ReaxFF Molecular Dynamic Simulations of ZnO Nanocluster and Films

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ReaxFF Molecular Dynamic Simulations of ZnO Nanocluster and Films in H Atmosphere 2

Shengen Zhang, Feng Cheng, Xiang He, and Zhaoxu Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07461 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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The Journal of Physical Chemistry

ReaxFF Molecular Dynamic Simulations of ZnO Nanocluster and Films in H2 Atmosphere Sheng-En Zhang†, Feng Cheng†, Xiang He‡, Zhao-Xu Chen*,† †

Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China. ‡ State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China.

ABSTRACT: Reactive molecular dynamics simulations were performed to explore the structural evolution of ZnO nanocluster and (0001) and (1010) surfaces under H2 atmosphere at different temperatures. The mechanisms of H2 dissociation and water formation were analyzed. Our simulations reveal that there are two pathways for H2 dissociation and three routes for water formation on the surfaces. The nanocluster is more active for H2 dissociation and water formation than the two surfaces. The gas-solid interactions lead to outward displacement of the substrate O atoms. While the O-terminated surface of the (0001) facet is active for H2 dissociation and water formation, the Zn-terminated one is inactive for the dissociation. Unlike (0001) surface which is more easily reduced, the (1010) surface is readily hydroxylated. Water formation and desorption results in surface oxygen depletion and Zn aggregation which leads to surface metallization, in accordance with the experimental observations. Our simulations show that Zn sites are not active for H2 dissociation. By fitting the obtained rate constants at different temperatures, we estimated the activation energy of the H2 dissociation over ZnO cluster to be 4.06 kcal/mol, in very 1

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good agreement with the experimental result of 5.0 kcal/mol.

INTRODUCTION Zinc oxide (ZnO) is an important material and is widely used in various fields

such as electronic devices, solar cells, gas sensing systems and catalysis.1–4 However, the properties and performances of ZnO are affected by its interaction with the atmosphere under which it works. For example, exposure to gases like H2 atmosphere can strongly influence the electrical properties of ZnO and improve its conductivity.3,5–7 It is also shown that ZnO (1010) surface would become metallic when exposed to hydrogen, methanol or even water,8,9 and its conductivity is influenced by atomic hydrogen that diffuses into the material.5,10 The interaction of ZnO with environmental gases always happens in heterogeneous catalysis when it serves as catalysts in water–gas shift reaction,11,12 methanol steam reforming13,14 and synthesis of methanol from syngas,15,16 where ZnO can directly react with the related reactants, products and intermediates. No doubt, investigations of the interaction and reaction of ZnO with environmental gases are of significance.

Many investigations have been devoted to adsorption and dissociation of H2 on ZnO. Both non-dissociative17 and dissociative18 adsorption modes have been reported. It is suggested that18–21 dissociative adsorption occurs via two different ways: Type I is rapid and reversible and responsible for the formation of OH and ZnH groups while type II results in Zn-H-Zn and O-H-O groups with a relatively slow and irreversible 2


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process. But the detailed mechanism for H2 dissociation is still not clear. Additionally, the surface O vacancies are supposed to be the catalytically active sites and can accelerate the H2 dissociation by lowering dissociative energy barrier.11,22–24

A number of theoretical investigations have also been carried out on the H2 dissociation over ZnO in the past decades.25–28 Embedded cluster self-consistent field (SCF) quantum chemical calculations revealed that (1010) of ZnO was unreactive toward H2 dissociation.26 On the contrary, periodic Hartree-Fock (HF) calculations supported the dissociative adsorption of H2 on (1010).27 HF study using (ZnO)6 cluster indicated that H2 dissociation took place on both the Zn and O sites, with a preference on the two-coordinated O sites.28 A recent reactive molecular dynamics (RMD) simulation of water dissociation on stepped ZnO surfaces29 shows that the surface H2O molecules can dissociate into surface hydroxyls, in agreement with previous reports.7,30–33 As indicated above, there are still some controversial and unclear issues with H2 dissociation on ZnO. In addition, to make better use of ZnO, it is necessary to understand the interaction with and response to the environmental atmosphere of ZnO nanocluster, because nanoclusters are essentially the existing forms of working heterogeneous catalysts and often exhibit quite different properties from those of bulk materials and flat surfaces.34–36 As far as we know, there are no papers dealing with the structural evolution of ZnO nanocluster in the H2 gas environment. However, calculations for such purposes are intractable with first-principles computational techniques because of the computationally extremely expensive cost of time and huge 3



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resources needed.

