The Doping Effect of 13-Atom Iron Clusters on Water Adsorption and O

Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Doping Effect of 13-Atom Iron Clusters on Water Adsorption and O−H Bond Dissociation Hongchao Zhang, Chaonan Cui,* and Zhixun Luo* State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China University of Chinese Academy of Sciences, Beijing 100049, P. R. China

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S Supporting Information *

ABSTRACT: Understanding the interactions between water and Fe-based clusters is necessary to unravel the micromechanics of the surface hydrophilic property and the corrosion process of iron-related materials. Herein, a theoretical study is conducted of water adsorption and dissociation on icosahedral Fe13 and Fe12X (X = Ti, V, Cr, Mn, Co, Ni) clusters. It is found that the doping atoms have significant influence on the geometric structures, magnetic moments, and electronic states of Fe12X clusters. The center-doped clusters X@Fe12 show higher stability than the shell-doped X−Fe12; Ni@Fe12 exhibits lower activation energy for the dissociation of H2O than all the others; Ti@Fe12 strikes a weak bonding energy and high activation energy for water dissociation. Also, a water dimer finds a decreased energy barrier for O−H dissociation, and the electronic states and metal−water interactions can be altered by the support effect. This information is helpful to those working on water chemistry, anticorrosion wading devices, and high-standard potable water utilization.

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INTRODUCTION Metal iron and its alloys have been used since ancient times and are widely applied in modern industry. They are used in engineering and magnetic applications and also used as building materials and surface coatings due to their mechanical and chemical properties and relatively low cost.1 The corrosion of Fe often raises serious issues of reliability in long-term utilization in various environments; therefore it is of technological importance to fully understand the interaction of water with Fe and Fe-based materials. Also, there are reasonable concerns about high-standard potable water utilization, where anticorrosion wading devices and magnetized water free of pollution but with oxygen enrichment are desired.2 For this purpose, the chemistry of water has received reasonable research interest recently. Water dissociation reactions on iron trace back to early studies utilizing ultraviolet photoemission spectroscopy on clean and oxidized Fe(110) surfaces.3 Also, in the gas phase, water dissociation and hydrogen desorption on neutral iron clusters were observed via molecule beam technique and timeof-flight mass spectrometry.4 Further, the interactions of cationic iron clusters with a water molecule Fen+−H2O were studied by infrared multiple photon dissociation (IR-MPD) spectroscopy. It was found that water dissociation readily takes place on the iron clusters and there is size dependence.5 With the use of argon matrices, a joint experimental and theoretical photochemistry study was conducted in the mid-IR and UV− visible regions, verifying the existence of HFeOH and HFe2OH indicative of a dissociated O−H bond on the iron atom and the dimer.6 On the other hand, there are a few previously © XXXX American Chemical Society

published theoretical studies addressing the interactions between water and small cluster Fen (n = 1−4),7 illustrating the determining role of the H-bond in stabilizing water molecules adsorbed on iron surfaces,8 as well as water activation on Fe(100).9−11 It is generally recognized that hydrogen evolution from iron clusters is energetically favorable but there are exceptions such as FeH2O+ and Fe4H2O+ cations, and occasionally the local and nonlocal correlation functionals could be sensitive to the calculation results.12 Among others, there are several studies revealing nanoscale materials of iron alloys to be highly efficient for water dissociation.13−16 It is necessarily important to examine the principles in typically larger clusters prior to a decisive mechanism for water adsorption and O−H bond dissociation on Fen clusters. Metal clusters of a dozen atoms provide complementary active sites for surface adsorption and reaction, with tailored properties where one atom site makes a difference. One of the prime objectives of cluster science is to lay the foundation for understanding the interactions between molecules and surfaces with nanometric roughness. Clusters with a “magic” number of 13 atoms have unique properties due to their typical geometric structure or closed electronic structure.17 Typically, Al13 and Al13− sharing an icosahedral structure of 13 aluminum atoms were unreactive toward oxygen.18,19 Some other metal clusters such as Ag13−,20 Au13,21 Pd13,22 and Pt13,23 also exhibit enhanced stability although their lowest energy structures are Received: March 6, 2019 Revised: May 17, 2019 Published: May 22, 2019 A

DOI: 10.1021/acs.jpca.9b02154 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

considered for cluster optimization in this study (see Supporting Information, Figure S1). It is found that the icosahedral configuration has the local lowest energy among the five structures, which is consistent with the previous studies.37−40 The Fe−Fe bond lengths between the outer Fe12 cage atoms and the center Fe atom of Fe13 are shown in Supporting Information Table S1, where the two Fe−Fe bonds at the opposite direction display a shorter bond length of 2.32 Å than the other bonds (2.42 Å), indicating the icosahedral structure is slightly squashed. The total magnetic moment of the icosahedral Fe13 cluster is 44 μB (average 3.08 μB for the outer Fe and 2.63 μB for the center Fe), similar to the results of all-electron DFT calculations39 and semicore DFT calculations.40 As described in Figure 1, the structures of doped Fe12X clusters are obtained by substituting the center atom (written

not icosahedral. In general, doping an atom in the cluster is an effective way to regulate the electronic structure and change the reaction activity of clusters. For example, recently we reported a study on the reactions of H2O molecules on Lewis acid sites of 13-atom Al clusters for hydrogen evolution, and found that the doping of a Ga atom into the Al13 cluster can reduce the transition state barrier for H2O dissociation.24 Actually, enhanced reaction activity was previously studied for iron nanoparticles after doping with 3d transition metal atoms such as Ti, V, Cr, Mn, Co, and Ni.25−29 These findings motivate us to conduct further study into their doping effect on the 13-atom iron clusters. In this study, we select the Fe13 cluster as a model on which we construct a few doped clusters Fe12X (X = Ti, V, Cr, Mn, Co and Ni) to investigate the doping effect for water adsorption and dissociation processes. By employing the density functional theory (DFT) method, the geometries, magnetic moment, and electronic state of Fe12X clusters are analyzed. Furthermore, an in-depth study on a basis of complementary active sites and support effect is conducted endeavoring to unravel how the water adsorption and dissociation on the iron clusters could be altered within the mechanism of electronic states and metal−water interactions.

