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Article

Comparison of H2O Adsorption and Dissociation Behaviors on Rutile (110) and Anatase (101) Surfaces Based on ReaxFF Molecular Dynamics Simulation

by
He Zhou
,
Heng Zhang
and
Shiling Yuan
*
Key Lab of Colloid and Interface Chemistry, Shandong University, Jinan 250100, China
*
Author to whom correspondence should be addressed.
Molecules 2023, 28(19), 6823; https://doi.org/10.3390/molecules28196823
Submission received: 29 August 2023 / Revised: 21 September 2023 / Accepted: 25 September 2023 / Published: 27 September 2023
(This article belongs to the Special Issue Advances in Molecular Modeling in Chemistry)
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Abstract

:
The relationship between structure and reactivity plays a dominant role in water dissociation on the various TiO2 crystallines. To observe the adsorption and dissociation behavior of H2O, the reaction force field (ReaxFF) is used to investigate the dynamic behavior of H2O on rutile (110) and anatase (101) surfaces in an aqueous environment. Simulation results show that there is a direct proton transfer between the adsorbed H2O (H2Oad) and the bridging oxygen (Obr) on the rutile (110) surface. Compared with that on the rutile (110) surface, an indirect proton transfer occurs on the anatase (101) surface along the H-bond network from the second layer of water. This different mechanism of water dissociation is determined by the distance between the 5-fold coordinated Ti (Ti5c) and Obr of the rutile and anatase TiO2 surfaces, resulting in the direct or indirect proton transfer. Additionally, the hydrogen bond (H-bond) network plays a crucial role in the adsorption and dissociation of H2O on the TiO2 surface. To describe interfacial water structures between TiO2 and bulk water, the double-layer model is proposed. The first layer is the dissociated H2O on the rutile (110) and anatase (101) surfaces. The second layer forms an ordered water structure adsorbed to the surface Obr or terminal OH group through strong hydrogen bonding (H-bonding). Affected by the H-bond network, the H2O dissociation on the rutile (110) surface is inhibited but that on the anatase (101) surface is promoted.

