Dissociative Adsorption and Aggregation of Water on the Fe(100

Article pubs.acs.org/JPCC

Dissociative Adsorption and Aggregation of Water on the Fe(100) Surface: A DFT Study Rafael R. Q. Freitas,*,† Roberto Rivelino,*,‡ Fernando de Brito Mota,*,† and Caio M. C. de Castilho*,†,§ †

Grupo de Física de Superfícies e Materiais, Instituto de Física, Universidade Federal da Bahia, Campus Universitário da Federaçaõ , 40170-115 Salvador, Bahia, Brazil ‡ Instituto de Física, Universidade Federal da Bahia, Campus Universitário da Federaçaõ , 40210-340 Salvador, Bahia, Brazil § Instituto Nacional de Energia e Ambiente − INCT-EA, Campus Universitário da Federaçaõ , Universidade Federal da Bahia, 40170-280 Salvador, Bahia, Brazil ABSTRACT: Aggregation, molecular adsorption, and dissociation of water on the Fe(100) surface were investigated using spin-polarized density functional calculations. The preferential sites for H2O, HO, O, and H were carefully investigated on this surface. Also, the dissociation of H2O into H + OH species, and further OH into O + H species, was examined. The charge transfer mechanism during these dissociation processes, as well as of small water aggregates at different orientations on the Fe(100) surface, was studied within the Bader charge analysis. The coverage dependence on the adsorption properties was examined by comparing the results of a (2 × 2) with a (3 × 3) supercell. These calculations predicted that H2O is weakly adsorbed (physisorption) on hollow, bridge, and on-top sites, with the on-top site being slightly preferred for both coverages of 0.11 and 0.25 monolayer. As expected, OH was predicted to be strongly adsorbed (chemisorption) on the Fe(100) sites, producing a large charge transfer from the surface to p-orbitals of the O atom. A dissociation barrier of about 1.0 eV, for the dissociation H2O → OH + H, was calculated from the on-top site to the next most stable bridge and hollow sites, respectively. In contrast, a smaller barrier of ca. 0.8 eV was calculated for the dissociation of OH. Regarding the adsorption of small water aggregates on Fe(100), the present study has demonstrated that they are strongly reoriented on the surface in comparison to the isolated structures, leading to stable adsorbates. Most interestingly, the dissociation of a water molecule, after the dimer formation, leads to an energy barrier of 1.25 eV, about 25% higher than the corresponding value of an adsorbed single water molecule.

1. INTRODUCTION Chemical attack on a metal surface is boosted by the adsorption of water, which can preserve favorable conditions for a variety of surface reactions. For this reason, the interaction and decomposition of water on single-crystal surfaces have been a long-time issue of intense study and debate.1−3 Of special interest, mainly because of the economic impact of corrosion, experimental and theoretical studies on the iron reactivity have received a great deal of attention. For instance, it is possible to mention, on the experimental approach, oxidation processes on Fe(100) using spin-polarized Auger spectroscopy,4 temperature programmed spectroscopy, low energy electron diffraction, and Auger spectroscopy for investigating adsorption and reaction of thiophene on the Fe(100) surface,5 adsorption and decomposition of formaldehyde on Fe(100),6 and more recent techniques such as scanning tunneling microscopy in different situations as reviewed by Okuyama and Hamada7 and references therein. On the theoretical side, important examples are those using density functional theory (DFT) calculations as, for example, to explore adsorption and hydrogenation of CHX compounds on Fe(100),8 hydrogen interstitial diffusion through iron,9 oxygen adsorption on Fe(100) and Fe(110),10 © 2012 American Chemical Society

and adsorption and dissociation of sulfur and H2S on iron surfaces.11−13 When in contact with some metal surfaces, water is adsorbed and can dissociate under certain conditions.2,14 Indeed, the dissociation of water can lead potentially to a variety of chemical species at surfaces. Thus, the reasons which determine whether water will dissociate, and what are the final products of dissociation, have been an issue of great interest in experimental and theoretical studies. The simplest products experimentally found for the water dissociation in several metal surfaces are adsorbed hydroxyl, atomic oxygen, and atomic hydrogen.2,15 Also, hydroxyl can dissociate, giving rise to adsorbed atomic hydrogen and oxygen. In this direction, it is known that the water dissociation is thermodynamically favored in the case of iron surfaces.15,16 Interestingly, the effect of aggregation and hydrogen bonding in water strongly influences the adsorption and dissociation process in such metal surfaces.17 All of these issues will be considered in the present work concerning the Received: April 17, 2012 Revised: August 31, 2012 Published: September 4, 2012 20306

