Water Oxidation Catalysis with Fe2O3 Constrained at the Nanoscale

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Water Oxidation Catalysis with FeO Constrained at the Nano-Scale Meir Haim Dahan, and Maytal Caspary Toroker J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12666 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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The Journal of Physical Chemistry

Water Oxidation Catalysis with Fe2O3 Constrained at the Nano-scale Meir Haim Dahan and Maytal Caspary Toroker* Department of Materials Science and Engineering, Technion - Israel Institute of Technology, Haifa 3200003, Israel Keywords: DFT, iron oxide, catalysis, OER, water splitting.

Abstract Photo-electrochemical cells containing iron (III) oxide (Fe2O3) have attracted extensive investigations due to their ability to convert solar energy into chemical energy by water splitting. Recently, fabrication of nano-scaled Fe2O3 have been adopted for photoelectrochemical cells in order to increase solar energy absorption and reduce slow diffusion length of charge carriers. In order to understand how nano-scaled confinement influences catalytic efficiency, we perform Density Functional Theory +U calculations of water oxidation on a thin slab of Fe2O3(0001). We consider possible hydrogen vacancies that may appear at high pH and voltage and find that promoting hydrogen vacancy formation improves catalytic efficiency. We also analyze the effect of geometrical strain on the slab that may result from deposition on a substrate. We conclude that nano-Fe2O3 should be grown on a substrate with a similar lattice constant in order to reduce strain and improve catalytic efficiency.

*Corresponding author: E-mail: [email protected] , Tel.: +972 4 8294298.

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1. Introduction Solar energy conversion with photo-electrochemical cells has attracted a significant amount of research and development.1-3 One of the most studied materials in the past couple of decades is iron (III) oxide (Fe2O3).4-7 The many advantages of Fe2O3 include having proper band edge positions and band gap for catalysis and solar energy absorption, respectively. The outstanding advantage which makes Fe2O3 superior to other suggested materials is long-lasting stability.8 Yet, a major limitation is low mobility and high electron-hole recombination rate.9-11 This limitation can be avoided by nano-structuring.12-15 Among the different nanostructures proposed for Fe2O3, nanoribbons are interesting architectures due to their low-dimensionality that may be useful for better solar energy absorption and conversion. There are reports showing that synthesizing thin nanoribbons of Fe2O3 is possible.16-19 As a result, theoretical investigations have also considered models for Fe2O3 nanoribbons.20 However, no study has evaluated how low-dimensionality would influence the water oxidation catalysis ability of Fe2O3. Lower dimensional materials are widely used for many catalytic applications.21 The main reason is the larger surface area that can more efficiently absorb solar energy. In addition, the larger surface contributes additional active sites. Other factors may play a role since the electronic structure also changes with the reduction of dimensionality. Some examples that have been investigated include graphene and transition metal dichalcogenide nanostructures.22-23 In this paper, we explore the contribution of nano-scaling to catalysis for Fe2O3. We constructed periodic slab models of thin nano-ribbons of Fe2O3(0001) and used Density Functional Theory + U (DFT+U) to model the water oxidation catalytic mechanism. Our results show that reducing the number of atomic layers does not change the overpotential required for water oxidation. However, hydrogen vacancies that may appear at smaller scales reduce the overpotential. In addition, we applied strain on the nano-ribbon in order to mimic the geometrical effect of a substrate and found that a large mismatch would not be favorable for catalysis.

2. Methods and Calculation Details Spin-polarized Density Functional Theory calculations were performed with the VASP program.24-25 The DFT+U formalism of Duradev et al. was used with the Perdew-Burke-Ernzerhof (PBE) functional and an on-site Coulomb repulsion26 with an effective U-J term of 4.3 eV for Fe, since we wish to compare to previous calculations with this methodology that was shown successful in predicting electronic structure27-28 and catalytic ability of Fe2O3.29-38 Projected-augmented wave (PAW) potentials replaced the core electrons of Fe 1s2s2p3s and O 1s.39-40