1. Introduction

TiO2 is an important photocatalyst in photocatalytic hydrogen production and photooxidation of organic pollutants [1,2,3]. The interaction of H2O with TiO2 in an aqueous environment plays an important role in many practical applications [4]. As a fundamental process, water splitting is one of the most important chemical reactions [5]. It affects the reactivity, surface chemistry, and overall performance of the material. For the dissociation of H2O by the reaction H2O → OH− + H+, reactive energy is required and gained from the catalyst or charge transfer [6,7].
Among surfaces relevant to heterogeneous catalysis, rutile and anatase have been extensively investigated for their interaction with water, owing to their photocatalytic activity. To study the adsorption and dissociation characteristics of H2O on rutile (110) and anatase (101) surfaces, extensive research using experiments and theoretical calculations has been carried out in the past decades [8]. Many scholars confirmed that H2O dissociated at the Ti5c site on the surface of defect-free rutile (110) [9,10]. Using scanning tunneling microscopy (STM), Tan et al. indicated that the dissociative state of H2O was more stable than the molecular state of H2O at the Ti5c site on the rutile (110) surface [11]. Additionally, the adsorption of H2O on the anatase (101) surface was in the molecular form rather than the dissociated form under the vacuum condition [12,13,14].
At the atomic level, it is still a great challenge to characterize the adsorption and dissociation behavior of H2O on the TiO2 surface under an aqueous environment in experiments. There are still some unsettled fundamental problems and controversies. For example, the nature of adsorbed water, the extent of H2O dissociation, and the generation of surface hydroxyl species on the perfect TiO2 surface remain controversial. Some experimental measurements and theoretical investigations suggested that molecular adsorption mechanism or mixed adsorption mechanisms may occur simultaneously at room temperature [12,15,16,17,18,19]. Additionally, Ángel et al. directly determined that the terminal hydroxyl group and the bridge hydroxyl group were easily formed on the anatase TiO2 surface in an aqueous environment [20,21]. Under this circumstance, even in the well-defined crystal plane, the adsorption and dissociation mechanism of H2O on the TiO2 surface remains incompletely elucidated [16]. Therefore, it is still necessary to clarify the adsorption and dissociation behavior of H2O on the TiO2 surface in the real environment.
In the real environment, there is an intermolecular interaction between adsorbed and non-adsorbed H2O in bulk water. By the first-principle calculation, double or triple layers of water can be found on TiO2 surfaces [22,23]. Additionally, some in situ experiments were also carried out. Using STM measurement, Tan et al. confirmed that the interfacial H-bond facilitated the dehydrogenation of H2O on the rutile (110) surface [11]. Yang et al. found that the dissociation of H2O on the rutile (110) surface was suppressed at high coverage [24,25]. However, the probability of H2O dissociation on the anatase (101) surface remained constant with the increase in H2O coverage [26]. An STM experiment showed that the H-bond network on the anatase (001) surface promoted hydrolysis dissociation [27]. However, the role of the H-bond network in H2O dissociation on rutile (110) and anatase (101) surfaces has not been completely revealed yet. Hence, it is essential to explore the role of the H-bond network on H2O adsorption and dissociation on rutile (110) and anatase (101) surfaces.
As mentioned above, there are still two problems that need to be solved. One is the adsorption and dissociation mechanism of H2O on H2O–TiO2 interfaces in an aqueous environment. The other is that the water structure formed on TiO2 surfaces may be affected by the strength of H-bonding. For this purpose, reactive molecular dynamics (RMD) with ReaxFF is employed to study the atomic and molecular behavior of H2O on H2O–TiO2 heterogeneous interaction in an aqueous environment. Here, we demonstrate the significance of subtle surface structures in water dissociation on rutile and anatase TiO2 in aqueous environments. This study will offer strategies to achieve efficient catalysis via matching proper surface structures with targeted reaction characteristics. Additionally, the double-layer model is proposed to describe the water structure on rutile (110) and anatase (101) surfaces theoretically. The result shows that the two problems mentioned above are illustrated clearly.
The rest of the paper is organized as follows. Section 3 introduces the main methods such as the ReaxFF force field. The results and discussion are given in Section 2. The conclusions are summarized in Section 4.