dx.doi.org/10.1021/jp303684y | J. Phys. Chem. C 2012, 116, 20306−20314

The Journal of Physical Chemistry C

Article

the maximum difference between the output and the input of each element of the density matrix in a self-consistent field cycle become smaller than 10−5. For the optimization of the parameters associated with the Fe(100) surface, a slab with five layers and a vacuum layer with 12.0 Å width were employed. This slab is the substrate for deposition in the following calculations. Both (2 × 2) and (3 × 3) unit cells were considered relative to the Fe(100) primitive one. During the relaxation process, the two top layers were allowed to relax while keeping the other three layers fixed, occupying the bulk positions. The unit cells, along the directions parallel to [100] and [010], have sizes 2 × 2 and 3 × 3 times the lattice parameter with a total number of atoms of 20 and 45, respectively. There are in fact two reasons for considering two different sizes for the unit cell. First, the possibility of adsorption of atoms/molecules on the surface and the study of the effects related specifically to this process would recommend minimizing the adsorbate × adsorbate interaction so that the lateral dimensions of the cell should be as great as possible. The second reason is to adequately investigate the differences in the results obtained by Eder and Terakura,21 using a (2 × 2) unit cell, and those obtained by Jung and Kang,22 using a (3 × 3) unit cell. A more recent work by Govender and collaborators23 has also employed a (2 × 2) unit cell. Thus, an interesting question to be answered in the present work is if the diffusion process calculated results are or are not dependent on the unit cell size. The climbing image nudged elastic band (CI-NEB) method36 was used to locate the minimum energy paths (MEPs) and the transition states for H2O and OH dissociation on Fe(100). Furthermore, the H2O dissociation into OH plus H after dimerization was also considered. The normal NEB37 was used for approximately 10 ionic steps to roughly converge the MEP, and then switch on to the climbing-image algorithm. The same force tolerance criterion for the transition search was employed, as done for structural relaxations. Additionally, the charge transfer between the water molecules and the Fe(100) surface by means of the Bader analysis24 on the charge density grid was calculated. The adsorption energies, as usually defined,9,23,38−41 were calculated by using the expression

dissociative adsorption and aggregation of water on the Fe(100) surface. More specifically related to the present work are the experimental findings concerning water reactions during adsorption of water on the Fe(100) surface18−20 and corresponding theoretical calculations.21−23 In recent work, Jung and Kang22 investigated the possibility of obtaining stable molecular sites of a single H2O molecule on Fe(100) and their dissociation mechanisms utilizing DFT calculations. Here, we have performed a systematic study for the adsorption of H2O, HO, O, and H, and the dissociation of H2O into H + OH species, as well as OH into O + H species on this iron surface. The charge transfer (CT) mechanism during the dissociation processes was carefully studied within the Bader charge analysis.24 We have also considered the cell size dependence on the adsorption properties, comparing results of using either a (2 × 2) or a (3 × 3) unit cell. Since recently hydrogen bond formation has been imaged on Cu(110),7 we have also investigated the interaction of small water clusters on Fe(100). Structure, charge transfer between substrate and adsorbate, and adsorption properties were calculated for clusters containing up to five water molecules at different orientations with respect to the surface sites. We have employed spin-polarized periodic DFT calculations in order to explore all the structural and electronic properties of water adsorbed on this surface. Our results were contextualized and compared with previous studies of water adsorbed on iron surfaces. Besides that, the effects of hydrogen bonding in the dissociation of water on the Fe(100) surface are also reported and discussed. After this Introduction, this paper contains the following sections: Computational Details, Results and Discussion, and Conclusion.