As a novel method, RMD simulations37 can handle systems of much larger size and events of much longer time, compared with first-principle calculations. Furthermore, reactive force field (ReaxFF) is more applicable to deal with transition state chemistry than other reactive potentials.38 With the above in mind, we recently performed RMD simulations to explore the mechanism of H2 dissociation and water formation on a ZnO nanocluster and two types of flat surfaces, and the structural evolution of the ZnO materials in the reactive atmosphere. The present paper is arranged as follows. Section 2 gives a brief description of the reactive force field method and the simulation models we adopted; Section 3 presents the results and discussion, where the difference of reactivity and the structural evolution of the three models are compared; Conclusions are given in section 4.

METHOD AND MODEL ReaxFF Force Field. Based on the distance-dependent bond order concept of a

chemical bond, the ReaxFF force field can be applied to model chemical reactions.39 According to the detailed characterization of ReaxFF energy terms proposed by van Duin et al.,39,40 atom-atom interactions can be calculated from the formula of bond order. The long-range van der Waals interactions between every pair of atoms are derived from a distance corrected Morse-potential while the electrostatic interactions are deduced from a shielded Coulomb potential with the atomic charges determined 4


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using the electron equilibrium method (EEM),41 including the charge transfer and geometry-dependent polarization during chemical reactions. ReaxFF force field is parameterized against the high-level first-principle calculations or experimental data, making it capable of simulating the bond breaking and formation of the reactive system. RMD simulations of larger systems and longer physical time events computationally cost much less than first-principle calculations37 and have been applied to investigate a variety of systems.40,42–44

Simulation Details. It is accepted that surface reactivity is closely related to morphology and nanoclusters are generally more active than flat surfaces.34,35 In this paper, we constructed three different ZnO models: one cluster and two slab models, to explore the morphology-dependent reactivity and surface structure evolution of ZnO materials in H2 atmosphere (Figure 1). Based on the Wulff construction theory,45 a 2 nm spherical cluster consisting of 173 O-Zn pairs was constructed out of the infinite ZnO bulk material with the optimized lattice parameters of α=β=90°, γ=120° and a=b=3.23 Å, c=5.14 Å (the experimental values are a=b=3.25 Å, c=5.21 Å). The cluster was then placed in a 100×100×100 Å3 cubic box. For the slab models we chose ZnO (1010) and (0001) surfaces. A slab of five Zn-O layers truncated from the optimized ZnO bulk was built for (1010) and (0001) respectively, with a vacuum spacing of 25 Å to separate the repeating slabs. The cell parameters are a=13.00 Å, b=20.82 Å, c=37.19 Å, α=β=γ=90° for (10 10) and a=b=16.25 Å, c=38.62 Å, α=β=90°, γ=120° for (0001), which has two terminations: O-terminated and Zn-terminated ones. Twenty randomly distributed H2 molecules were put in the 5



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surrounding space of each model and H2 can access both upper and lower surfaces. All the RMD simulations were performed in the ReaxFF modeling suite as implemented in the Amsterdam Density Functional (ADF) program46 under NVT ensemble (i.e. constant particle number N, cell volume V and temperature T) with a total time of 2.5 ns. The Velocity Verlet algorithm47 was employed to update each atom’s position and velocity after a timestep of 0.25 fs. The Berendsen thermostat48 was adopted to hold the set temperature constant with a damping coefficient of 100 fs. Zn/O/H ReaxFF force field parameters were taken from reference.49 Before each RMD simulation, conjugate-gradient energy minimization was performed for all the initial systems with a convergence criterion of 0.01 kcal/mol/angstrom.