2. Results and Discussion

2.1. Adsorption and Dissociation Mechanism of H2O on Rutile (110)

Among various rutile surfaces, rutile (110) is the most stable one and it is widely discussed [28]. Figure 1 shows the RMD simulation snapshot of the water distribution with the coverage of 2.0 ML on the rutile (110) surface. Here, the adsorption and dissociation of H2O at the Ti5c site are observed. In Figure 1, the blue ball represents the dissociated H2O at the Ti5c site. As shown in Figure 2a,c, the result shows that the mixed state containing the molecular and dissociative adsorption of H2O on the rutile (110) surface is favorable. For molecular adsorption, the O atom of H2Oad forms a coordination bond with the surface Ti5c atom, and the length of the Ti5c-Oad bond is 2.29 Å in Figure 2a. The terminal H2Oad forms an intermolecular H-bond with the surface Obr, in which the distance of the H-bond is 1.35 Å. The interfacial H-bond can effectively assist proton transfer and exchange across the surface. Figure 2b shows that the generated OH group (OHad) is stably adsorbed at the Ti5c site, where the length of the Ti5c-O bond is 1.80 Å. This result is consistent with other research using STM experiments and DFT calculations [11,29]. For dissociative adsorption, the terminal free H2O (H2Of) can easily combine with the surface Obr, resulting in a free OH group (OHf) and an ObrH, as shown in Figure 2c,d. The formation of ObrH promotes the dissociation of H2Of. Subsequently, the generated OHf group is stably adsorbed at the adjacent Ti5c site with the length of the Ti5c-O bond being 2.04 Å. Alternatively, the OHf group recombines with the ObrH. Additionally, the indirect dissociation mechanism is also observed on the rutile (110) surface. As shown in Figure 2e,f, the H2O in the second layer donates an H-bond to an adjacent Obr and transfers its proton to the Obr. Simultaneously, the H2O in the second layer receives a proton from the H2Oad at the Ti5c site. This is consistent with other results of DFT studies [30]. However, this indirect proton transfer involves at least two proton transfers, whose sequential occurrence is less likely than the direct proton transfer between the H2Oad and the surface Obr.
To manifest the interfacial character more clearly, Figure 3 shows the radial distribution function (RDF) of Ti5c-Ow (Ow represents the O from H2O) at the coverage of 3.0 ML. The first peak of r(Ti5c-Ow) is about ~1.85 Å, which corresponds to terminal hydroxyl groups (OHad) from dissociative H2O molecules on the rutile (110) surface. The second peaks of r(Ti5c-Ow) appear at about ~3.55 Å and ~3.75 Å. They correspond to the H2O in the second layer connected with the surface Obr and terminal OH through the H-bond network. The water above the second layer can be regarded as bulk water. These results are consistent with the simulation data obtained by Předota et al. [31]. They observed the first layer of oxygen from the terminal OH group at the top of Ti5c sites with a distance of ~1.9–2.4 Å, and the second layer of water appears at about ~3.8 Å. The double-layered structure of water is also obtained by Mamontov and co-workers [32,33]. As illustrated on the right of Figure 3, the dashed red line represents the H-bond network between the second layer of water and surface Obr or OH groups, which affects water dissociation.
To investigate the effects of the H-bond network for H2O dissociation on the rutile (110) surface, the RMD simulation under different initial coverage is shown in Figure 4a,b. The result shows that the amount of water dissociation (AWD) reaches a maximum value at 1.5 ML coverage with the increase in water coverage. The result is consistent with the previous ab initio MD simulation by Bandura et al. [34]. The possible reason is that the H-bond network formed by the second layer of water inhibits the direct dissociation of H2O to a certain extent. At low coverage (<1.5 ML), there is a direct proton transfer between the terminal H2Oad molecule and a nearby surface Obr. At higher coverage (>1.5 ML), H2O in the second layer shares the H-bonding with the surface Obr and OH group. The direct dissociation of H2Oad is inhibited because the surface Obr is occupied by the H2O in the second layer. However, the possibility of H2Oad dissociation via the indirect proton transfer is relatively low. This demonstrates that the dissociation of H2O will be suppressed at high coverage on the rutile (110) surface. These results confirm the observation results of the STM experiment by Yang et al. [24]. They suggested that the reaction of H2O dissociation was strongly suppressed as the coverage of water increases on the rutile (110) surface. Through the statistics and ensemble averaging of these microscopic processes, our RMD results predict macroscopic properties well.