2. COMPUTATIONAL DETAILS First-principles calculations were carried out in order to investigate the H2O and HO adsorption, diffusion, and dissociation, as well as water aggregation, on Fe(100). These are based on spin-polarized periodic DFT25,26 as implemented in the VASP code.27−30 These calculations utilize the projector augmented wave (PAW) potentials31 and the generalized gradient approximation (GGA).32 The exchange-correlation functional was included, based on the approach of Perdew− Wang (PW91),32 since, as noted in the study of adsorption of the water molecule on the graphene surface,33 this functional has given more reliable values for the adsorption energy. Furthermore, PW91 is known to yield acceptable interaction energies for water clusters.34 The adopted cutoff energy was of 400 eV. In order to obtain the lattice parameter of bulk Fe, it was considered a primitive cell (one Fe atom per unit cell) which was three-dimensionally propagated according to the bcc symmetry. The optimized lattice parameter was calculated using a bcc unit cell sampled with a 15 × 15 × 15 Monkhorst− Pack k-point grid. The theoretical value thus obtained was 2.826 Å which can be compared with the most accepted experimental value of 2.87 Å,35 a difference less than 2.0% smaller. The total energy calculation and the surface structural relaxation were performed by sampling the Brillouin zone with a 6 × 6 × 1 Monkhorst−Pack grid. All of the considered geometries were fully relaxed so that the forces could become smaller than 0.01 eV/Å. Additionally, it was adopted, as a convergence criterion, that self-consistency is achieved when

Eads = E Total − E Fe(100) − Eadb

(1)

where the rhs first term is the total energy of the system, the second term is the energy associated to the isolated surface, and the last term (Eadb) corresponds to the energy of the isolated species, H, O, OH, H2O, and (H2O)n in the present case. In this work, the adsorption energy is referenced to gaseous hydrogen and oxygen molecules and also their atomic species. In cluster cases, the adsorption energies were calculated considering the cluster energies of the fully relaxed structures in vacuum and also in the adsorbate structure. This, consequently, allows an estimation of the cluster relaxation energy.

3. RESULTS AND DISCUSSION This work reports results for the clean iron surface, for separate adsorption of hydrogen, oxygen, and hydroxyl, water adsorption and dissociation. The final part of this section reports the results for adsorption of small water molecule aggregates on Fe(100) and also for the water dissociation into OH plus H in a dimer structure. 20307



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The structure and energy associated with the isolated hydroxyl radical were independently determined, using a box with vector sizes equal to the ones used in the five-layer cell, so that it was kept the same dimension for the box. This procedure was also adopted for exploring the adsorbed atoms of hydrogen and oxygen. The OH distance, after relaxation, was found as being 0.987 Å. For the results related to the isolated water molecule, the distances reported by Freitas and collaborators were simply used.29 Three different possible sites for the adsorbates were considered and are schematically displayed in Figure 1, being denoted by T (top), B (bridge), and H (hollow).

Table 1. Adsorption Energy (Eads), Average Distance (d) from the Adsorbate to the Fe Surface, and Charge Transfer (CT) to H, O, and OH at different sites, top (T), bridge (B), and hollow (H), of Fe(100)a Θ = 0.25 H

Θ = 0.11

B

T

H

B

T

−0.35 −2.66

−0.32 −2.60

0.18 −2.08

0.43

1.20

1.56

0.38 0.49

0.39

0.14

−3.97 −6.84

−3.32 −6.19

−2.67 −5.55

0.58

1.32

1.62

0.61 1.15

0.85

0.77

−3.94

−3.98

−3.54

H Eads (eV)b Eads (eV)c Eads (eV)21 Eads (eV)22 Eads (eV)23 d (Å) d (Å)21 d (Å)22 CT (e)

Figure 1. Top (1a) and profile (1b) schematic views of the considered adsorption positions of H, O, OH, and H2O units on the Fe(100) surface. In 1b, the B position is not displayed, since it is just behind the H position.