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COMPUTATIONAL DETAILS All calculations were conducted with the plane wave-based pseudopotential code of Vienna Ab initio Simulation Package (VASP).30 The electron−ion interaction was described with the projector augmented wave (PAW) method,31 as the exchange and correlation energies were described using the generalized gradient approximation and Perdew−Burke− Ernzerhof functional (GGA-PBE).32 Spin-polarized calculations were performed to account for the magnetic properties of iron and transition-atom doped clusters. A cutoff energy of 400 eV was used for the plane wave basis set. The zero-point energy (ZPE) corrections were obtained from the vibrational frequencies calculated by treating 3N degrees of freedom for every adsorbate. All calculations were performed in a cubic box of 17 Å in each direction with the use of a single k point (Γ point) for the Brillouin-zone integration. To check the accuracy of the settings, tests were also calculated using a cubic box of 20 Å with a larger number of k points, for example, 3 × 3 × 3, but negligible difference was found in the results. All atoms were relaxed during the geometric optimization until the atomic forces converged to 0.02 eV/Å and the energy difference in an electronic self-consistent cycle was smaller than 10−6 eV. To calculate the bonding-energy of every cluster on the basis of the normal method, we have checked out the energy of each free atom using an orthorhombic box with dimensions of 17 × 17.25 × 17.5 Å3 in order to avoid lateral interactions under a single k point. The transition states were determined by combining the climbing image nudged elastic band (CI-NEB) and Dimer methods.33,34 Each transition state was verified as a first-order saddle point by vibrational analysis. Additionally, Bader charge analysis was conducted to investigate the charge transfer between water molecules and Fe12X clusters.35,36

Figure 1. Doping positions (red color) on Fe13 and possible adsorption sites on Fe12X. For center-doped X@Fe12 clusters. Top (T), bridge (B), and hollow (H) adsorption sites are marked as circle, rectangle, and triangle, respectively. While the adsorption sites of the shell-doped clusters X-Fe12 are marked as doping atom X, ortho-Fe, meta-Fe, and para-Fe.

as X@Fe12) or a shell atom (presented as X-Fe12) of the icosahedral Fe13 with another 3d transition atom (X = Ti, V, Cr, Mn, Co, and Ni), which has similar atomic radius ranging from 1.6 to 2 Å. All optimized structures of the Fe12X clusters are shown in Figure 2. The charge redistribution, magnetic moments, and bonding energies of the clusters are investigated and listed below for each structure. The bonding energies of clusters are calculated by the following equation: ΔE b = E Fe12X − E Fe13 − (E X − E Fe)

(1)

where EFe13 and EFe12X present the energies of Fe13 and Fe12X clusters, and EX and EFe are the energies of the doped atom and Fe, respectively. It is energetically favorable for the Fe13 cluster to replace the central Fe atom instead of shell atoms, except for Mn@Fe12 and Ni@Fe12, while negligible difference in energy is presented for Ni-doped Fe13 at the center or shell positions. What is more, positive values of ΔEb in V−Fe12, Cr@Fe12, Cr−Fe12, Mn@Fe12, and Mn−Fe12 indicate energetically unfavorable binding of the doping atoms in Fe13 clusters, which is consistent with the previous study.41 It is found that the doping atoms have important influences on the geometries as obvious distortions can be observed after the doping. The bond length between the doping atom X and the surrounding Fe atoms are shown in the Table S1, embodying the structure distortion of these Fe12X clusters. For Fe12Ti and Fe12Co clusters, the icosahedral structure is stretched along the central axis. Also, the icosahedrons are distorted in Fe12V and Fe12Cr, but Fe12Mn and Fe12Ni retain the icosahedral structure. Considering the existence of both attractive forces that hold

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RESULTS AND DISSOCIATION Structure Optimization. On the basis of previous studies of the cluster structures of light transition metals,37−40 five Fe13 configurations with a highly symmetric ground-state were B


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Figure 2. Optimized structures of the center-doped and shell-doped Fe12X clusters (shown as X@Fe12 and X-Fe12, respectively). The Bader charge (QB) and the local magnetic moment (in parentheses) of the doping atom, total magnetic moment (M), and the bonding energy (ΔEb) of each cluster are listed below every structure.