2.2. Adsorption and Dissociation Mechanism of H2O on Anatase (101)

Anatase (101) is the lowest-energy surface of the anatase TiO2 polymorph [35]. Figure 5 shows the equilibrium trajectory snapshot of the water distribution over anatase (101) at the coverage of 2.0 ML, in which the blue ball represents the dissociated H2O at the Ti5c site. As shown in Figure 6a, the simulation result shows that molecular adsorption is favorable on the anatase (101) surface, which matches with the observation in the experimental result [12]. For molecular adsorption, the O atom of H2Oad forms a coordination bond with the surface Ti5c atom, and the length of Ti5c-Oad is 2.03 Å in Figure 6a. The H2O in the second layer shares the H-bond with both the H2Oad and surface Obr, and forms a water layer in close contact with bulk water. The simulated snapshot in Figure 6b describes the dissociation progress of H2Oad on the anatase (101) surface. The H2O in the second layer provides a cascaded channel for the proton transfer from the H2Oad to the surface Obr. The RMD simulation result is consistent with the DFT calculation by Selloni et al. [18]. However, during the reaction on the anatase (101) surface, no direct proton transfer is observed between the terminal H2Oad molecule and a nearby surface Obr. This behavior is contrary to that of the rutile (110) surface investigated in this paper. As shown in Figure 6a and Figure S3, the H-bond between the H2Oad and the H2O in the second layer is 1.52 Å, and that between H2Oad and the nearby Obr is 2.50 Å. The larger distance between H2Oad and the Obr site makes the direct proton transfer unfavorable on the anatase (101) surface, which is consistent with the calculation result by others [18,36]. The varied behavior of the water dissociation is found to be related to the subtle structure difference of surface Ti5c and Obr sites on rutile (110) and anatase (101). As shown in Figure 7a,b, the rutile (110) surface is flat with Obr or Obr2− bound to 6-fold-coordinated Ti cations and is projected out of the surface plane, whereas the anatase (101) surface has a terraced structure and exposes Obr or Obr2− at the step edge. The distance between Ti5c and Obr is about 3.55–3.56 Å on the rutile (110) surface, while that distance is about 3.85 Å on the anatase (101) surface. The larger distance between Ti5c and Obr makes the transfer of H atom from H2Oad to Obr difficult on the anatase (101) surface. Therefore, for the dissociation of H2Oad on the anatase (101) surface, it is necessary to assist proton transfer through the H-bond network.
In order to illustrate interfacial characteristics more clearly, Figure 8 shows the RDF of Ti5c-Ow on the anatase (101) surface at the coverage of 3.0 ML. The first peak of r(Ti5c-Ow) is estimated at ~1.85 Å, corresponding to the terminal OHad group. The second peak of r(Ti5c-Ow) appears at about ~3.85 Å, which corresponds to the second layer of H2O. The H2O in the second layer is connected to the surface Obr and terminal OH group through the H-bond network. As shown in Figure 8, the dashed red line represents the H-bond network between the first layer and the second layer of water. The water above the second layer can be regarded as bulk water. The double-layer model explains the experimental and theoretical results well [37,38,39]. These results are consistent with the simulation data obtained by Sumita and co-workers [39]. They showed that the O atom in the first layer was consistent with dissociated H2Oad at the Ti5c site, and the RDF of the first peak was at ~1.82 Å by DFT.
To investigate the effect of the H-bond network for H2O dissociation on the anatase (101) surface, Figure 9a,b exhibit the change of the AWD with different initial coverage in the RMD simulation. However, it is worth noting that the AWD monotonically increases with the increase in coverage. The result suggests that the H2O in the second layer participates in and assists the dissociation of H2Oad. This phenomenon is contrary to the results of the rutile (110) surface studied in this paper. The main reason is that the dissociation of H2O happens in different ways on rutile (110) and anatase (101) surfaces. The dissociation of H2O on rutile (110) is mainly driven by the proton transfer directly to the surface Obr, and the H-bond network between the first layer of water and the second layer of water may greatly reduce the dissociation of H2O, while the indirect proton transfer on the anatase (101) surface needs to be assisted by the H2O in the second layer.