3.1. Hydrogen, Oxygen, and Hydroxyl Adsorption on Fe(100). The adsorption of an isolated hydrogen atom was considered in all of the three proposed adsorption sites, i.e., H, B, and T. After relaxation of the system (for the adsorbed atom and first two layers of the substrate), the adsorption energy (Eads), the equilibrium distance between adsorbed atom and substrate surface (d), and the charge transfer (CT) from substrate to adsorbed atom were calculated. These values, together with the ones specifically related to the energy and geometry, as calculated by Eder and Terakura,21 those by Jung and Kang,22 and the ones obtained by Govender et al.,23 are indicated in Table 1. The value of Θ, as mentioned in Table 1, corresponds to the ratio between the number of occupied sites and the number of available sites. Thus, Θ = 0.25 corresponds to the situation of one adsorbate in the (2 × 2) unit cell, while Θ = 0.11 corresponds to one adsorbate in a (3 × 3) unit cell. From Table 1, it is possible to conclude that the most stable site for hydrogen adsorption is the H site, with energies of −0.42 and −0.35 eV (referenced to the molecular species) and −2.70 and −2.66 eV (referenced to the atomic species) for the systems with Θ = 0.25 and Θ = 0.11, respectively. In both cases, for Θ = 0.25 and Θ = 0.11, the B site is slightly less stable than the H site (a difference of 0.03 (0.06) eV and 0.05 (0.06) eV, respectively), while the T site is locally unstable if we consider the energies calculated with reference to the molecular species and less stable considering the energies calculated with reference to the atomic species. The distance between adsorbed atom and substrate is 0.37 and 0.43 Å for the systems with Θ = 0.25 and Θ = 0.11, respectively. The present results indicate that the calculated adsorption of hydrogen is sensitive to the cell size in the 0.04−0.07 energy range. The present results, shown in Table 1, can be compared to the ones obtained by Eder and Terakura21 and by Govender and collaborators23 for a (2 × 2) unit cell. However, there is a difference in what refers to the site stability for which, according to the present results, the most stable one is the H site either

−0.42 −2.70 −0.35

−0.37 −2.64 −0.36

0.24 −2.03 0.17

−2.69 0.37 0.35

−2.71 1.09 1.08

−2.69 1.55 1.63

0.40

0.37

0.18

−0.42

Eads (eV)b Eads (eV)c Eads (eV)21 Eads (eV)22 Eads (eV)23 d (Å) d (Å)21 d (Å)22 CT (e)

−3.95 −6.85 −3.72

−3.30 −6.20 −3.16

O −2.62 −5.55 −2.19

−6.59 0.59 0.63

−6.09 1.43 1.32

−5.28 1.61 1.67

1.17

1.01

Eads (eV) Eads (eV)21 Eads (eV)22 d (Å) d (Å)21 d (Å)22 CT (e)

−3.84 −3.86

−4.01 −4.12

0.75 OH −3.54 −3.71

1.29 1.22

1.62 1.56

2.08 1.99

1.40

−3.95 1.56

2.14

0.69

0.69

0.67

0.72

1.59 0.68

0.59

−3.47

a

The present results are also compared with those obtained in previous works. bValues referenced to the hydrogen and oxygen molecules. cValues referenced to atomic hydrogen and oxygen.

for the (2 × 2) unit cell as well as for the (3 × 3) unit cell, while Eder and Terakura found that the H and B sites are essentially equivalent. Similarly, Govender and collaborators23 have found that the sites B and H are equally stable. It is possible to observe that there are differences, in the values of the distance between the atom and the substrate, if comparing the results of this work with those by Eder and Terakura.21 Probably this difference results from the fact that in the present work a PAW approach is used, while in ref 21 ultrasoft pseudopotentials were used. The preferential adsorption site, as calculated here (see Table 1), is exactly the same as that identified by Jung and Kang,22 with small differences in energy and in distance from adsorbed atom and substrate. It appears that these differences come from the fact that, in the present work, as a convergence criterion, a value of 0.01 eV/Å is used for the maximum force, while in ref 22 a value of 0.05 eV/Å was used. The values obtained for the charge transfer, using Bader’s analysis, indicate that charge is transferred from the substrate toward the adsorbed atom. As expected, the Fe(100) surface acts as an electron donor, while the hydrogen atom acts as an electron acceptor. Pairwise with the stability, the CT is higher for adsorption in the H site, being successively reduced for B and T sites. On the other hand, there is no significant difference 20308



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Table 2. Adsorption Energy (Eads), Average Distance (d) from the Water Molecule to the Fe Surface, and Charge Transfer (CT) to the Water Molecule at Different Sites, Top (T), Bridge (B), and Hollow (H), with the Orientations Up (u) and Parallel (n) to Fe(100)a Θ = 0.25 H Eads (eV) Eads (eV)21 Eads (eV)22 d (Å) d (Å)21 d (Å)22 CT (e) a

Θ = 0.11

B

T

T

H

B

u

n

u

n

u

n

u

u

u

n

−0.25

−0.37

−0.26 −0.35

−0.36

−0.04 −0.26

−0.38

−0.23

−0.27

−0.01

−0.34

3.15

2.20

2.10 1.87

2.38

2.45 2.29

2.29

2.95

−0.26 2.10

2.45

−0.39 2.28

0.03

0.07

0.08

0.04

0.06

0.05

0.02

2.01 0.04

0.04

2.29 0.05

The present results are also compared with those obtained in previous works.