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The bulk unit cell structure of Fe2O3 was cleaved at the (0001) facet since this is one of the stable facets at aqueous environment and since we wanted to compare to previous literature that studied this facet.29-30 However, in this work we chose a single Fe2O3 monolayer of Fe atoms, fully coordinated with oxygen atoms and hydroxyl groups. The chemical components of the sheet are: Fe2O6H6, that is a fully hydroxylated Fe2O3. Each one of the two Fe atom is octahedrally coordinated with six hydroxyl groups. Two of the hydroxyl groups are chemically bonded to both Fe atoms. Other parameters were the same as in ref. 29-30 for comparison: the vacuum length was taken to be 10 Å, the energy cutoff and k-point grid were chosen to be 700 eV and a Gammacentered k-point mesh of 3x3x1, respectively. The ion positions in the slabs were converged until the force components on all ions were less than 0.03 eV/Å. Final geometries obtained are provided in the Supporting Information (SI). Vacancy formation energy was calculated by the formula:41 1 ∆𝐸𝑓 (𝐻) = 𝐸ℎ𝑜𝑠𝑡 − 𝐸𝑑𝑒𝑓𝑒𝑐𝑡 − 𝐸(𝐻2 ) 2 where 𝐸ℎ𝑜𝑠𝑡 is the energy of the host surface cell (denote *OH, i.e. terminating hydroxyl group) with no H vacancy or reaction intermediate adsorbed, 𝐸𝑑𝑒𝑓𝑒𝑐𝑡 is the energy of the surface unit cell with a H vacancy, and 𝐸(𝐻2 ) is the energy of a hydrogen diatomic molecule. The surface energy is calculated from the formula: ∆𝐸𝑠𝑢𝑟𝑓 = [𝐸𝑠𝑢𝑟𝑓 − 𝐸𝑏𝑢𝑙𝑘 − 3 ∙ 𝐸(𝐻2 𝑂)]/2𝐴 where 𝐸𝑠𝑢𝑟𝑓 is the total energy of the surface, 𝐸𝑏𝑢𝑙𝑘 is the total energy of the Fe2O3 bulk, three is the number of H2O groups binding to the surface and forming hydroxyl groups on the surface, and A is the surface area. We considered slabs representing reaction intermediates of the following fivestep mechanism that has been studied in refs. 29-30, but modified to reflect alkaline operating conditions, * + H2O  *H2O

(1)

*H2O + h+ + OH-  *OH + H2O

(2)

*OH + h+ + OH-  *O + H2O

(3)

*O + h+ + OH-  *OOH

(4)

*OOH + h+ + OH-  * + O2 + H2O (5) where intermediate "*" is a slab with an oxygen vacancy termination available for adsorption and, for example, intermediate "*H2O" is a slab with an adsorbed H2O molecule. Overall, this reaction mechanism includes one adsorption reaction and four



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deprotonations. In practice, we remove an H atom rather than a proton in each step since under electrochemical conditions the excess electron is also removed from the surface as a result of applying voltage. The free energy for each reaction was calculated by subtracting reactants and products and adding our calculated Zero Point Energy (ZPE) corrections and entropic contributions. In the supporting information we provide details of ZPE calculation for all slabs, showing negligible differences from either removing atomic layers, adding hydrogen vacancies, or applying strain; hence, we ultimately used ZPE correction from the thick slabs. The applied voltage is accounted for by adding a constant energy -eU of 1 eV to the free energies of reactions 1-5 with respect to the normal hydrogen electrode (NHE). The pH is incorporated by adding −𝐾𝐵 𝑇 ∙ 𝑙𝑛10 ∙ 𝑝𝐻 (𝐾𝐵 is Boltzmann constant and 𝑇 is the temperature of 298.15 K) to the free energy calculations. The overpotential is defined as voltage needed to add to the calculated electrochemical potential so that all reaction free energies are negative.29 The free energies were calculated in the presence of added hydrogen vacancies or strain, i.e. contraction or elongation of the surface lattice constants horizontal to the surface plane. Hydrogen vacancies were added to each reaction intermediate slab at various locations. As seen in Figure 1, we considered both H vacancy below and aside the active site. The unit cells of all slabs are obtained from the bulk unit cell and normally only the ion positions are optimized. In some cases mentioned explicitly, full lattice vectors is also accounted for in order to consider surface reconstruction that appears at the nano-scale. The contraction or elongation of the slab volume by X% was done by multiplying the surface lattice vectors by √𝑋.

Figure 1. Unit cell for the nano-scaled Fe2O3(0001) surface showing locations of hydrogen vacancies for the first reaction intermediate denoted as *vac. Red, brown, and white spheres represent oxygen, iron, and hydrogen atoms, respectively.

3. Results In this section we present calculated free energies of water oxidation for a thin "ribbon" of Fe2O3(0001) in the presence of hydrogen vacancies and strain (see Figure 2). Hydrogen vacancies are anticipated to have strong dominance for a thin material at operating conditions of high pH and voltage. Our calculations reveal that hydrogen vacancy formation energy for a thin nanoribbon is 0.56 eV under pH=14 and E=1 Volt. Furthermore, geometrical strain may be induced by a substrate. We explain that


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hydrogen vacancies charge the surface and could improve catalytic performance, while geometrical strain alters the workfunction and has an unfavorable effect on catalysis.