2.3. The Roles of the H-Bond Network in Water Dissociation

As discussed above, the H-bond network, which is ubiquitous in a practical aqueous environment, plays a crucial role in the dissociation of H2O on rutile (110) and anatase (101) surfaces. This paper suggests the double-layer model on rutile (110) and anatase (101) surfaces: the first layer is the dissociated H2Oad at the Ti5c site, and the second layer is defined as the H2O adsorbed onto the Obr or terminal OH group through H-bonding. Simulated snapshots in Figure 10a,b show the ordered H-bond network geometry of the second layer of water on the rutile (110) surface at 2.6 ps and the anatase (101) surface at 3.5 ps, respectively. It is observed that the H2O in the second layer adsorbed on Obr through strong H-bonding interaction. To further understand the effect of strong H-bonding in the dissociation of H2O on TiO2 surfaces, this paper goes on to investigate the property of the H-bond network in the second layer of water. Figure 10c,d and Figure 11a show the RDFs of O-H on the rutile (110) surface, anatase (101) surface, and bulk water, respectively. The first peak represents the distance of the intramolecular O-H bond. The second peak of r(O-H) corresponds to the strong H-bonding between the H2O in the second layer and the surface Obr or terminal OH group. For convenience, the second peak of r(O-H) is labeled as r2(Oad-Hw). As shown in Figure 10c,d, the r2(Oad-Hw) of rutile (110) and anatase (101) is about ~1.63 Å. For comparison, the r2(Oad-Hw) in bulk water is estimated at ~1.78 Å in Figure 11a. The shorter r2(Oad-Hw) on rutile (110) and anatase (101) surfaces suggests a stronger H-bonding interaction between the H2O in the second layer and the surface Obr or terminal OH group.
Figure 11b–d, respectively, calculate the RDFs of O-O on the rutile (110) surface, anatase (101) surface, and bulk water. The first peak of r(O-Ow) represents the distance from the surface Obr or Oad to the H2O in the second layer on rutile (110) and anatase (101) surfaces. As shown in Figure 11b, the first peak of r(Ow-Ow) is located at about ~2.78 Å in the bulk water, which is in accord with the experimental measurement and DFT calculations [40,41]. Compared with that in the bulk water, the first peak of r(O-Ow) on rutile (110) is estimated at ~2.68 Å in Figure 11c, and that on anatase (101) is about ~2.63 Å in Figure 11d. The shorter r(O-Ow) indicates the strong interaction between the H2O in the second layer and the surface Obr or OH group on rutile (110) and anatase (101) surfaces. Therefore, the H-bonding between the surface Obr or terminal OH group and the second layer of water appears to be stronger than the H-bonding of ordinary H2O-H2O in bulk water.
Figure 12 shows the change of surface Obr and Oad atomic charge during the reaction process. As illustrated in Figure 12a,d, the mean charge of surface Obr atoms is close to −0.65 e on the rutile (110) surface, and that on the anatase (101) surface is about −0.70 e before the dissociation reaction of H2O. As shown in Figure 12b,c,e,f, the mean charge of Obr and Oad atoms dramatically decreases until the reaction is completed. The charge of surface Obr and Oad atoms is roughly stable after the reaction. The mean charge of Obr and Oad on rutile (110) is −0.77 e and that on anatase (101) is about −0.80 e. In Figure 12c,f, surface Obr and Oad atoms are more electron-rich than Ow in bulk water (−0.71 e). Correspondingly, the polarization of surface Obr and Oad is enhanced. It is indicated that the strong H-bonding between the surface Obr or Oad group and the second layer of water is formed. These results provide insights to reveal the role of the H-bond network for water dissociation on rutile (110) and anatase (101) surfaces. More specifically, the H-bond network inhibits the dissociation of H2O at high coverage on the rutile (110) surface. But the H-bond network may facilitate the dissociation of H2O by linking the proton transfer channel on the anatase (101) surface.