in what refers to the charge transfer for systems with Θ = 0.25 and Θ = 0.11. Similarly to what was done for hydrogen, we have investigated the adsorption of an oxygen atom in all of the three sites considered, as summarized in Table 1. As experimentally demonstrated earlier,42,43 oxygen atom is predicted to strongly adsorb on the hollow site. Also, as it is easy to notice, in most of the cases, there are no significant differences in the results obtained for Θ = 0.25 and Θ = 0.11, with respect to the equilibrium distance between adsorbate and substrate, being equal to 0.59 and 0.58 Å, respectively. Therefore, in the case of a stronger adsorption (chemisorption), as calculated for an oxygen atom, we note that the process is practically independent of the coverage. The differences are restricted to the distance between atom and substrate and CT in the case of the B site. In the case of the oxygen atom, the most stable adsorption site was found as being the H site, which coincides with the conclusions of Eder and Terakura,21 the ones of Jung and Kang,22 and also the results by Govender and collaborators.23 Again, these differences in energy and in the distance from adsorbate to substrate can be attributed to the same differences in the calculation procedures and convergence criteria, as already pointed out here. From the analysis of the results relative to the CT, once again, the substrate acts as a donor and in this case the oxygen atom behaves as an electron acceptor and again the charge transfer increases in the same direction of site stability. Despite the similarities with the case of hydrogen adsorption, it is possible to observe an important increase in the charge transfer, being now, for the H site, 1.17 and 1.15e, for Θ = 0.25 and Θ = 0.11, respectively. It is possible to interpret this as being a result of the metallic character of iron and to the high electronegativity of oxygen. The calculated CT are essentially the same either using a (2 × 2) or a (3 × 3) unit cell. The most significant difference is related to the adsorption in the B site. Following the same lines of the previously described procedures for the adsorption of hydrogen and oxygen atoms, the hydroxyl adsorption was also considered and the results are summarized in Table 1. It is worth mentioning that, in this case, the distance between the adsorbate and substrate is defined in terms of a weighted average, taking into account the atomic number as the weight. It is possible to notice from Table 1 that, differently from the previous cases, now the most stable site is the B site with an adsorption energy of −4.01 eV, followed by the H and T sites with energies of −3.84 and −3.54 eV,

respectively. The average distance from adsorbate to substrate, when in equilibrium, is 1.62 or 1.56 Å, depending if we consider Θ = 0.25 or Θ = 0.11, respectively. In general, the several values found using Θ = 0.25 or Θ = 0.11 are quite close. It is interesting to notice here that the OH is predicted to be strongly chemisorbed on Fe(100), with the O atom in the bridge site and the OH bond oriented perpendicular to the surface (slightly off 90°), due to a large charge transfer from the surface to the O p-orbitals. The present results for OH adsorption are well comparable with the ones by other authors. One important point that perhaps deserves to be mentioned is related to the definition of distance between adsorbed molecule and substrate. As previously mentioned, in this work, a weighted average distance is adopted with the atomic number being the weight. However, in refs 21−23, the distance in Table 1 is calculated between the oxygen atom and the substrate. In what refers to the mechanism of charge transfer in this surface, the donor behavior of the substrate still remains. However, the amount of CT is not in pace with the stability, as can be seen from the values listed in Table 1. Besides that, the CT values in this case are higher than those for hydrogen but less than those calculated for oxygen. 3.2. Adsorption of a Single Water Molecule on Fe(100). We consider now the adsorption process of a single water molecule. Similarly to what was done for hydrogen and oxygen atoms and also for the hydroxyl radical, the equilibrium position of a water molecule adsorbed on the H, B, and T sites of the Fe(100) surface was determined. Besides this, the equilibrium orientation of the water molecule was explored relative to the surface, starting from three initial orientations: (i) water molecule with the O−H bonds parallel to the surface (denoted by n), (ii) O−H bonds oriented upward relative to the surface (u), and (iii) O−H bonds oriented downward relative to the surface (d). The molecular system, as before, was left to relax together with the atoms of the first two layers of the substrate, starting in all of the sites, sequentially for the three possible initial orientations of the water molecule, as already mentioned. In all of the situations, when the molecule starts with an orientation of the O−H bonds downward toward the surface (d), the relaxation process leads the molecule to an orientation such that the O−H bonds become parallel to the surface (n). The adsorption energies, equilibrium distance from molecule to substrate, and charge transfer after relaxation are given in Table 2, together with previous results. As can be seen from the 20309