Figure 1. Initial configuration of the three ZnO-H2 systems under study (purple: Zn, red: O, white: H)

RESULTS AND DISSCUSSION H2 Dissociation. Hydrogen dissociation is the first reaction for reduction of ZnO

in H2 atmosphere, and the generated H species can accelerate the entire reduction reaction. We start our discussion from H2 dissociation and water formation on the cluster model at 1200K. Generally H2 dissociation can occur in the gas phase through H2 intermolecular collision (pathway I) or on the interface where H2 impacts on a 6


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surface O (pathway II), OH (pathway III) and Zn (pathway IV). Pathway I denoted in Equation (1) was not observed in our simulation at 1200K, likely due to the high barrier of H2 dissociation in the gas phase. Indeed, when temperature is high enough, 2000K, for example, it does take place. With time going on, pathways II and III take place. In pathway II, after overcoming a ZnO…H…H approximate transition state structure, an H atom desorbs into the gas phase, producing a surface hydroxyl (Figure 2). This process can be simplified as Equation (2). In pathway III an H2 molecule first collides with a surface hydroxyl, and then a surface water is formed after one of the H atoms of the H2 molecule desorbs into the gas phase, Equation (3). The produced water may eventually desorb into the gas phase. Previous studies20,50 suggested that H2 dissociation occurred on both O and Zn sites. However, pathway IV was not observed in our simulation, indicating that Zn sites are at least not as active as O sites for H2 dissociation, consistent with Wilson and Barnard’s point of view.51 On the two slab models hydrogen dissociation follows the similar processes described for Pathways I to III on the cluster. It is worth mentioning that with the (0001) surface we only observed H2 dissociation on the O-terminated surface, implying that Zn sites are inactive for the dissociation. H2(g) → 2H(g)

(1)

H2(g) + ZnO(s) → ZnO…H…H(s) → ZnO-H(s) + H(g)

(2)

ZnO-H(s) + H2(g) → Zn O …H…H(s) → Zn- O ―H (s) + H(g) → Zn(s) + ｜｜ H H H2O(g) + H(g)

(3) 7



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Figure 2. Illustration of hydrogen dissociation via H2 (g) + ZnO (s) [see Equation (2)] on the ZnO nanocluster model. Panels from left to right represent the initial state, approximate transition state and final state respectively.

Figure 3 compares H2 dissociation over different models at 1200K. As can be seen from the figure, the dissociation takes place within 50 ps on the ZnO (1010) and (0001) surfaces while it begins at ~500 ps on the cluster model, which seems to indicate that the cluster is less reactive than the surfaces for H2 dissociation. However, beyond 500 ps, the number of H2 per surface area decreases more quickly on the cluster than on the two surfaces, meaning that the cluster is more active towards H2 dissociation than the surfaces after an initiation stage. This phenomenon can be rationalized by the content of exposed O atoms since H2 dissociation occurs on these sites. We found that initially there are 8.1/nm2 surface O atoms on the cluster whereas the values are 5.5/nm2 and 5.9/nm2 for the (0001) and (1010) surfaces respectively. The initiation stage, which is longer with the cluster model than with the two flat surfaces, is due to the lower H2 pressure used in the cluster model. To confirm this, we increased the pressure by adding H2 molecules from 20 to 100 and performed the simulations. In this case, the dissociation occurred within 50 ps. For (1010) and (0001) surfaces, the slopes of the dissociation curves (red and black in Figure 3) are comparable, indicating that these two surfaces possess similar reactivity for H2 dissociation. 8


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Figure 3. Time variation of number of H2 (g) divided by the surface area of the ZnO (1010), (0001) and cluster models at 1200 K.