3. Methods

In this paper, molecular dynamic simulation with ReaxFF force field [42,43] is employed to investigate the behavior of H2O on rutile (110) and anatase (101) surfaces. The force field parameters are determined from quantum mechanics (QM) based on training sets and experimental results, which can ensure the accuracy of the RMD simulations. The ReaxFF method developed by van Duin uses the bond order relation obtained from the interatomic distance [43,44], which is updated at each RMD or energy minimization step. It allows continuous bond dissociation for all orders at the same time. Therefore, the ReaxFF can be used to describe chemical reactions, including bond formation and bond breaking [42].
In the ReaxFF reactive force field, the total (system) energy is given by [45]
E system = E bond + E over + E under + E lp + E val + E vdWaals + E coulomb
The terms in Equation (1) include bond energies (Ebond), the energy to penalize over-coordination of atoms (Eover), the energy to stabilize under-coordination of atoms (Eunder), lone-pair energies (Elp), valence-angle energies (Eval), van der Waals interactions (Evdwaals) and terms to handle nonbonded Coulomb (Ecoulomb), respectively.
This paper employs the Ti/O/H ReaxFF interatomic potential, which is developed by Kim et al. [45]. The force field is carefully used and validated in previous research on the H2O–TiO2 interface [46,47,48,49]. Using the large-scale atomic/molecular massively parallel simulator (LAMMPS) package [50,51], the RMD calculation with ReaxFF is capable of simulating systems larger than 106 atoms in nanosecond time scales.
The general flow chart of the molecular dynamics simulation is shown in Figure 13. Rutile (110) and anatase (101) surfaces are carved from bulk rutile and anatase TiO2 crystals, respectively. The dimensions of simulation cells are 59.18 Å (x) × 64.97 Å (y) and 67.96 Å (x) × 61.26 Å (y) for rutile (110) and anatase (101) surfaces in Figure S1a,b, respectively. Cleaved rutile (110) and anatase (101) surfaces are composed of four-layer TiO2 slabs. Their bottom two layers are fixed in the bulk configuration to simulate the bulk-like environment. There are 200 and 216 Ti5c reactive sites on rutile (110) and anatase (101) surfaces, respectively. In all discussions that follow, one monolayer (ML) is defined as the number of Ti5c sites on rutile (110) and anatase (101) surfaces. H2O molecules are placed over rutile (110) and anatase (101) surfaces with a coverage of 0.25, 0.50, 0.75, 1.0, 1.5, 2.0, and 3.0 ML. The simulation box is constructed with periodic boundary conditions in both the X and Y directions. And the fixed boundary condition is applied along the Z direction. To avoid interaction between H2O and the bottom of TiO2, a reflecting wall is used at the top of the box along the Z direction. The RMD simulation is performed in the canonical ensemble (NVT) with the time step of 0.25 fs. The conjugate gradient (CG) approach is used to minimize energy. During the simulation, the ambient temperature of 300 K is constantly controlled by the Nosé–Hoover thermostat with a 50 fs damping constant [52]. The velocity Verlet algorithm is employed to calculate Newton’s equation of motion. The atomic charge is equilibrated at every time step using the QEq (charge equilibration) model. Ovito is employed to generate snapshots of the simulation. In this paper, all systems reach equilibration after 200 ps, which can be monitored by the convergence of potential energy in Figure S2.

4. Conclusions

In this paper, the ReaxFF RMD simulation is employed to investigate the adsorption and dissociation mechanisms of H2O on rutile (110) and anatase (101) surfaces in an aqueous environment. Furthermore, this paper explores the vital role of the H-bond network in understanding the underlying mechanisms for water dissociation at a deeper level. Here are several significant findings and conclusions from this paper:
(1)
There is a mixed adsorption trend with both molecular and dissociative adsorption on the rutile (110) surface. Compared with that on the rutile (110) surface, molecular adsorption is dominant on the anatase (101) surface.
(2)
The dissociation of H2O is mainly the direct dissociation on the rutile (110) surface. The interfacial H-bond between the adsorbed H2Oad molecule and the surface Obr promotes proton transfer for H2O dissociation on the rutile (110) surface. Compared with that on the rutile (110) surface, the dissociation of H2O is dominated by indirect proton transfer on the anatase (101) surface. This different catalytic function is solely determined by the distance between Ti5c and Obr on the surface, which determines the behavior of water dissociation.
(3)
The H-bond network plays a crucial role in the dissociation of H2O on rutile (110) and anatase (101) surfaces. At high coverage (>1.5 ML), the H-bond network structure of the second layer of water on the rutile (110) surface inhibits the dissociation of H2O to some extent. Compared with that on the rutile (110) surface, the RMD simulation shows that H-bond could assist the proton transfer on the anatase (101) surface. In an aqueous environment, the dissociation of H2Oad is promoted by the enhanced H-bond network structure of the second layer of water on the anatase (101) surface.
Overall, this paper provides a meaningful insight to understand the behavior of H2O adsorption and dissociation on TiO2 surfaces in an aqueous environment. It is hoped that the findings reported here will motivate further experimental and theoretical work to achieve a complete understanding of this technologically relevant interface.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28196823/s1. Figure S1. Initial models of simulation are established. (a) H2O molecules on the rutile (110) surface at the coverage of 2.0 ML. (b) H2O molecules on the anatase (101) surface at the coverage of 2.0 ML. Grey, red, and white balls represent Ti, O, and H atoms, respectively. The upper H2O is represented by a stick model. Figure S2. The time evolution of potential energy on (a) the rutile (110) surface and (b) the anatase (101) surface during the NVT RMD simulation of water dissociation. Figure S3. The molecular absorption of H2O on the anatase (101) surface.