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results of Table 2, the most stable site for the water molecule adsorption is the T site, with the molecule in the orientation n, exhibiting an adsorption energy equal to −0.38 eV and an average distance equal to 2.29 Å for a coverage Θ equal to 0.25. For Θ equal to 0.11, the values are −0.34 eV and the average distance is equal to 2.28 Å. The present results and the ones by Jung and Kang22 indicate the most stable adsorption position as the T site, while the results by Eder and Terakura21 suggest the most stable one as being the B site. As expected for the adsorption of H2O on Fe(100), a weak physisorption was calculated, in contrast to the OH adsorption. However, this process is dependent on the water orientation relative to the surface and also on the adsorption site. The values obtained indicate a small amount of charge transfer from the surface to the adsorbed molecule, as can be seen in Table 2, if we compare with the cases of H, O, and OH. Moreover, the amount of CT is almost independent of the adsorption site. It is worth mentioning that, different from the case of water adsorbed in graphene,33 in this work, the amount of CT does not depend on the water molecule orientation with respect to the surface. 3.3. Dissociative Processes on the Fe(100) Surface. Dissociative processes, from H2O to OH and H, as well from OH into O and H, were also studied hereby. In order to obtain the MEP, knowledge of the initial and final states of the adsorbed systems is necessary and this study was done by employing the NEB and CI-NEB methodologies.36,37 As already mentioned here, the most stable site for adsorption of the water molecule was the site denoted by T, with an orientation parallel to the surface (n). On the other hand, the most favorable sites for isolated adsorption of H and OH were, respectively, the H and B sites, while for atomic oxygen the most stable site is H. We have determined this dissociative process by comparing results of using either a (2 × 2) as well as a (3 × 3) unit cell. The general results for both unit cells are summarized in Figures 2−5. The initial and final states, together with an intermediate state, are displayed in Figures 2 and 4 for the dissociation of water and in Figures 3 and 5 for the dissociation of OH. Starting with the initial and final states, a linear interpolation was performed in order to obtain the intermediate states which were separately converged toward local minima, using the same convergence criteria of force and energy. After local convergence in the several stages of the reaction process, a curve of minimum energy is obtained, i.e., the MEP for the water molecule dissociative process, as displayed in Figures 2 and 4. By examining the MEP, the transition from the water molecule to the dissociated products occurs at the fourth point of Figures 2 and 4, corresponding to an energy barrier of 1.04 eV (for a coverage of 0.25 monolayer) and 0.94 eV (for a coverage of 0.11 monolayer). This barrier of approximately 1.0 eV has been calculated as the necessary energy that must be given to the system for occurrence of the dissociation of a H2O molecule into the OH and H separated species. In ref 21, the authors have found an energy barrier for this dissociation process of 1.1 eV, although both the initial and final states were in the B site considering a (2 × 2) unit cell. Differently, in ref 22, the authors have found an energy barrier of only 0.35 eV but considering the initial state of water on a T site and the final states of OH and H on B sites. They have also considered a similar route to our calculated MEP, having found an energy

Figure 2. Three stages of the dissociation process for a water molecule into isolated H and OH in a coverage of 0.25. The portion indicated by (a) represents the most stable position for a water molecule, (b) indicates a transition state, while (c) indicates the final positions of H and OH.

barrier of only 0.75 eV, i.e., 25% lesser than the present results, as well as those of ref 21. The dissociation of the hydroxyl radical into adsorbed O and H was also considered in this work. As stated before, the preferred adsorption site for OH is the B site, while for atomic oxygen and also for hydrogen the most stable site is H. As in the previous case, this necessary information for determining the MEP is schematically displayed in Figures 3 and 5. The same steps and convergence criteria, necessary for studying the H2O dissociation, were followed in this case. According to the MEP, the transition state from the adsorbed radical to the state where the hydroxyl components are dissociated corresponds to the fourth point of Figure 3 and the third point of Figure 5, exhibiting a potential barrier of 0.84 eV in a (2 × 2) unit cell and 0.75 eV in the case of (3 × 3). Similarly to the calculation for the dissociation barrier of H2O, an increase of 0.1 eV was noticed for a larger unit cell. The transition state corresponds to the OH bond breaking, which occurs on the B site. This compares well with the calculations of ref 22, which gives an energy barrier of 0.79 eV. After obtaining the potential barrier for the dissociation of the water molecule and, subsequently, of the hydroxyl, it is possible to describe the whole process of dissociation of the water molecule by adsorption on the Fe(100) surface, which is illustrated in Figure 6. From the schematic diagram, it is possible to notice that, initially, the water molecule is adsorbed at a T site with an adsorption energy of −0.38 eV for Θ = 0.25 20310