Generally speaking, the produced atomic H may have three fates: (a) combining with a surface O atom to form a hydroxyl group or bonding to a surface hydroxyl to form a surface water; (b) combining with a surface Zn atom to form Zn-H group; (c) diffusing into the interior of ZnO cluster or the surfaces. However, (b) and (c) were not detected in our simulations. It was reported that at temperature higher than 550K, Zn-H and bulk H species would lose the H atom and only surface OH species remained.8,10 To examine the temperature influence, we performed simulations at lower temperatures from 100K to 500K. We found H2 hardly dissociate below 200K. Although H2 dissociation happens above 300K, we still have not detected Zn-H group and bulk H species. Thus more work needs to be done to clarify this issue. The surface hydroxyl is mainly formed through combining an H atom with the surface O atom (Equation (4)) or from water dissociation (Equation (5)) that eventually produces two surface hydroxyls. The latter has been verified experimentally.7,27–32 In addition, migration of the surface H atom from one surface O atom to another or migration of a surface OH group from one Zn atom to another can also happen, though the above migration processes do not change the extent of surface 9



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hydroxylation. According to our simulations, on the (0001) surface most of the dissociated H atoms convert into H2O which tends to desorb into the gas phase (Figure 4 (a)) and there are fewer OH groups on either the Zn or O termination. On the other hand, the dissociated H atoms prefer to exist in hydroxyl on the (1010) surface (Figure 4 (b)). This result exemplifies the different responses of the two surfaces to the environment: (0001) is easily to be reduced while (1010) shows the propensity to be hydroxylated. Owing to surface reactions and water desorption, a change in crystal structure was observed. Detailed discussion is presented in the section Structural Evolution of Different ZnO Models. ZnO(s) + H(g) → ZnO…H(s) → ZnO-H(s)

(4)

ZnO(s) + H2O(g) → H2O … ZnO(s) → HO … ZnO … H(s) → HO-ZnO-H(s) (5)

Figure 4. Side view of (0001) (a) and (1010) (b) surfaces after 2.5 ns simulation at 1200 K in the presence of H2.

Water Formation. Water is the final product of the H2 dissociative reaction on ZnO. According to our simulations, there are three different mechanisms of H2O formation. The first mechanism (Figure 5 (a) and Equation (6)) happens at the early 10


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stage of the simulation in which an H2 molecule approaches an O atom, leading to water formation after overcoming a triangular transition structure. The second mechanism shown in Figure 5 (b) and Equation (7) takes place when the surface hydroxylation reaches to a certain extent, in which a gaseous atomic H collides with the O atom of a surface hydroxyl and then a surface water is generated which later may desorb into the gas phase. The third mechanism (Figure 5 (c)) is illustrated in the Equation (3) in which H2 attacks an OH group and ends with the formation of a water molecule and an active H atom. ZnO(s) + H2(g) → ZnO…H2(s) → Zn (s) + H2O(g)

(6)

ZnO-H(s) + H(g) → Zn O …H(s) → Zn(s) + H2O(g) ︙ H

(7)

Figure 5. Illustration of H2O formation from ZnO (s) + H2 (g), i.e. Equation (6), (a), ZnO-H (s) + H (g) i.e. Equation (7), (b) and ZnO-H (s) + H2 (g), i.e. Equation (3), (c) respectively. Panels from left to right represent the initial state, approximate transition state and final state respectively.

Figure 6 displays the variation of number of gaseous water per surface area (per nm2) with simulation time. As can be seen in Figure 6, water appears first on the (101 11



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0) surface (black curve). It oscillates significantly, implying that the formed water re-adsorbs on the surface and even dissociates. In fact, as mentioned above and shown in Figure 4 (b), there are hydroxyl groups on the (1010). Hence, we believe most re-adsorbed water dissociates on this surface. Beyond ~750 ps, there are notable gaseous water, although re-adsorption still happens. Water formation begins at 250 ps and there is a sharp increase on the (0001), indicating that as soon as water is formed, it desorbs into the gas phase. The red curve for the (0001) fluctuates much less than the black one for the (1010), which can be rationalized by the ready desorption of water from the (0001) surface (see above and Figure 4 (a)). Recall there is an initiation phase of ~500 ps for H2 dissociation on the cluster (Figure 3), and after that phase H2 dissociates more quickly than on (1010) and (0001). The same initiation phase appears in Figure 6 (blue curve). After the phase the gaseous water content increases linearly, with a slope higher than those for the two surfaces, which is consistent with higher activity of the cluster than that of the surfaces.