Author Contributions

Conceptualization, H.Z. (He Zhou) and S.Y.; Methodology, H.Z. (Heng Zhang); Software, H.Z. (Heng Zhang); Formal analysis, S.Y.; Data curation, H.Z. (He Zhou); Writing—original draft, H.Z. (He Zhou); Writing—review & editing, H.Z. (He Zhou). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by [Natural Science Foundation of Shandong Province] grant number [ZR2021MB040].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Not applicable.

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Molecules 28 06823 g001
Figure 1. The RMD simulation snapshot of water on rutile (110) at the coverage of 2.0 ML. The blue ball represents the O atom from the adsorbed and dissociated H2O.
Figure 1. The RMD simulation snapshot of water on rutile (110) at the coverage of 2.0 ML. The blue ball represents the O atom from the adsorbed and dissociated H2O.
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Molecules 28 06823 g002
Figure 2. Snapshots of H2O adsorption and dissociation on the rutile (110) surface: (a,b) represent direct dissociation of the H2Oad molecule at the Ti5c site; (c,d) represent the dissociation and adsorption process of H2Of molecule at the Ti5c site; (e,f) represent the indirect dissociation of H2Oad at the Ti5c site. Red, grey and pink balls represent O, Ti and H atoms, respectively.
Figure 2. Snapshots of H2O adsorption and dissociation on the rutile (110) surface: (a,b) represent direct dissociation of the H2Oad molecule at the Ti5c site; (c,d) represent the dissociation and adsorption process of H2Of molecule at the Ti5c site; (e,f) represent the indirect dissociation of H2Oad at the Ti5c site. Red, grey and pink balls represent O, Ti and H atoms, respectively.
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Figure 3. On the left is the RDF of Ti5c-O. On the right is the cartoon illustration of the H-bond network on the rutile TiO2 (110) surface. The red dotted line represents the enhanced H-bond between the first layer and the second layer of water, and the black dotted line represents the H-bond of ordinary H2O-H2O.
Figure 3. On the left is the RDF of Ti5c-O. On the right is the cartoon illustration of the H-bond network on the rutile TiO2 (110) surface. The red dotted line represents the enhanced H-bond between the first layer and the second layer of water, and the black dotted line represents the H-bond of ordinary H2O-H2O.
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Figure 4. (a) The amount of water dissociation varies with time under the different coverage. (b) The AWD changes with the coverage of water on the rutile (110) surface.
Figure 4. (a) The amount of water dissociation varies with time under the different coverage. (b) The AWD changes with the coverage of water on the rutile (110) surface.
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Molecules 28 06823 g005
Figure 5. The RMD simulation snapshot of water distribution on the anatase (101) surface at the coverage of 2.0 ML. The blue ball represents the O atom from the adsorbed H2O.
Figure 5. The RMD simulation snapshot of water distribution on the anatase (101) surface at the coverage of 2.0 ML. The blue ball represents the O atom from the adsorbed H2O.
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Figure 6. Snapshots of H2O adsorption and dissociation on the anatase (101) surface: (a) the molecular adsorption of H2Oad at the Ti5c site, and (b) the indirect dissociation of H2Oad at the Ti5c site.
Figure 6. Snapshots of H2O adsorption and dissociation on the anatase (101) surface: (a) the molecular adsorption of H2Oad at the Ti5c site, and (b) the indirect dissociation of H2Oad at the Ti5c site.
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Figure 7. The surface structures of (a) rutile (110) and (b) anatase (101).
Figure 7. The surface structures of (a) rutile (110) and (b) anatase (101).
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Figure 8. On the left is the RDF of Ti5c-O. On the right is the cartoon illustration of the H-bond network on the anatase TiO2 (101) surface. The red dotted line represents the enhanced H-bond between the first layer and the second layer of water, and the black dotted line represents the H-bond of ordinary H2O-H2O.