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Figure 3. Three stages of the dissociation process for an OH radical into isolated H and O in a coverage of 0.25. The part indicated by (a) represents the most stable position for hydroxyl, (b) indicates a transition state, while (c) indicates the final positions of H and O.

Figure 4. Three stages of the dissociation process for a water molecule into isolated H and OH for a coverage of 0.11. The portion indicated by (a) represents the most stable position for a water molecule, (b) indicates a transition state, while (c) indicates the final positions of H and OH.

and −0.34 eV for Θ = 0.11. The water molecule can then be dissociated into H and OH components provided an energy of about 1.0 eV (potential barrier) could be overcome. The dissociated species, H and OH, adsorbed, respectively, on the H and B sites, with an adsorption energy of −1.14 eV for Θ = 0.25 and −1.11 eV for Θ = 0.11. At last, OH dissociates with both components, H and O, adsorbed on the H site and an energy gain of 1.62 eV for Θ = 0.25 and 1.75 eV for Θ = 0.11, both cases relative to the free water molecule. 3.4. Adsorption and Dissociation of Small Water Aggregates on Fe(100). Understanding the stability and composition of small water aggregates adsorbed on iron is an important step in describing wetting and the oxidation process. Thus, we have examined the properties of small water aggregates containing up to five molecules adsorbed on the Fe(100) surface (see Figure 7). We have considered different orientations and the CT mechanism between adsorbate and substrate. The initial distance between aggregate and surface was estimated according to the most stable position of an isolated water molecule at equilibrium when adsorbed. In order to determine the energetics of the adsorption process, it was necessary to calculate the energy associated with the clean Fe(100) surface. Hence, the water−iron interaction is calculated using the concept of adsorption energy, which is expressed by eq 1.

The adsorption energies were referenced to the cluster structures fully relaxed in vacuum. We have also estimated the effect of relaxation considering the cluster structures relaxed on the Fe(100) surface. The calculated adsorption energies are presented in Table 3. We have considered two distinct orientations for the water dimer, trimer, tetramer, and pentamer (structures 2a, 2b, 3a, 3b, 4a, 4b, 5a, and 5b in Figure 7) with respect to the Fe(100) surface. All of these structures were obtained as being moderately bound to the iron surface with an adsorption energy per molecule in the 60−340 meV range. In general, these values do not depend significantly on the orientation and size of the cluster with respect to the Fe(100) surface. On the other hand, it is also known that the binding energy of water aggregates with a metallic surface depends on the number of water molecules which form hydrogen bonds.7 The relaxation effects contribute with values of the order of 10−2 eV. The major difference however was obtained in the structure 4a (0.28 eV). In fact, the tetramer 4a (displayed in Figure 7) undergoes the largest structural change to be accommodated on the iron surface. The highest total adsorption energy calculated in the present work was obtained for dimer 2b. However, as a result of the number of hydrogen bonds formed in the cluster structure, the corresponding energy per molecule was of about 340 meV. 20311



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Figure 6. Schematic energy diagram for the complete dissociation of a water molecule on the Fe(100) surface: (a) Θ = 0.25; (b) Θ = 0.11.

Figure 5. Three stages of the dissociation process for an OH radical to isolated H and O in a coverage of 0.11. The part indicated by (a) represents the most stable position for hydroxyl; (b) indicates a transition state, while (c) indicates the final positions of H and O.

