Article pubs.acs.org/JPCC
Engineering Band Edge Positions of Nickel Oxyhydroxide through Facet Selection Vicky Fidelsky† and Maytal Caspary Toroker*,‡ †
The Nancy and Stephen Grand Technion Energy Program and‡Department of Materials Science and Engineering, TechnionIsrael Institute of Technology, Haifa 3200003, Israel S Supporting Information *
ABSTRACT: A promising material for catalysis should have appropriate band edge positions. Unfortunately, the band edge positions are unknown for nickel oxyhydroxide (NiOOH), one of the best water oxidation catalysts. We present here first-principles calculations of band edge positions for surfaces of pure NiOOH. Specifically, the band edge positions of NiOOH (001), (100), and (01̅5) were calculated using density functional theory (DFT) + U, PBE0, and G0W0 methods by using slab models for the surfaces with a vacuum region and periodic boundary conditions. The band edge positions were determined by calculating the band gap center using DFT + U and by accounting for the band gap using G0W0. This approach was validated with other methods. Our results show that due to the polarity of NiOOH, the valence band position is especially sensitive to surface orientation: facets with O−H bonds parallel to the normal of the surface have the highest valence band edge. The following relation between valence band edges is obtained: EVBM(001) > EVBM(01̅5) > EVBM(100). As a result, the (100) surface should be the most active, while the other surfaces may be less efficient in enabling the oxygen evolution reaction. Our results suggest that chemical activity of polar materials can be controlled through facet selection.
1. INTRODUCTION Solar energy conversion through water splitting is one of the most popular fields in recent years.1 In water splitting, water is oxidized at the anode to form oxygen gas (oxygen evolution reaction, OER), and water is reduced at the cathode to form hydrogen fuel (hydrogen evolution reaction, HER).2 Therefore, efficient anode and cathode materials at low cost are essential.3 An outstanding inexpensive catalyst that is considered to be one of the best for OER is nickel oxyhydroxide (NiOOH). As a result, many recent studies have included NiOOH in photoelectrochemical and other solar cell devices, such as Fedoped NiOOH structures, 4−7 Fe 2 O 3 /NiOOH, BiVO 4 / NiOOH, perovskite/NiOx heterostructures,8−10 and mixed iron−nickel oxide thin films.11 In attempt to understand why NiOOH is so successful, several theoretical studies have focused on modeling the electronic structure and catalytic activity of NiOOH.12−15 Despite these cutting-edge advances, no study has analyzed the electronic band edge positions of NiOOH, which is a critical characteristic for catalysis feasibility. Therefore, we present first principle calculations of the band edge positions of NiOOH and find strong dependence on facet selection. This work contributes to understanding the nature of NiOOH’s success.
were done with several exchange-correlation (XC) functionals. The Perdew−Burke−Ernzerhof (PBE) functional with on-site Coulomb repulsion18 of Duradev et al. DFT + U formalism was used with an effective U-J term of 5.5 eV for Ni.19 Previous literature reported deriving this U-J value from linear response theory, as well as employing this value for NiOOH.12−14 Projected-augmented wave (PAW) potentials replaced the Ni 1s2s2p3s3p and O 1s core electrons.20,21 The bulk unit cell structure was built according to the more commonly reported crystal structure of NiOOH.13,22−25 We obtained the bulk structure from the Supporting Information of ref 13. We note that we focus this study on the β-phase of NiOOH, which may be one of the chemically active phases of NiOOH.4,26 The unit cell has three stoichiometric units of NiOOH that are arranged in three different layers (see Figure 1). The bulk was cleaved at three different surface facets: (001), (100), and (015̅ ) (see Figure 2). The (001) and (100) were selected since they were observed in previous experiments to be stable.27,28 The (01̅5) slab was also built since this high-index facet was interpreted to be chemically active.13 The vacuum length was taken to be 10 Å for all slabs in order to converge the valence band edge position. Specifically, instabilities in the integrated potential energy appeared in the
2. COMPUTATIONAL DETAILS All calculations were performed with the VASP program.16,17 Spin-polarized density functional theory (DFT) calculations
Received: January 8, 2016 Revised: March 4, 2016
© XXXX American Chemical Society
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DOI: 10.1021/acs.jpcc.6b00214 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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the relative size of the bulk cell. The ion positions in the slabs were converged with DFT + U until the force components on all ions were less than 0.03 eV/Å. Final geometries obtained are provided in the Supporting Information. Although DFT + U has been used before for NiOOH,13,15 validating accuracy using PBE0 is important since DFT + U fails to predict a band gap for NiOOH.12 Therefore, the band edges were calculated also with PBE030,31 at the DFT + U geometry since the choice of functional was shown to have little effect on NiOOH geometry. 