Figure 8. On the left is the RDF of Ti5c-O. On the right is the cartoon illustration of the H-bond network on the anatase TiO2 (101) surface. The red dotted line represents the enhanced H-bond between the first layer and the second layer of water, and the black dotted line represents the H-bond of ordinary H2O-H2O.
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Molecules 28 06823 g009
Figure 9. (a) The amount of water dissociation varies with time under the different coverage. (b) The AWD changes with the coverage of water on the anatase (101) surface.
Figure 9. (a) The amount of water dissociation varies with time under the different coverage. (b) The AWD changes with the coverage of water on the anatase (101) surface.
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Molecules 28 06823 g010
Figure 10. (a) A simulated snapshot of local water distribution at 2.6 ps on the rutile (110) surface with the coverage of 3.0 ML; (b) a simulated snapshot of local water distribution at 3.5 ps on the anatase (101) surface with the coverage of 3.0 ML; (c,d) RDFs of O-H on rutile (110) and anatase (101) surfaces, respectively.
Figure 10. (a) A simulated snapshot of local water distribution at 2.6 ps on the rutile (110) surface with the coverage of 3.0 ML; (b) a simulated snapshot of local water distribution at 3.5 ps on the anatase (101) surface with the coverage of 3.0 ML; (c,d) RDFs of O-H on rutile (110) and anatase (101) surfaces, respectively.
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Figure 11. RDFs of (a) Ow-Hw in bulk water; (b) Ow-Ow in bulk water; (c) O-Ow on the rutile (110) surface; and (d) O-Ow on the anatase (101) surface.
Figure 11. RDFs of (a) Ow-Hw in bulk water; (b) Ow-Ow in bulk water; (c) O-Ow on the rutile (110) surface; and (d) O-Ow on the anatase (101) surface.
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Molecules 28 06823 g012
Figure 12. Snapshot views of the local structures of the rutile (110) surface: (a) before the interaction at 0 ps, (b) after the reaction occurs at 10 ps, and (c) after the reaction occurs at 200 ps with the coverage of 1.5 ML. Snapshot views of the local structures of the anatase (101) surface: (d) before the interaction at 0 ps, (e) after the reaction occurs at 10 ps, and (f) after the reaction occurs at 200 ps with the coverage of 1.5 ML. The atom is colored by charge. Green, yellow and blue balls represent O atoms. Smaller and bigger brown balls represent H and Ti atoms, respectively.
Figure 12. Snapshot views of the local structures of the rutile (110) surface: (a) before the interaction at 0 ps, (b) after the reaction occurs at 10 ps, and (c) after the reaction occurs at 200 ps with the coverage of 1.5 ML. Snapshot views of the local structures of the anatase (101) surface: (d) before the interaction at 0 ps, (e) after the reaction occurs at 10 ps, and (f) after the reaction occurs at 200 ps with the coverage of 1.5 ML. The atom is colored by charge. Green, yellow and blue balls represent O atoms. Smaller and bigger brown balls represent H and Ti atoms, respectively.
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Molecules 28 06823 g013
Figure 13. Schematic diagram of the calculation step.
Figure 13. Schematic diagram of the calculation step.
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Zhou, H.; Zhang, H.; Yuan, S. Comparison of H2O Adsorption and Dissociation Behaviors on Rutile (110) and Anatase (101) Surfaces Based on ReaxFF Molecular Dynamics Simulation. Molecules 2023, 28, 6823. https://doi.org/10.3390/molecules28196823

AMA Style

Zhou H, Zhang H, Yuan S. Comparison of H2O Adsorption and Dissociation Behaviors on Rutile (110) and Anatase (101) Surfaces Based on ReaxFF Molecular Dynamics Simulation. Molecules. 2023; 28(19):6823. https://doi.org/10.3390/molecules28196823

Chicago/Turabian Style

Zhou, He, Heng Zhang, and Shiling Yuan. 2023. "Comparison of H2O Adsorption and Dissociation Behaviors on Rutile (110) and Anatase (101) Surfaces Based on ReaxFF Molecular Dynamics Simulation" Molecules 28, no. 19: 6823. https://doi.org/10.3390/molecules28196823

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