Indeed, in order to have a better description of the water−iron surface interactions, it would be necessary to consider a large coverage of water adsorbates. As it is expected, the adsorption energy tends to higher values by increasing the number of water molecules in the aggregate, being also dependent on the structure of each aggregate. In what refers to aggregates, with the same number of molecules but different orientations, the largest difference in the adsorption energy between 2a and 2b was obtained. In fact, there are many local minima on the Fe(100) surface and the exact orientation of the dimer is much more sensitive to the position of the adsorption sites than in the case of larger aggregates. Analyzing the charge transfer between the water aggregates and Fe(100), we have noticed a significant increase of this quantity in the largest clusters. For example, in the case of the water dimer and trimer, we have calculated the CT values (from iron to the water molecules) between 0.070 and 0.112 e (Table 3). However, in the case of the tetramer, the CT increases to the 0.105−0.135 e range, while in the case of the pentamer the CT increases to 0.136 e. This can be an indication that the larger the adsorbed water aggregate, the larger the CT from the Fe(100) surface. Thus, a large amount of water adsorbed to the iron surface could affect the redox process. Furthermore, as discussed previously in the case of water on graphene,33 for the adsorbed monomer, the Bader charge analysis associated with the GGA schemes leads to small CT values.

Figure 7. Small water aggregates after relaxation. Labels 2, 3, 4, and 5 indicate the number of water molecules, while a, b, and c indicate the orientation of the water aggregates relative to the Fe(100) surface.

The dissociative process of a water molecule after the dimer formation was also investigated, i.e., (H2O)2 → H2O(OH) + H. The calculated MEP for this process is displayed in Figure 8. By examining the transition from the water dimer into the dissociated products, it can be noticed that the energy barrier 20312



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CONCLUSION In this work, we have investigated the adsorption, dissociation, and aggregation of water on the Fe(100) surface using a firstprinciples methodology. The calculations were performed within a reliable periodic DFT scheme which has been proven to be useful for these processes.21−23,33 For the H2O, OH, H, and O adsorption, as well as for the H2O and OH dissociation, our study considered distinct coverage degrees (Θ = 0.11 and Θ = 0.25) and demonstrated that they play a mild influence on the adsorption and dissociation energetics. It is worth mentioning that in the present work very tight criteria for convergence in the calculations were considered. Our results have indicated that a single H2O molecule is weakly physisorbed at an on-top site, with the molecule in the orientation n, exhibiting the highest adsorption energy for both considered coverages. In contrast, OH is strongly chemisorbed on Fe(100), exhibiting a large charge transfer from the iron surface to the p-orbitals of the O atom. The calculated diffusion barrier of H2O, from the on-top site, into H + OH species, at hollow and bridge sites, respectively, is in the 0.94−1.04 eV range, depending on the coverage. These results indicate that the water dissociation on iron surfaces requires a systematic investigation of both adsorption sites and molecular orientation with respect to the surface. Considering the adsorption of small water aggregates on Fe(100), the present study has demonstrated that they are strongly reoriented on the surface, in comparison to the isolated structures, leading to stable adsorbates. Most interestingly, the dissociation of a water molecule, after the dimer formation, leads to an energy barrier of 1.25 eV, about 25% higher than the corresponding value of an adsorbed single water molecule.

Table 3. Adsorption Energy (Eads) and Charge Transfer (CT) from the Fe(100) Surface to the Water Aggregatesa # mol H2O orientation 2 2 3 3 4 4 5 5

a b a b a b a b

Eads (eV)

Eads /mol (eV)

CT (e)

CT/mol (e)

−0.59 −0.69 −0.53 −0.59 −0.62 −0.44 −0.32 −0.49

−0.29 −0.34 −0.17 −0.19 −0.15 −0.11 −0.06 −0.09

0.082 0.070 0.093 0.112 0.135 0.105 0.134 0.136

0.041 0.035 0.031 0.037 0.034 0.026 0.028 0.027

Article

a

Labels 2, 3, 4, and 5 indicate the number of water molecules, while a, b, and c indicate the orientation of the water aggregates relative to the Fe(100) surface. The labels in the first two columns correspond to the ones as displayed in Figure 7.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.R.Q.F.); [email protected] (R.R.); [email protected] (F.d.B.M.); [email protected] (C.M.C.d.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Drs. L. C. O. Dacal and G. P. Thim and Mr. F. W. Fernandes for suggestions and discussions during the early stages of this work. This work has been supported by CNPq and FAPESB, Brazilian agencies. The authors also acknowledge the computational support from CENAPAD-SP.

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Figure 8. Three stages of the dissociation process for (H2O)2 into H2O(OH) + H in a coverage of 0.11. The part indicated by (a) represents the most stable position for the dimer, (b) indicates a transition state, while (c) indicates the final positions of H2O(OH) and H.

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Dissociative Adsorption and Aggregation of Water on the Fe(100

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