12 The hybrid functional calculation was done with the same k-point grid and the same q-point mesh for the Fock potential. We used the scheme for calculating band edge positions that we have presented in our previous publication.29 Accordingly, the band gap center (BGC) is calculated from the slab with DFT + U or PBE0 and then half of the band gap is subtracted (or added) to obtain the valence band edge position (or conduction band edge position). The band gap is calculated from a G0W0 method,32,33 which is directly comparable to the experimental photoemission/inverse photoemission (PES/ IPES) band gap measurements. As opposed to calculating the valence band edges directly from the slab with DFT + U, this approach of calculating the bang gap center carries the advantage of canceling errors related to band gap underestimation.29 The bulk structure for the G0W0 calculation is a 2 × 2 × 1 supercell in order to reduce errors obtained from reducing the k-points to 1 × 1 × 1. This technique of increasing cell size to reduce k-points was done in a previous study of NiOOH.12 Prior to the G0W0 calculation, the energy cutoffs of 600 eV and a Gamma-centered k-point grid of 1 × 1 × 1 were chosen in order to converge the total energy to EVBM (100). The relative trend of band positions does not change using either PBE0 hybrid functional or DFT + 5.5 (see Table 2). According to our scheme for calculating band edge positions that we have proposed in a recent study,29 the band gap center is calculated at the slab unit cell with either DFT + 5.5 or PBE0. B
DOI: 10.1021/acs.jpcc.6b00214 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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notably for the (001) surface that would have a valence band edge of −5.51 and −5.76 eV, for selecting the PBE0 or experimental band gaps, respectively, which is close to the energy required for water oxidation (−5.7 eV). Again, as seen in Table 2, the overall trend between facets is not affected by the method choice. We performed another calculation scheme with the conventional approach where the valence band edge is calculated first using PBE0, and then the PBE0/G0W0 band gap is added. In this case, as seen in Table 3, the valence band edge of the (001)
Table 2. Band Gap Center Positions of NiOOH(001), NiOOH(015̅ ), and NiOOH(100) Calculated with DFT + U and PBE0; Units Are eV (001) (015̅ ) (100)
DFT + 5.5
PBE0
−2.31 −4.77 −6.35
−3.88 −4.33 −6.44
The valence band edge is then calculated by subtracting half of the band gap value calculated from the bulk unit cell with a G0W0 method. As we have shown in our previous work,29 the advantage of calculating the band gap center is robustness with respect to different choices of XC functionals; the advantage of calculating the band gap center is the cancellation of error associate with the choice of XC functionals, as demonstrated for several transition metal oxides. Among several methods exemplified,29 DFT + U is most attractive in terms of computational efficiency and accuracy balance for calculating the band gap center. We also validate the calculation against PBE0 since, as far as the band gap in concerned, so far PBE0 has brought the best performance in obtaining the band gap.12 A G0W0 correction is important to include since the latter gives a quasiparticle band gap that can be directly comparable to the experimental band gap. Hence, the band edge positions of Table 1 were calculated with DFT + U for the band gap center and PBE0/G0W0 for the band gap. The band gap calculated with DFT + U/G0W0 is 2.76 eV and with PBE0/G0W0 method is 3.12 eV. The latter is similar to the band gap of 3.25 eV calculated with PBE0 and in agreement with one of the experimental values of 3.75 eV36 (although another value of 1.7−1.8 eV was also measured).37 Although G0W0 usually increases the band gap,29 here there is a decrease from the PBE0 result; nevertheless, the PBE0/G0W0 value is similar to PBE0 within the 0.1 eV numerical accuracy. Although the PBE0 value is slightly closer to experiment, we prefer to use the one calculated with PBE0/G0W0 since the latter method has formal justification for comparison to the experimental observable.29 The detailed projected density of states obtained by the PBE0/G0W0 method is shown in Figure 3, where a band gap separates band edges comprising hybridized Ni and O states. We note that if the PBE0 or experimental gap had been chosen, then the band edge positions would all be consistently lower. Furthermore, if the PBE0 band gap center had been selected then the valence band edges would change, most
surface is also above the energy required for water oxidation. However, the valence band edge of the (01̅5) surface is slightly above the energy required for water oxidation. Although the individual values for the band edge positions change with the method choice, the overall trend between facets is still significant. Hence, we find that the valence band position lowers as the angle of the O−H chemical group relative to the surface is smaller. For example, in the (001) facet, the valence band edge is highest and the O−H group is perpendicular to the surface: the dipole along the O−H bond creates an electrostatic force that drives electrons along the axis of this bond toward the surface. In contrast, the O−H group is parallel to the (100) facet and creates zero dipole for electrons to be attracted toward leaving the surface. As a result, the (001) facet is the least effective in inducing holes to participate in water oxidation. In fact, within our scheme of calculating the band gap center with DFT + U and the band gap with PBE0/G0W0, the valence band edge is located above the free energy needed for water oxidation (see Figure 4). Therefore, the (001) facet may not be suitable for water oxidation catalysis. In comparison with the (001) facet, a smaller angle of OH group relative to the surface makes the electron more available
Figure 3. Projected density of states for NiOOH obtained with the PBE0/G0W0 method.