Figure 6. Time variation of number H2O (g) divided by the surface area of the ZnO (1010) and (0001) surfaces and the cluster at 1200K.

Temperature Influence on Surface Reactions. Above analyses show that the cluster has the highest activity towards H2 dissociation. In the following we investigate the temperature effect on the reactions over the cluster. Simulations were 12


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performed at 600K, 900K and 1200K. To reduce the fluctuation of the simulation results, 10 independent simulations were carried out at each temperature to get an averaged simulation result. Figure 7 shows the initial structure (IS) after energy minimization and the final structures (FS) of the ZnO-H2 system selected from one simulation at three different temperatures. The time evolution of the average number of gaseous atomic H (g) species and H2O (g) molecules is depicted in Figure 8. As expected, more H2O molecules and surface hydroxyl groups are yielded at 1200K, followed by the 900K system. Zn aggregation is observed only at 1200K, while no such phenomenon is detected with the other two systems at lower temperatures, due to the simulation time.

Figure 7. Comparison of the initial and final structures of the ZnO nanocluster in the presence of H2 at different temperatures after 2.5 ns simulation.

The first atomic H species appears the earliest at 1200K, followed by that at 900K (See the upper panel in Figure 8), meaning that the higher the temperature, the more quickly the H2 molecules dissociate. The lifetime of the produced gaseous atomic H must be short, because the number of H (g) atoms are less than 2.0 even at 1200K. Remember gaseous H2 decreases fast after 500 ps on the cluster (see Figure 3). Hence the fewer gaseous atomic H must be owing to formation of water and/or hydroxyl. Indeed, water appears simultaneously with the occurrence of atomic H 13



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atoms (see the same initiation time in the upper and lower panels in Figure 8). Beyond ~1.0 ns, gaseous water roughly increases linearly with both temperature and time. For example, there are ~7 molecules in the box (~ 0.012 mol/L) at 1200K, 3.5 at 900K and 1.5 at 600K at 2500 ps.

Figure 8. Time evolution of the number of atomic H (g) and H2O (g) produced from the nanocluster ZnO system at 600K, 900K and 1200K. The number of H (g) and H2O (g) were averaged from10 independent RMD simulations.

Estimation of the Rate Constants. First-order kinetics of the dissociative adsorption of H2 molecules over ZnO cluster is assumed to study the H2 consumption rate constant at six different temperatures from 500K to 1000K with an interval of 100K. According to the first-order model, the rate constant k is proportional to the ሾு ሿ

value ݈݊ ሾுమሿ೟ where the arbitrary time t’s concentration [H2]t is estimated with the మ బ

remaining number of H2 molecules at time t. Based on Arrhenius equation, the activation energy of H2 dissociation over ZnO cluster is obtained by fitting the attained rate constants at different temperatures. 14


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Figure 9. Time variation of number of the H2 (g) molecules at various temperatures for the nanocluster ZnO system. The H2 (g) number was averaged for 10 independent RMD simulations.

Figure 9 depicts the time evolution of the gaseous H2 number by averaging 10 independent RMD simulations from 500K to 1000K. As shown in Figure 9, the number of H2 molecules remains to be 20 until ~750 ps. Beyond ~750 ps, it begins to decrease as a function of the temperature. The higher the temperature, the more drastically H2 decreases. The estimated rate constants of H2 dissociation are displayed in Table 1 for each temperature. An activation energy, 4.06 kcal/mol, derived from the fitting of Arrhenius equation (as shown in Figure 10) is in very good accordance with the experimental value of 5.0 kcal/mol,20 indicating that our models and the methodology adopted are reasonable. Previously a DFT calculated barrier of 11.5 kcal/mol for the elementary reaction is reported.25 Here one can see that the apparent activation energy and elementary step activation energy differ notably. Using the same procedure we estimated the rate constants of water formation to be 0.64, 1.80 and 4.21 ns-1 for 600, 900 and 1200K respectively and an activation energy to be 4.02 kJ/mol.

Table 1. Fitted rate constants of H2 dissociation at different temperatures T/K

500

600

700

800

15


900

1000


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k*10-8/s-1

1.56

2.01

4.47

5.93

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7.93

11.20

Figure 10. Relationship between reciprocal temperature (1000/T) and the rate constants (lnk) for the dissociation of H2 on the ZnO nanocluster system.