atoms to Fe atoms on the Fe12X (X = Ti, V, Cr and Mn), while conversely transfer from Fe atoms to the doping atom for Fe12Co and Fe12Ni. This is consistent with the element sequence of the light transition metals, of which the after-Fe metals (i.e., Co and Ni) bear relatively larger electrophilicity. What is more, relatively more charge transfer is obtained for center-doped clusters than the shell-doped one, resulting in higher binding energy for the center-doped clusters. It is anticipated that the 12-coordination could induce larger electron density changes for the central metal atom in X@ Fe12 comparing with the 6-coordinated doping atom in X-Fe12. The positive charge obtained on the doping atom X from Fe12Ti to Fe12Mn is gradually reduced according to the sequence of periodic table of the elements, while Co and Ni atoms in their correlative clusters are negatively charged by −0.11 to −0.24|e|. As the center-doped X@Fe12 clusters have higher stability and larger charge rearrangement than the shelldoped ones for most systems, hereafter we focus on the X@ Fe12 clusters to study the water dissociation process. To gain more insights into the electronic structures of the doped clusters, the electronic partial density of state (PDOS) of the center-doped X@Fe12 clusters is studied. Figure 3 displays the PDOS of the ground state of X@Fe12 projected onto the 4s and 3d orbitals of the cage shell Fe12 atoms. A clear shift of s orbitals (between −6 and −7 eV) to the low-energy level is seen from Ti to Ni. Also, the d-band center moves closer to the Fermi level after doping. These features indicate that the doping atoms modify the delocalization of valence electron states of the outer Fe atoms, which may influence the reaction activity of X@Fe12 as discussed below. Water Adsorption and Dissociation. For water adsorption, three initial orientations for water molecules relative to the clusters are studied, with the O−H bonds parallel, upward, or downward to the shell surface, which are optimized with full relaxation to find the energy minima (see Figure S2). It is found that H2O prefers to adsorb on the top site (as shown in Figure 1) with O atom bonding to Fe for all the clusters, thus this top-site configuration is chosen for the subsequent calculations. For the center-doped X@Fe12

together the atoms and electrostatic repulsive forces that tend to segregate, the slightly varied atomic radius and d-electrons of the light transition metals could account for the altered binding energies and likely structure distortion of these clusters.41 We have also studied the magnetic moments for these clusters, as shown in Figure 2. The total magnetic moment is defined in the Russell−Saunders scheme as μ = (2S + L) μB, where μB is the Bohr magneton, S and L are the total angular spin and angular moments, respectively. The total spin magnetic moment is estimated according to M = 2S μB = (nα − nβ) μB, where nα and nβ are the numbers of the majority spin and minority spin electrons, respectively.37 While Fe13 finds a magnetic moment of 44 μB, that of Fe12X clusters decreases after the doping expected for Mn−Fe12 which has a slightly larger magnetic moment of 45 μB. Cr−Fe12 possesses the same magnetic moment as Fe13. For Fe12Ti, Fe12V, and Fe12Co clusters, interestingly the same magnetic moments are found for the two substitution situations. The magnetic moments of the center-doped clusters gradually decrease from Ti@Fe12 to Mn@Fe12 and also reduced for Ni@Fe12 comparing with Fe13. Among these doped clusters, Co@Fe12, Ni@Fe12, Co−Fe12, Ni−Fe12, Cr−Fe12, and Mn−Fe12 are ferromagnetic; while the other clusters including Cr@Fe12 and Mn@Fe12 are antiferromagnetic. The antiferromagnetic coupling effect of the local spin moments between the doped atom and Fe atoms in certain clusters is associated with relatively higher energy occupation in the β-spin than in the αspin. Interestingly, while the antiferromagnetic coupling occurs in Cr@Fe12 and Mn@Fe12 clusters, the Cr and Mn atoms combined with six Fe atoms in Cr−Fe12 and Mn−Fe12 clusters exhibit lower β-bonds (instead of α-3d-electrons) hence pertaining to ferromagnetic properties.41 Electron structures of the doped clusters are anticipated to play an important role in water reactions. In view of this, Bader charge analysis is conducted to figure out the charge transfer between the Fe and doping atom X. The relative charge obtained on the doping atom is shown in Figure 2, where it is shown that the electron prefers to transfer from the doping C