Figure 2. Catalytic cycle of water oxidation with Fe2O3(0001) having one hydrogen vacancy (denoted hydrogen 1 in the text). The geometries are fully optimized. Red, brown, and white spheres represent oxygen, iron, and hydrogen atoms, respectively. Our central result regarding the addition of hydrogen vacancies is that they slightly decrease the free energy of reaction 3 (see Table 1), which determines the overpotential in the stoichiometric case. As a result, the overpotential may decrease. However, when the concentration of hydrogen vacancies is large or when the vacancies are close to the active site (hydrogen vacancy 2 is closer than hydrogen vacancy 1 to the active site), the decrease in the free energy of reaction 3 is accompanied by an increase in the free energy of reaction 4, which is now determining and increasing the overpotential. We note that the free energies of a thicker slab (with the reaction happening on both sides of the slab in order to correct for inter-unit cell dipole interactions) are similar to the free energies for the thin nano-Fe2O3; hence, reducing the number of layers has little effect on catalysis. Table 1. Free energies and overpotentials for Fe2O3(0001) with or without hydrogen vacancies at pH=14 and E=1 Volts. Units are in eV. Results for thick slabs are obtained from ref. 34. The reaction in all thin slabs occurs on one side of the slab, while the reaction on the thick slab occurs at two sides. Free energies without potential and pH corrections are given in the supporting information. Reaction

1 2 3

No No H vacancies, vacancies, vacancy thick slab thin slab 1

-1.78 -1.86 -0.01

-2.18 -1.99 0.05

-1.98 -0.73 -0.16

H vacancy 2

-1.60 -0.78 -0.38


H H vacancies vacancies 1 and 2 2 and 3

-1.56 -0.36 -0.40

-1.59 -0.36 -0.28


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4 5 overpotential

-0.15 -0.92 0.71

0.03 -0.62 0.77

-0.16 -1.69 0.56

0.27 -2.23 0.99

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0.09 -2.48 0.81

-0.12 -2.37 0.60

The changes in the free energies as a result of hydrogen vacancies can be visualized by observing the cumulative free energies (Figure 3). The most dominant feature is the rise of the cumulative free energy in reaction 2. We observed a similar rise in reaction 2 in our previous study,30 when considering Fe vacancies in thick Fe2O3. Both Fe and H vacancies generate excess holes in the lattice that inhibit reaction 2, which involves creating additional holes. The hole generated by a H vacancy is located on an Fe atom according to the magnetization changes observed: the magnetization on an Fe atom reduced from 4.3 to 3.6 Bohr magneton as a result of an excess hole). The surface charging as a result of hydrogen vacancies can explain the trends in the free energies. Hydrogen vacancies create holes and destabilizes the surface. Specifically, intermediates *vac, *OH2, and *OOH are now less neutral and stable. Therefore, the free energy of reactions 2 and 4 increase, while reaction 5 decreases.

Figure 3. Cumulative free energies for water oxidation reaction for Fe2O3(0001) in the a) pure, b) H vacancy, and c) 2H vacancy case. The locations of the vacancies are denoted as "H vacancy 1" and "H vacancies 1 and 2" as shown in Figure 1, respectively. In addition to hydrogen vacancies, strain may have a large effect on nanostructure functionality. In order to explore the effect of strain on catalysis, we considered both geometrical compression and expansion of the lattice vectors (see catalytic cycle in Figure 4). We find that the best performance occurs when there is no geometrical strain. As seen in Table 2, the free energy of reaction 3 (that determines the overpotential at no strain) decreases when the lattice vectors are shorter. However, this favorable decrease comes at the expense of an increase in the free energy of reaction 4 and as a result, an increase in the overpotential. The increase in overpotential is also observed when the lattice constants increase after allowing full relaxation (0.87 Volt in Table 2).


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Figure 4. Catalytic cycle of water oxidation with Fe2O3(0001) having strain of 10% contraction. The geometries are fully optimized. Red, brown, and white spheres represent oxygen, iron, and hydrogen atoms, respectively. The atomic distance 𝛿 shown at the bottom of each intermediate (in Å) refers to the smaller distance that results from contraction between, for example, the oxygen atoms marked with the yellow crosses.