Structural Evolution of Different ZnO Models. Our simulations reveal that the three ZnO models behave quite differently in the presence of hydrogen. Both upper and lower surfaces of the (1010) are active for H2 dissociation because of existence of exposed O sites, and the formed H2O can either desorb into the gas phase or convert into surface hydroxyls. The depletion of the surface oxygen atoms deforms the surface morphology of (1010). The average height difference of Zn- and O- layer increases from 0.0 Å to 0.12Å with the Zn- layer higher than the O-layer, and the maximum difference increases to 3.04Å compared to 0.94Å in the initial structure. Dissociation of H2 and formation of H2O only happen on the O-terminated (0001) surface. Water formation and desorption leads to the formation of O vacancies and of Zn-Zn bonds. The average height difference of Zn- and O- layer reduces to 0.54/0.47Å from 0.61 Å and maximum height difference increases by 1.08/0.83Å for the O-/Zn- terminated surfaces. Diffusion of H atom into the sub-surface or interior bulk has not been observed with both of the slab models. 16


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Regarding the cluster model, there is an increasing tendency of Zn atoms aggregation with desorption of water. In one of our simulations at 1200K, there are 8 water molecules in the gas phase at 2.5 ns, meaning 8 O atoms have been removed from the ZnO cluster. It can be expected that with surface oxygen depleted, Zn atom will dominate the surface, indicating that ZnO surface tends to be metallized in the H2 atmosphere. To verify this deduction, we simulated the cluster model with 100 H2 molecules for 100ns (Figure 11). In this case most of the surface area is occupied by Zn atoms. In other words, the surface is going to be metallic. It can be expected that more Zn atoms will aggregate with higher H2 pressure at longer simulation time. It is reported that8 the ZnO surface becomes metallic when exposed to the atomic hydrogen atmosphere. Here our simulation results furnish a direct evidence of surface metallization at the atomic scale.

Figure 11. Illustration of surface metallization or surface Zn aggregation of the ZnO nanocluster after 2.5 ns simulation at 1200K with 20 initial H2 molecules (a) and after 100 ns simulation at 1200K with 100 initial H2 molecules (b).

CONCLUSIONS We simulated ZnO in H2 atmosphere using ReaxFF method. Our ReaxFF

molecular dynamics simulations reveal that there are two pathways for H2 17



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dissociation and three routes for water formation on the surfaces. At variance with previous conclusion that both Zn and O sites are active for H2 dissociation, we found Zn sites are not active for the dissociation. On the (0001) surface, H2 dissociation and water formation happens on the O-termination. The Zn-terminated surface can inhabit the formed water, though it is inactive for H2 dissociation. While the (1010) surface is favorable for surface hydroxyl inhabitation, the (0001) surface tends to be reduced because of easy desorption of water. Our simulations demonstrated that the cluster is more active for the dissociation and water formation than the two surfaces. It is revealed that surface O atoms tend to displace outwards in the two surface models and water desorption depletes the surface O atoms, leading to Zn aggregation and surface metallization, in agreement with experiments. Based on the first-order kinetics model, we estimated the rate constants and activation energy of the H2 dissociation and water formation. The result, 4.06 kcal/mol for H2 dissociation, agrees nicely with the experimental value of 5.0 kcal/mol.

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. (Z. X. Chen) ORCID

Zhao-Xu Chen: 0000-0002-5444-776X Notes

The authors declare no competing ﬁnancial interest.

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ACKNOWLEDGEMENTS Prof. van Duin is greatly acknowledged for the helpful guidance of ReaxFF. The

authors also thank the financial support from the Major Research Plan of the National Natural Science Foundation of China (No. 91545118) and the Natural Science Foundation of China (No. 21273103). Calculations were carried out at the high performance computing center of Nanjing University and National Supercomputing Center in Shenzhen.

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TOC Graphic

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ReaxFF Molecular Dynamic Simulations of ZnO Nanocluster and Films

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