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between the lone pair electron of the O atom and the shell Fe atom. The red shift of the vibrational frequency of the O−H bond also indicates a weakening of the O−H bond upon water adsorption, based on the concept of empirical Badger rules.42,43 The details of bond length, angles, and vibrational frequencies for water adsorption are shown in Table S3. We have also investigated the adsorption behavior of hydrogen atom and −OH group on different sites to determine the stable configuration after water dissociation, and found that the H adatoms on Bridge (B) and Hollow (H) sites exhibit similar adsorption energies (−0.51 and −0.54 eV, respectively), while on T site the H adatom is much less stable. For the OH group, the most stable adsorption configuration is on the B site (−0.92 eV), while on the H site the configuration with the vertical orientation is less stable (−0.73 eV). These results are consistent with the previous studies of water adsorption on the Fe surface.8−10 In Figure 4, the charge density difference of each cluster is also presented to understand the atomic interaction of X@ Fe12−H2O systems. A more positive charge is located mainly on the adsorption site of Fe which bonds to the O atom, while the charge on the other shell Fe atoms have smaller magnitude. Notably, significant charge depletion on the para-site Fe is presented on Co@Fe12 and Ni@Fe12 after H2O adsorption, especially for Ni@Fe12. Bader charge analysis has been used to measure the charge transfer among the systems (see Supporting Information Table S4). We find that the adsorbed water molecule tends to donate minor electrons to the clusters and the Fe atom bound with H2O (i.e., Feads) becomes positively charged. A similar result has been found for water interacting with the other transition-metal 13-atom clusters,43 which is expected due to the lower electronegativity of the Fe atom than of the O atom. However, the doping atoms could induce a rearrangement of the electron distribution and adjust the electronegativity of the clusters. Figure 5 shows the PDOS of Ti@Fe12, Cr@Fe12, Fe13, and Ni@Fe12 clusters with free or adsorbed water molecules. More PDOS of the other systems are shown in the Figure S3. Positive and negative values represent spin-up and spin-down contributions, respectively. All the molecular orbitals of H2O, denoted as 1b1, 3a1, and 1b2, are significantly shifted away from Fermi level after adsorption onto clusters. The shift can be attributed to the strong interaction of water and clusters after adsorption, with a small distortion of H2O molecules. It is noticeable that the 1b1 orbital, corresponding to the oxygen lone-pair which is perpendicular to the molecular plane,9,44 is different after water adsorption for the varied systems. The 1b1 orbital overlaps with the 4s orbital of the Fe atom and changes into two bands in Ti@Fe12−H2O, Cr@Fe12−H2O, and Fe13− H2O systems, with distinct shapes, while it tends to overlap slightly with the 3d orbital for Ni@Fe12. This demonstrates that the interaction between oxygen lone pair and the 3d and 4s orbitals of Fe could be regulated by doping different atoms. On the other hand, the water adsorption also induces changes of the Fe-derived PDOS as the hybridization of the d orbital with 3a1 orbital.9 This phenomenon that the water adsorption energy correlates to the overlap of the 1b1 orbital of water with the 4s orbital of the Fe atom is also seen in X−Fe12−H2O (X = Ti, Ni) systems (Figure S9). We also analyzed the relation between the d-band center of the X@Fe12 clusters and the water adsorption energy (Figure S4); however, the linear relation is very weak, which may be due to the discrete electronic structure and high spin state of the d orbitals. The

Figure 3. DOS of the ground state of X@Fe12 projected on the 4s (red and blue) and 3d (green and orange) orbitals of the cage shell Fe12 atoms. Positive and negative values represent spin-up and spindown contributions, respectively. The d band center is marked by gray solid line, and the Fermi level is set to be zero.

clusters, we assume that all the shell Fe atoms are identical to each other; for shell-doped X-Fe12 clusters, we have tested the water adsorption on the doping atom X, ortho-Fe, meta-Fe, and para-Fe sites. The adsorption energy Eads is defined as Eads = E Fe12X + H 2O − E Fe12X

(2)

where EFe12X + H2O and EFe12X are the energies of water adsorbed on the Fe12X and the bare cluster. Details of adsorption configurations and energies are shown in Figure 4. It is found

Figure 4. Adsorption energy of H2O onto X@Fe12 with their charge density difference. Insets showing the charge accumulation (depletion) given with yellow/cyan isosurfaces. Oxygen atoms are in light red, hydrogen in white, and iron in light blue. The inner doped atoms corresponding to Ti, V, Cr, Mn, Fe, Co, Ni.

that, although there is a similar adsorption configuration, the electronic state after water adsorption is largely different from that of the nascent metal cluster. On the basis of eq 2, the Eads of water adsorbed on Fe13 is calculated to be −0.55 eV, which is consistent with the previous studies of water adsorption on the Fe surface.8,10 In comparison, the adsorption of water adsorbed onto Cr@Fe12 is much stronger than that of Ti@ Fe12, which may result from the specific properties of different doping atoms. The O−H bond of water is weakened after the adsorption on X@Fe12, by lengthening the O−H bond and enlarging the angle of H−O−H, due to the interaction D


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Figure 5. PDOS for the H2O and the adsorbed Fe atom in (a) Ti@Fe12−H2O; (b) Cr@Fe12−H2O; (c) Fe13−H2O; and (d) Ni@Fe12−H2O systems. The area filled with green and orange represent the spin up (alpha) and spin down (beta) states of d orbitals of Fe atoms, respectively. The area filled with red and blue represent the spin up (alpha) and spin down (beta) states of s orbitals of Fe atoms, respectively. Red and purple solid line represent the p orbital of the O atom. The Fermi level is set to be zero.

Figure 6. (a) Reaction pathway of H2O adsorption and dissociative on X@Fe12. (b) Brønsted-Evans−Polanyi relationships of H2O dissociative on X@Fe12. Plots are activation energies as a function of the reaction energies.

where EIS, ETS, and EFS present the energies of IS, TS, and FS, respectively. Interestingly, it is difficult for water to dissociate on Ti@Fe12 and Mn@Fe12 clusters, while an improved performance is found by doping with Ni and Co. Especially, both Ea and Er decrease obviously for water dissociation on Ni@Fe12 comparing to that on Fe13. Notably, the Er and Ea values are significantly magnified for Ti@Fe12, indicating resistance for water dissociation. As presented in the Bader charge analysis (shown in Table S4), negative charges are found for the shell Fe atoms in the Ti@Fe12 cluster, whereas they are inverse in the Ni@Fe12 cluster (also Co@Fe12). Considering that the initial interaction in these systems is the nucleophilic attack of water on the cluster surface,45 it is illustrated that the negatively charged Fe atoms could hinder water dissociation, while the gain of minor positive charge can accelerate water dissociation for center-doped clusters.46,47 Conversely, for shell-doped clusters, the activation energy for water adsorbed on the Ti site of Ti−Fe12 is much lower than that on Ni of Ni−Fe12 (for details see Table S8). As seen in Figure 6b, there is a strong linear correlation between the Er

hybridization of 1b1 orbital of water with the 4s orbital of the Fe atoms could contribute to the adsorption energy of water. The PDOS analyses in Figure 5 (and Figure S3) reveal that the greater overlap between the orbitals of water and Fe atoms corresponds to a larger adsorption energy of water, indicating the influence of doping atoms on the electronic structures of X@Fe12 clusters, and thus on water dissociation. The dissociation of H2O to form the OH and H species was studied on each X@Fe12 cluster. The initial state (IS), final state (FS), and transition state (TS) of water adsorption and the dissociation step are displayed in Figure 6a. Geometric parameters of the TSs are provided in Table S5. As mentioned above, the most stable site for the adsorption of water is T, while for OH and H it is B and H, respectively. The activation energy Ea is defined as Ea = E TS − EIS