Table 2. Free energies and overpotentials for Fe2O3(0001) with or without strain at pH=14 and E=1 Volts. Values without operating conditions are given in SI. "Lattice relaxed" refers to fully relaxing the ions and lattice constants of the slab. The percentage is given for the expansion or contraction of the volume. Units are in eV. Free energies without potential and pH corrections are given in the supporting information. Reaction

No strain

1 2 3 4 5 overpotential

-1.78 -1.86 -0.01 -0.15 -0.92 0.71

Lattice 5% 10% 5% 10% 15% relaxed contract contract expand expand expand

-1.55 -1.82 0.15 -0.48 -1.02 0.87

-1.62 -1.97 0.01 0.15 -1.29 0.88

-1.68 -1.88 -0.04 0.27 -1.38 0.99

-2.15 -1.98 0.09 -0.10 -0.59 0.81

-1.65 -1.94 0.14 -0.35 -0.91 0.86

-1.59 -1.89 0.20 -0.50 -0.94 0.92

20% expand

-1.53 -1.82 0.27 -0.54 -1.09 0.99

The effect of strain on the overpotential is apparent in the cumulative free energy curves. As seen in Figure 5, there is a monotonous decrease in reaction step 3 and a rise in reaction step 4. The effect of strain on the reaction free energies can be explained from the changes in the work function. As seen in Figure 6, the valence band edge position decreases (work function increases) when the lattice constants of the slab are elongated. The larger distance between Fe-O bonds localized the atomic orbitals and as a consequence the energy required to extract an electron is larger. The larger work function inhibits *OH slab deprotonation and electron extraction that are occurring in reaction 3, and therefore the free energy of this reaction is larger (Table 2). This result implies that expansion stabilized intermediate *OH relative to intermediate *O, and therefore the free energy of reaction 3 increases while reaction 4 decreases.



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Figure 5. Cumulative free energies for water oxidation reaction for Fe2O3(0001) in the a) pure, b) %10 contraction, and c) 10% expansion.

Figure 6. Valence band edge position for Fe2O3(0001) under applied strain for reaction intermediate *OH. The red single dot shows the valence band edge with a H vacancy and no strain. We note that a major advantage of Fe2O3 is the proper band gap and stability. We calculated the band gap on the Fe2O3 nanoribbon and found a value of 2.6 eV, which is higher than the bulk band gap of 2.1 eV,27 consistent with the quantum size effect. The surface energy is negative -0.31 J/m2 as a result of terminating the surface with hydroxyl groups and indicates the creation of the surface is favorable and therefore stable. Finally, we considered a combination of having hydrogen vacancy 1 and performing geometrical changes. The corresponding free energies for reactions 1-5 are -0.31, 1.11, 1.59, 1.81, and 0.23 eV when contracting 5% of the volume and having a


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hydrogen vacancy. After full geometry optimization with a hydrogen vacancy, the free energies are -0.04, 1.62, 2.01, 1.47, and -0.63 eV. We observe that the free energy required for reaction 2 rises as a result of vacancy-generated holes charging the surface. At 5% contraction with a hydrogen vacancy, the free energy of reaction 3 decreases while that of reaction 4 rises. But elongation resulting from full geometry optimization increases the free energy of reaction 3 and the overpotential.

4. Conclusions The water oxidation reaction was modeled for a nano-ribbon of Fe2O3(0001). The model included hydrogen vacancies and geometrical strain that may be dominant at the nano-scale. We considered H vacancies at different positions, as well as contraction and expansion of the unit cell's lattice vectors. Our calculations reveal that H vacancies may reduce the overpotential required for water oxidation. The surface charging with vacancy-generated holes makes the first deprotonation step difficult, but the second deprotonation step requires less energy and the overpotential is lower. We further find that strain is unfavorable for catalyzing water oxidation. Elongating the lattice vectors localizes the atomic orbitals and increases the work function. With a larger work function, electron extraction during reaction 3 is prohibited and the overpotential increases. In contrast, contracting the lattice vectors decreases the work function and reaction 3 becomes more plausible, but in turn reaction 4 has a higher free energy that increases overpotential. Since any strain, contraction or expansion, increases the overpotential, we suggest to grow nano-Fe2O3 on a substrate with similar lattice constants in order to reduce mismatch and strain for maintaining good catalytic ability.

Acknowledgements This research was supported by the Nancy and Stephen Grand Technion Energy Program, the I-CORE Program of the Planning and Budgeting Committee, and The Israel Science Foundation (Grant No. 152/11). This work was supported by the post LinkSCEEM-2 project, funded by the European Commission under the 7th Framework Programme through Capacities Research Infrastructure, INFRA-2010-1.2.3 Virtual Research Communities, Combination of Collaborative Project and Coordination and Support Actions (CP-CSA) under grant agreement no RI-261600.

Supporting information available Further details on free energy calculations and unit cells used for the surfaces are provided in the supporting information.



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Table of Contents Image

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Table of Contents Image Table of Contents Image 177x127mm (300 x 300 DPI)



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Water Oxidation Catalysis with Fe2O3 Constrained at the Nanoscale

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