reconstruc-

coverage of 5/8 ML for GaN and 3/4 ML for ZnO, at which the antibonding state is just depleted entirely. This prediction is consistent with the calculation results revealed in Figure 3a and Figure 4a. A maximum adsorption energy of 1.138 and 0.569 eV/H2O is obtained for GaN and ZnO, respectively. The small fluctuation of Eads after saturation (at lower coverage) is from the interaction between water molecules. The half-dissociative adsorption of OH + H exists at 0.5 ML (0.25 ML) and lower coverage for GaN(ZnO), and they are more stable in energy than molecular adsorption. That is because the dissociated species can passivate more dangling bonds at surface. It is easy to understand that the highest coverage is 0.5 ML for GaN, where the surface Ga’s are all covered by OH or H. The case for ZnO is different, for which the OH and H will recombine to H2O after relaxation when the coverage is 0.5 ML. The highest coverage for ZnO we found is 0.25 ML, implicating that at least one-half the surface Zn is bare for the dissociative adsorption of OH + H. The dissociation processes of H2O marked by green arrows in Figure 3a and Figure 4a are calculated, and the energy profiles of them are revealed in Figure 3b and Figure 4b. It is found that the H2O on GaN(0001) and ZnO(0001) surface is easy to release the first H atom. The responding activation barrier (red curves) is 0.14 and 0.38 eV, respectively. The value for GaN is well consistent with the result (0.10 eV) reported by Chen et al..33 The small energy barriers indicate that the adsorption morphology of OH + H is very probable at low coverage. This conclusion for GaN agrees well with the experimental result of Bermudez and Long,31 where dissociative adsorption with a saturated coverage of 0.46 ML was obtained. However, our result for ZnO conflicts with that of Casarin et al.,23 where low-coverage models were used but molecular adsorption was found to be more stable. The discrepancy is deemed to be that the cluster models they used are too small to describe the polar surface. In general the coverage dependence property of Eads for dissociative adsorption can be understood by the electron counting rule.43 The Eads should saturate at 3/8 ML for GaN and 1/4 ML for ZnO (at which the half-filled orbitals of H and OH are just fully occupied by the electrons from the dangling bond of surface Ga and Zn). However, the fact presented in Figure 3a and Figure 4a is that obvious increase of Eads maintains when the coverage is lower than these values. The issue is attributed to the electrostatic attraction between the negatively charged adsorbates (OH and H) and bare surface cations. When the half-filled orbital of OH and H couples with a dangling bond of surface cation, the bonding state cannot be fully occupied without the electrons collected from the bare dangling bonds around. Therefore, the adsorption center is negatively charged, and the bare cations are more positive. The strength of electrostatic interaction between them is not ignorable. It is worth to note that the orientation of charge shift here is contrary to that of molecular adsorption. The structure of half-dissociative adsorption for GaN is conventional, with OH and H species lying atop surface Ga atoms. The one for ZnO is interesting, as shown by the inset in Figure 4a for 1/4 ML coverage, where the released H atom induces an atomic transport of the sublayer O to surface to form an OH group and leaving Zn−Zn bond at subsurface. This phenomenon had been observed in experiment44 and suggested by theoretical calculation19 for hydrogen adsorption on ZnO(0001) surface. The origin of how the triangular pattern of clean ZnO(0001) surface is blurred by water F



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Notes

tion shown in Figure 2 will present gradually. Another situation is that the semiconductor surface is exposed in humid condition, e.g., acting as catalyst, and the water molecules are not all isolated and several may touch the surface together. This time the dissociation process may be prevented by the interaction between water molecules and the (2 × 1) pattern appears directly in some districts of the surface. Lots of similarities and differences between GaN and ZnO have been seen in the above discussion. Another obvious difference between them is that the adsorption energies for GaN are wholly larger than those for ZnO and the activation barrier of water dissociation on GaN surface is smaller. That is because the essence of chemical adsorption is to shift the electrons from the surface states to lower levels. The dangling bonds of GaN(0001) surface are occupied by more electrons and have higher energy eigenvalues. When these electrons are transferred to lower levels, more energy is released consequently.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the China National Natural Science Fund with grant no. 61176079, 11204231, and 11074200.

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4. CONCLUSION By using the first-principles calculation methods, the structure, energetics, and mechanism of water adsorption on ZnO(0001) and GaN(0001) surface are investigated systematically. The stable adsorption structure is found to be greatly dependent on the coverage. At full monolayer coverage, the adsorption structures with (1 × 1) symmetry are eliminated; the energetically favorable configurations are (2 × 1) reconstructed. It is definitely a (2 × 1) molecular adsorption for ZnO, while a (2 × 1) dissociative adsorption, composed of H2 molecules and a hydroxyl adlayer, is also possible for GaN. For these highcoverage adsorptions, both covalent bonding interaction between adsorbate and substrate and the hydrogen bonding interaction between water molecules contribute to the adsorption. At low coverage (⩽0.5 ML for GaN and ⩽0.25 ML for ZnO), the adsorbed H2O can easily release one H atom resulting in dissociative adsorption. The adsorbed OH and H locate at top sites on the GaN surface whereas the dissociative H atoms have significant disturbance to ZnO(0001) surface. At the coverage ⩽1/6 ML, the adsorbed OH on GaN surface can further decompose to H and O atoms under some conditions, but this process is impossible for ZnO. The calculated energy barrier for H2O to release the first H atom on GaN(ZnO) surface is 0.14(0.38) eV, and the one to release the second H atom on GaN surface is 1.42 eV. These energy barriers may be reduced in real experiment. Besides the covalent bonding interaction, for dissociative adsorption the electrostatic attraction between adsorbate and bare surface cation is an important stabilization mechanism. For the coverage between 0.5(0.25) and 1 ML coverage, molecular adsorption is found to be stable by theoretical calculation, but the real structure in experiment is greatly dependent on the chemical condition. A mixed adsorption morphology may be more general. Besides above similarities and differences, a whole feature is that the adsorption energy for GaN is always larger than that for ZnO under the same condition, and the reaction barrier of H2O dissociation on GaN surface is always lower. These results give a detailed and systematic description of the interaction between the first water adlayer and ZnO(GaN)(0001) surface.

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Structures and Mechanisms of Water Adsorption on ZnO(0001) and

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