(3)

while the reaction energy Er is defined as Er = E FS − E IS

(4) E


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The Journal of Physical Chemistry A and Ea of water dissociation on different X@Fe12 clusters, known as Brønsted−Evans−Polanyi (BEP) relationships.48,49 Note that Ni and Ti stand at the both ends pertaining to the distinguishable water adsorption/reaction of the Ni and Ti doped clusters. Difference of Water Dimer. As previously found in Alm(H2O)n systems,50,51 the presence of a second water enables a molecular relay mechanism, in which the second water acts as a “bridge” to translate the H atom of the first water molecule to the cluster surface and stabilize the system. To check this relay effect of a second water for O−H dissociation on X@Fe12, a H-bonded water dimer adsorption and dissociation on typical Ti@Fe12, Fe13, and Ni@Fe12 clusters were studied. As shown in Figure 7, the second

the defect site shows exceptional stability, which agrees with the previous studies on 13-atoms clusters.54,55 Figure 8 shows the side view of supported structures and the electron density difference analysis results of X@Fe12/DG (X =

Figure 8. Electron density difference of (a) Ti@Fe12/DG, (b) Fe13, and (c) Ni@Fe12/DG on defective graphene. Yellow (cyan) isosurfaces correspond to charge accumulation (depletion).

Ti, Fe, Ni). It can be seen that the C atoms bonding with X@ Fe13 clusters have raised out of the DG plane due to the strong interaction at the interface, which is a common phenomenon for defective graphene supports.55−57 It is clear that a large amount of electrons transferred at the interface, leading to a strong charge depletion on the cluster and charge accumulation around carbon atoms, indicating the cluster is positively charged on defective graphene, which was supported by the Bader charge analysis shown in Figure S6. The electron rearrangement on cluster provides more active sites that may enhance the catalytic activity for water dissociation. The adsorption and dissociation process of water on the top, subtop, and bottom sites on DG supported clusters are studied. As seen in Table 1, the Fe−O bond length shortens by water Figure 7. Structures and relative energies (in eV) of water dimer adsorption and dissociation onto X@Fe12 (X = Ti, Fe, Ni).

Table 1. Calculated Parameters for the Adsorption and Dissociation of Water on Top, Subtop, and Bottom Sites of DG-Supported Ti@Fe12, Fe13, and Ni@Fe12 clusters. The Insets Are Corresponding Structures

water interacts with the adsorbed water by the H-bond. During the water dissociation process, the second water stabilizes the OH species on the T site and lowers the activation energy by combining with the hydrogen atom from the adsorbed H2O and releasing one hydrogen to the cluster surface. The improvement of activity is more pronounced on Ti@Fe12 cluster as the activation energy of the O−H bond dissociation is significantly lowered compared to the reaction with only one water, and even lower than Ea on the Fe13 cluster. These results illustrate that the two-water relay system appears more efficient for O−H bond dissociation than the single water system. Support Effect. We have also studied the behavior of water on the X@Fe12 clusters which are supported on defective graphene. Graphene has been demonstrated as a promising material in a variety of applications, because of its unique high electron mobility and enhanced mechanical and electrical properties.52 Defective graphene with monovacancies is considered as a normal substance for catalysis and substrate materials.53 We tested three different positions of X@Fe12 (X = Ti, Fe, and Ni) clusters supported on defective graphene (X@ Fe12/DG) with a monovacancy. The bonding energies of supported clusters are calculated as E L = Egra + Fe12X − Egra − E Fe12X

adsorption on the bottom site as compared to that on the top and subtop site, indicating a strong interaction with the cluster, and also leading to a large distortion of the cluster and hindering the dissociation of water to form the stable OH+H species. As for Fe13/DG, the active energy is similar on the subtop and top sites (0.27 and 0.26 eV, respectively), both are lower than that in the gas phase (0.36 eV). Contrarily for Ti@ Fe12/DG, the activation barrier (0.47−0.48 eV) of the O−H bond dissociation is higher than that on the gas phase Ti@Fe12 (0.42 eV). This may be due to more electrons transferred from the substance to the adsorption site of Ti@Fe12, resulting in higher activation barrier for O−H bond dissociation. For Ni@ Fe12/DG, the reactivity of water dissociation is greatly

(5)

where Egra+Fe12X, Egra, and EFe12X are the energies of Fe12X supported on defective graphene, bare defected graphene, and gas-phase Fe12X, respectively. The relative EL of different clusters supported on DG are shown in Figure S5. A negative value of EL indicates the landing progress is exothermic. It is found that the H1 configuration with one Fe atom adsorbed on F



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enhanced by adsorption on the subtop site rather than on the top site. For comparison, the activation energies of O−H bond dissociation on pure X@Fe12 clusters with one or two water molecules, or on X@Fe12/DG with one water are summarized in Figure 9. The Ni-doped cluster is found to be the most

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.9b02154. More details of structures, bond length of Fe12X, Bader charge analyses results, PDOS for water and Fe atom, preferred magnetic state, and water dissociation on the X−Fe12 cluster (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhixun Luo: 0000-0002-9819-9155 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 21722308, No. 21802146), and supported by Beijing Natural Science Foundation (2192064), CAS Key Research Project of Frontier Science (CAS Grant QYZDB-SSW-SLH024), and Frontier Cross Project of national laboratory for molecular sciences (051Z011BZ3). Z. Luo acknowledges the national Thousand Youth Talents Program.