Figure 4. Band edge positions for NiOOH(001), NiOOH(100), and NiOOH(01̅5). The dashed and dotted lines represent the free energy required for water oxidation and reduction, respectively (denoted H2O/O2 and H2O/H2). The colored rectangles represent the band gap position for each facet.
Table 3. Valence and Conduction Band Edge Positions of NiOOH(001), NiOOH(01̅5), and NiOOH(100) Calculated with the Conventional Approach of Referencing the PBE0 Valence Band Edge to the Vacuum and Adding the Band Gap of PBE0/G0W0; Units Are eV (001) (01̅5) (100)
C
valence band edge
conduction band edge
−4.15 −5.36 −7.79
−1.03 −2.24 −4.67
DOI: 10.1021/acs.jpcc.6b00214 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C for catalysis. This angle reduction is demonstrated in (015̅ ) and (100) surfaces: the angles are ∼45° and 0°, respectively. At smaller angle configurations, the dipole moment along the zaxis of the slab in OH group is smaller, leading to lower valence band edge values. As a result, according to the scheme of calculating the band gap center with DFT + U and the band gap with PBE0/G0W0, the valence band position values for (01̅5) and (100) are lower than the energy required for water oxidation (−5.7 eV) and therefore permit the OER. Hence, these planes should be preferred over (001) in catalysis with NiOOH. The relative valence band edge positions of different facets indicate that the (100) surface is most promising for OER. The larger difference between the free energy for OER (−5.7 eV) and the valence band edge acts as a driving force for holes to transfer from NiOOH to perform surface catalysis. Since NiOOH is often used as a photoanode for water oxidation, our analysis focuses on the OER capabilities of NiOOH surfaces. However, our results show that facet selection can control the position of the conduction band edge relative to the free energy required for the HER. Water reduction may be feasible only under the necessary condition that the conduction band is higher than the free energy for the HER (−4.4 eV). According to all calculation schemes considered, the NiOOH(001) and NiOOH(015̅ ) surfaces should be capable of performing the HER. Hence, the results indicate that for NiOOH, which has an intrinsic dipole, the surface facet has a large influence on the band edge positions and therefore on the availability of charge carriers for catalysis.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +972 4 8294298. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Morantz Energy Research Fund, 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). The COST Action IC1208 is acknowledged for funding travel that promoted this research. This work was supported by the post LinkSCEEM-2 project, funded by the European Commission under the seventh Framework Programme through Capacities Research Infrastructure, INFRA2010-1.2.3 Virtual Research Communities, Combination of Collaborative Project and Coordination and Support Actions (CP-CSA) under grant agreement no RI-261600. V.F. acknowledges scholarship by the Jacob Isler Foundation.
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REFERENCES
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4. CONCLUSIONS The band edge positions were calculated for NiOOH using first principles. We found that by choosing the surface facet orientation, the valence band edge position changes dramatically and as a result may improve the OER reaction. This result is robust since using different XC functionals does not change the trend of valence band edge positions of NiOOH: EVBM(001) > EVBM(015̅ ) > EVBM(100). Hence, among the facets studied, the NiOOH(100) surface should be most active for OER. We obtained several additional results on the electronic structure of bulk NiOOH. First, we calculated the band gap with PBE0/G0W0 and found a value of 3.1 eV, which is in good agreement with experiment. Second, the chemical character of the band edge positions are a mixture of O and Ni states. Obtaining the band gap of the bulk aided in determining the band edge positions. The sensitivity of band edge positions to facet selection is pronounced since the crystal structure of NiOOH shows that this material has a unique polarity determined by the location of H atoms and O−H bonds; the direction and polarity of the O−H bond has a direct effect on the valence band edge position. We suggest performing experiments that compare different facets of NiOOH. Moreover, we anticipate that facet selection could be exploited for controlling chemical activity of polar materials.
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Unit cells used for bulk and surfaces (PDF)
ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00214. D
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