Figure 9. A histogram of the activation energy of water dissociation in different conditions.

reactive catalyst among the three systems; also, the water dimer adsorbed on Ni@Fe12 achieves the lowest activation energy of 0.20 eV. This is associated with the large shift of the 4s and 3d orbitals of Fe atoms and strong hybridization of Fe-3d with O2p orbitals (see Figure 5). Furthermore, two-water relay systems appear more efficient for water dissociation than the single one-water reaction as a result of the different mechanism. The second water serves as a “bridge” and stabilizes the intermediate species for water dissociation. The support effect is complicated in different systems. It enhances the catalytic activity of Fe13/DG and Fe12Ni/DG but restrains the water dissociation process for Fe12Ti supported on DG. All in all, whether doping heteroatoms or being coated on specific substrate, the process plays an important role in water adsorption and O−H bond dissociation performance.

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REFERENCES

(1) Suryanarayana, C.; Inoue, A. Iron-based bulk metallic glasses. Int. Mater. Rev. 2013, 58, 131−166. (2) Emran, K. M.; Al-Harbi, A. K. Comparison of the corrosion resistance behaviour for two metal-metal glassy alloys in neutral solution with chloride impact. J. Alloys Compd. 2018, 767, 753−762. (3) Dwyer, D. J.; Kelemen, S. R.; Kaldor, A. The Water Dissociation Reaction on Clean and Oxidized Iron (110). J. Chem. Phys. 1982, 76, 1832−1837. (4) Weiller, B. H.; Bechthold, P. S.; Parks, E. K.; Pobo, L. G.; Riley, S. J. The Reactions of Iron Clusters with Water. J. Chem. Phys. 1989, 91, 4714−4727. (5) Kiawi, D. M.; Chernyy, V.; Oomens, J.; Buma, W. J.; Jamshidi, Z.; Visscher, L.; Waters, L. B.; Bakker, J. M. Water Dissociation upon Adsorption onto Free Iron Clusters Is Size Dependent. J. Phys. Chem. Lett. 2016, 7, 2381−7. (6) Deguin, V.; Mascetti, J.; Simon, A.; Ben Amor, N.; Aupetit, C.; Latournerie, S.; Noble, J. A. Photochemistry of Fe:H2O Adducts in Argon Matrixes: A Combined Experimental and Theoretical Study in the Mid-IR and UV-Visible Regions. J. Phys. Chem. A 2018, 122, 529−542. (7) Gutsev, G. L.; Mochena, M. D.; Bauschlicher, C. W. Interaction of water with small Fen clusters. Chem. Phys. 2005, 314, 291−298. (8) Liu, S. L.; Tian, X. X.; Wang, T.; Wen, X. D.; Li, Y. W.; Wang, J. G.; Jiao, H. J. Coverage Dependent Water Dissociative Adsorption on the Clean and O-Precovered Fe(111) Surfaces. J. Phys. Chem. C 2015, 119, 11714−11724. (9) Jung, S. C.; Kang, M. H. Adsorption of a water molecule on Fe(100): Density-functional calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 115460−7. (10) Wang, W. J.; Wang, G. P.; Shao, M. H. First-Principles Modeling of Direct versus Oxygen-Assisted Water Dissociation on Fe(100) Surfaces. Catalysts 2016, 6, 29−11. (11) Freitas, R. R. Q.; Rivelino, R.; Mota, F. D.; de Castilho, C. M. C. Dissociative Adsorption and Aggregation of Water on the Fe(100) Surface: A DFT Study. J. Phys. Chem. C 2012, 116, 20306−20314.

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CONCLUSIONS The adsorption and dissociation of water on Fe13 and Fe12X (X = Ti, V, Cr, Mn, Co, Ni) clusters are studied by first-principles DFT calculation. We fully demonstrate the doping effect upon the lowest-energy structures, substitution sites, electron transfer inclination, and magnetic moments. This is associated with the s orbital of iron which gets altered in the presence of a heteroatom. Nevertheless, the putative global energy-minimum configurations of Fe12X clusters with an icosahedral geometry are only slightly affected by the adsorption of water. H2O prefers to adsorb on the top site of Fe12X with a pair of unoccupied orbitals of the iron clusters to host the lone pair electrons of oxygen in water. It is found from the PDOS analysis that the more overlap there is between the 1b1 of water and the 4s orbital of Fe atom, the larger is the adsorption energy among X@Fe12 systems. Ni@Fe12 exhibits lower activation energy for H2O dissociation among all the systems, and further lowered activation energy (∼0.20 eV) is achieved for a water dimer dissociation on Ni@Fe12, illustrating the importance of the water dimer mechanism for O−H bond dissociation. G


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(30) Kresse, G.; Furthmuller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15−50. (31) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (32) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (33) Henkelman, G.; Jonsson, H. A dimer method for finding saddle points on high dimensional potential surfaces using only first derivatives. J. Chem. Phys. 1999, 111, 7010−7022. (34) Henkelman, G.; Jonsson, H. Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points. J. Chem. Phys. 2000, 113, 9978−9985. (35) Henkelman, G.; Arnaldsson, A.; Jonsson, H. A fast and robust algorithm for Bader decomposition of charge density. Comput. Mater. Sci. 2006, 36, 354−360. (36) Yu, M.; Trinkle, D. R. Accurate and efficient algorithm for Bader charge integration. J. Chem. Phys. 2011, 134, 064111−8. (37) Gutsev, G. L.; Weatherford, C. A.; Jena, P.; Johnson, E.; Ramachandran, B. R. Structure and properties of Fen, Fen−, and Fen+ clusters, n = 7−20. J. Phys. Chem. A 2012, 116, 10218−28. (38) Yuan, H. K.; Chen, H.; Kuang, A. L.; Tian, C. L.; Wang, J. Z. The spin and orbital moment of Fen (n = 2−20) clusters. J. Chem. Phys. 2013, 139, 034314−9. (39) Bobadova-Parvanova, P.; Jackson, K. A.; Srinivas, S.; Horoi, M. Density-functional investigations of the spin ordering inFe13 clusters. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 66, 195402−10. (40) Piotrowski, M. J.; Piquini, P.; Da Silva, J. L. F. Density functional theory investigation of3d,4d, and 5d 13-atom metal clusters. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 155446−14. (41) Gutsev, G. L.; Johnson, L. E.; Belay, K. G.; Weatherford, C. A.; Gutsev, L. G.; Ramachandran, B. R. Structure and magnetic properties of Fe12X clusters. Chem. Phys. 2014, 430, 62−68. (42) Badger, R. M. A Relation Between Internuclear Distances and Bond Force Constants. J. Chem. Phys. 1934, 2, 128−131. (43) Zibordi-Besse, L.; Tereshchuk, P.; Chaves, A. S.; Da Silva, J. L. Ethanol and Water Adsorption on Transition-Metal 13-Atom Clusters: A Density Functional Theory Investigation within van der Waals Corrections. J. Phys. Chem. A 2016, 120, 4231−40. (44) Zhao, J. Y.; Zhao, F. Q.; Xu, S. Y.; Ju, X. H. DFT studies on doping effect of Al12X: adsorption and dissociation of H2O on Al12X clusters. J. Phys. Chem. A 2013, 117, 2213−22. (45) Roach, P. J.; Woodward, W. H.; Castleman, A. W., Jr.; Reber, A. C.; Khanna, S. N. Complementary Active Sites Cause Size-Selective Reactivity of Aluminum Cluster Anions with Water. Science 2009, 323, 492−495. (46) Liu, P.; Rodriguez, J. A. Catalysts for hydrogen evolution from the [NiFe] hydrogenase to the Ni2P(001) surface: The importance of ensemble effect. J. Am. Chem. Soc. 2005, 127, 14871−14878. (47) Zou, X.; Zhang, Y. Noble metal-free hydrogen evolution catalysts for water splitting. Chem. Soc. Rev. 2015, 44, 5148−80. (48) Bligaard, T.; Nørskov, J. K.; Dahl, S.; Matthiesen, J.; Christensen, C. H.; Sehested, J. The Brønsted−Evans−Polanyi relation and the volcano curve in heterogeneous catalysis. J. Catal. 2004, 224, 206−217. (49) Logadottir, a.; Rod, T. H.; Norskov, J. K.; Hammer, B.; Dahl, S.; Jacobsen, C. J. H. The Brønsted−Evans−Polanyi Relation and the Volcano Plot for Ammonia Synthesis over Transition Metal Catalasts. J. Catal. 2001, 197, 229−231. (50) Alvarez-Barcia, S.; Flores, J. R. How Fast Do Microhydrated Al Clusters React: A Theoretical Study. J. Phys. Chem. C 2011, 115, 24849−24857. (51) Alvarez-Barcia, S.; Flores, J. R. The oxidation of Al atoms embedded in water clusters: a dynamical study of the relay (Grotthuss-like) mechanism. J. Chem. Phys. 2011, 134, 244305. (52) McAllister, M. J.; Li, J. L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.;

(12) Bao, J. L.; Yu, H. S.; Duanmu, K.; Makeev, M. A.; Xu, X.; Truhlar, D. G. Density Functional Theory of the Water Splitting Reaction on Fe(0): Comparison of Local and Nonlocal Correlation Functionals. ACS Catal. 2015, 5, 2070−2080. (13) Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H. An advanced Ni-Fe layered double hydroxide electrocatalyst for water oxidation. J. Am. Chem. Soc. 2013, 135, 8452−5. (14) Han, L.; Dong, S.; Wang, E. Transition-Metal (Co, Ni, and Fe)Based Electrocatalysts for the Water Oxidation Reaction. Adv. Mater. 2016, 28, 9266−9291. (15) Jia, X. D.; Zhao, Y. F.; Chen, G. B.; Shang, L.; Shi, R.; Kang, X. F.; Waterhouse, G. I. N.; Wu, L. Z.; Tung, C. H.; Zhang, T. R. Ni3FeN Nanoparticles Derived from Ultrathin NiFe-Layered Double Hydroxide Nanosheets: An Efficient Overall Water Splitting Electrocatalyst. Adv. Energy Mater. 2016, 6, 1502585−6. (16) Wang, D. Y.; Gong, M.; Chou, H. L.; Pan, C. J.; Chen, H. A.; Wu, Y.; Lin, M. C.; Guan, M.; Yang, J.; Chen, C. W.; Wang, Y. L.; Hwang, B. J.; Chen, C. C.; Dai, H. Highly active and stable hybrid catalyst of cobalt-doped FeS2 nanosheets-carbon nanotubes for hydrogen evolution reaction. J. Am. Chem. Soc. 2015, 137, 1587−92. (17) Luo, Z.; Castleman, A. W. Special and general superatoms. Acc. Chem. Res. 2014, 47, 2931−40. (18) Luo, Z.; Grover, C. J.; Reber, A. C.; Khanna, S. N.; Castleman, A. W., Jr. Probing the magic numbers of aluminum-magnesium cluster anions and their reactivity toward oxygen. J. Am. Chem. Soc. 2013, 135, 4307−13. (19) Leuchtner, R. E.; Harms, A. C.; Castleman, A. W. Thermal Metal Cluster Anion Reactions - Behavior of Aluminum Clusters with Oxygen. J. Chem. Phys. 1989, 91, 2753−2754. (20) Luo, Z.; Gamboa, G. U.; Smith, J. C.; Reber, A. C.; Reveles, J. U.; Khanna, S. N.; Castleman, A. W., Jr. Spin accommodation and reactivity of silver clusters with oxygen: the enhanced stability of Ag13. J. Am. Chem. Soc. 2012, 134, 18973−8. (21) Shafai, G.; Hong, S.; Bertino, M.; Rahman, T. S. Effect of Ligands on the Geometric and Electronic Structure of Au13 Clusters. J. Phys. Chem. C 2009, 113, 12072−12078. (22) Koster, A. M.; Calaminici, P.; Orgaz, E.; Roy, D. R.; Reveles, J. U.; Khanna, S. N. On the ground state of Pd13. J. Am. Chem. Soc. 2011, 133, 12192−6. (23) Watari, N.; Ohnishi, S. Atomic and electronic structures of Pd13 and Pt13 clusters. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 1665−1677. (24) Chen, J.; Luo, Z. Single-point Attack of Two H2O Molecules towards a Lewis Acid Site on the GaAl12 Clusters for Hydrogen Evolution. ChemPhysChem 2019, 20, 499−505. (25) Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts. Nat. Mater. 2012, 11, 550−7. (26) Liang, G.; Huot, J.; Boily, S.; Van Neste, A.; Schulz, R. Catalytic effect of transition metals on hydrogen sorption in nanocrystalline ball milled MgH2-Tm (Tm = Ti, V, Mn, Fe and Ni) systems. J. Alloys Compd. 1999, 292, 247−252. (27) Wu, Z.; Wang, X.; Huang, J.; Gao, F. A Co-doped Ni−Fe mixed oxide mesoporous nanosheet array with low overpotential and high stability towards overall water splitting. J. Mater. Chem. A 2018, 6, 167−178. (28) Long, X.; Li, G.; Wang, Z.; Zhu, H.; Zhang, T.; Xiao, S.; Guo, W.; Yang, S. Metallic Iron-Nickel Sulfide Ultrathin Nanosheets As a Highly Active Electrocatalyst for Hydrogen Evolution Reaction in Acidic Media. J. Am. Chem. Soc. 2015, 137, 11900−3. (29) Kleiman-Shwarsctein, A.; Hu, Y. S.; Forman, A. J.; Stucky, G. D.; McFarland, E. W. Electrodeposition of alpha-Fe2O3 doped with Mo or Cr as photoanodes for photocatalytic water splitting. J. Phys. Chem. C 2008, 112, 15900−15907. H


Article

The Journal of Physical Chemistry A Prud’homme, R. K.; Aksay, I. A. Single sheet functionalized graphene by oxidation and thermal expansion of graphite. Chem. Mater. 2007, 19, 4396−4404. (53) Banhart, F.; Kotakoski, J.; Krasheninnikov, A. V. Structural defects in graphene. ACS Nano 2011, 5, 26−41. (54) Song, W.; Jiao, M.; Li, K.; Wang, Y.; Wu, Z. Theoretical study on the interaction of pristine, defective and strained graphene with Fen and Nin (n = 13, 38, 55) clusters. Chem. Phys. Lett. 2013, 588, 203−207. (55) Lim, D. H.; Negreira, A. S.; Wilcox, J. DFT Studies on the Interaction of Defective Graphene-Supported Fe and Al Nanoparticles. J. Phys. Chem. C 2011, 115, 8961−8970. (56) Li, Y. F.; Zhou, Z.; Yu, G. T.; Chen, W.; Chen, Z. F. CO Catalytic Oxidation on Iron-Embedded Graphene: Computational Quest for Low-Cost Nanocatalysts. J. Phys. Chem. C 2010, 114, 6250−6254. (57) Lim, D. H.; Wilcox, J. Mechanisms of the Oxygen Reduction Reaction on Defective Graphene-Supported Pt Nanoparticles from First-Principles. J. Phys. Chem. C 2012, 116, 3653−3660.

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The Doping Effect of 13-Atom Iron Clusters on Water Adsorption and O

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