Determining the Band Alignment at the BiVO₄|NiOOH Interface Using the Hybrid DFT Technique

José C. Conesa ^*

Instituto de Catálisis y Petroleoquímica, CSIC, Marie Curie 2, 20049 Madrid, Spain

* Correspondence: José C. Conesa

Academic Editors: Robert S. Weber and Ali Abdelhafiz

Special Issue: Recent Advances in Electrocatalysis for Sustainable Energy and Green Chemistry

Received: November 29, 2021 | Accepted: February 08, 2022 | Published: February 21, 2022

Catalysis Research 2022, Volume 2, Issue 1, doi:10.21926/cr.2201005

Recommended citation: Conesa JC. Determining the Band Alignment at the BiVO4|NiOOH Interface Using the Hybrid DFT Technique. Catalysis Research 2022;2(1):10; doi:10.21926/cr.2201005.

© 2022 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

BiVO₄ is a photocatalyst that has received considerable attention since it was first reported over 20 years ago [1]. Since its first mention, more than 3100 articles have been published on its photocatalytic properties. It is a visible light-active photocatalyst (its bandgap is E_g≈2.4 eV) and exhibits a relatively high hole mobility [2], facilitating the process of hole transport to the surface to improve the efficiency of oxidation reactions. Furthermore, it is an n-type semiconductor, which can potentially promote oxidation reactions in a photoelectrochemical setup. The electron mobility is low [3], but this can be compensated by increasing the electron concentration following the process of doping [4]. The positions of its valence and conduction bands (VB and CB, respectively) in contact with water are also known [5,6]. This indicates that the CB lies below the NHE electrode. As it is an n-type semiconductor, its Fermi energy will be a few tenths of eV below that. This implies that H₂ production using the semiconductor will require either the application of voltage or the exploitation of a Z-scheme similar to the one observed in the case of natural photosynthesis. Analysis of the position of VB reveals that this material can be potentially used for the efficient photo-oxidation of water. A similar observation has already been reported in a paper which reported a photocatalytic reaction [1].

It has been observed that to realize an efficient photoinduced water oxidation reaction, the addition of a co-catalyst may be needed. Several co-catalysts have been reported in the literature, most of them based on Ru or Ir oxides. Hydroxides and salts of Co, Ni, or Fe have also been used as co-catalysts. One of the best co-catalysts, not based on expensive metals, is nickel oxide. Some of the (oxy)hydroxides of Ni have also been used as cheap co-catalysts [7]. This article recognizes β-NiOOH as a likely active component of the system. It has been recently reported that when Fe is used for doping, the activity of NiOOH (used for O₂ evolution) is significantly improved [8].

However, the combination of BiVO₄ and NiOOH has been studied only recently, and the combination has been used to realize a photoanode structure [9,10,11,12]. Methods to improve the stability of the anode in alkaline media (by saturating the solution with V⁵⁺ or decorating the system with TiO₂ overlayers) have been proposed [13,14]. However, studies have not been conducted to understand to what extent the band positions of BiVO₄ and NiOOH favor the O₂ photo-evolution reactions at these electrodes. We used the (hybrid) DFT technique to address this issue.

2. Computational Methods

VASP (version 5.4.4) was used for data analysis [15]. The Projector augmented wave method was used [16,17] and the valence structures 1s¹, 2s²2p⁴, 3p⁶3d⁵, 3p⁶3d¹⁰, and 5d¹⁰6s²6p³ for H, O, V, Ni, and Bi, respectively, were considered. The threshold for geometry relaxation was below 10^–4 eV in all cases. Spin polarization, i.e. full separation of spin-up and spin-down components, was used for NiOOH. U values for V and Ni (4.0 eV and 5.3 eV, respectively) were used, and dispersion forces were taken into account [18,19] for both materials. The known experimental value for BiVO₄ (E_g = 2.4 eV) was used for the calculations. For NiOOH, the value resulting in the generation of the minimum energy value (previously reported by this author; E_g = 1.45 eV) was used for the calculations [20]. The k meshes considered were (6 6 8) and (6 8 6) for BiVO₄ and NiOOH, respectively. Spin-orbit coupling was not considered for BiVO₄, as the resulting difference in the bandgap values was negligible. The HSE06 method [21,22] was then used to adjust the experimental or theoretically computed values.

For building the interfaces, the (100) interface was considered for BiVO₄, as it is nonpolar according to Tasker’s criterion [23]. For NiOOH, a 4x(001) interface (nonpolar according to Tasker’s criterion) [23] was used. This allowed the generation of an interface of 11.70 × 5.09 Å for BiVO₄ and an interface of 11.30 × 5.29 Å for NiOOH. It is believed that a good epitaxial match can be obtained under these conditions as the differences are <4%. The k mesh considered for the interface (both for the combined slabs and the slabs against vacuum) was (2 1 1), and this helped simplify the calculations.

Interfaces were built following the guidelines laid down by van de Walle and Martin [24]. They determined methods to compute the Hartree potential and the experimental (or theoretically computed) values of the valence and conduction bands (in the following, VB and CB) with respect to the mentioned Hartree potential were located. Finally, a Bader analysis was carried out using the software developed by Henkelman and co-workers [25].

3. Results

3.1 BiVO₄

The bandgap of this material can be reproduced using HSE06, with a Fock exchange α value of 0.045. The (100) interface (which is drawn vertical and perpendicular to the plane of the drawings in all cases) is considered, and the result is shown in Figure 1a). Please note that the Hartree profile and its maximum potential are represented in black in all cases (to minimize the uncertainties attributable to inaccuracies in the grid used). The position of VB is represented in red, and the position of CB is represented in blue.

Figure 1 Positions of the Hartree potential, VB, and CB with respect to the a) BiVO₄ levels and b) NiOOH levels.

3.2 NiOOH

The computed bandgap, determined by analyzing the most stable case (reported previously by this author: E_g = 1.45 eV), could be reproduced using HSE06. The Fock exchange coefficient α was 0.20. The resulting Hartree potential and VB and CB positions are shown in Figure 1b).

3.3 Compound BiVO₄|NiOOH Interface

Eleven layers of BiVO₄ and five layers of NiOOH were used for the studies. Figure 2 presents the results. Only one-half of the unit cell is used as the compound slab is characterized by inversion symmetry. The figure reveals that the VB of NiOOH lies slightly above the CB of BiVO₄.

Figure 2 Results obtained for the combined slab model.

3.4 BiVO₄ and NiOOH Facing Vacuum

Now let us see what happens when BiVO₄ and NiOOH are subjected to vacuum conditions. Figure 3 presents the case where 11 layers of BiVO₄ and 5 layers of NiOOH are studied. Here, the dimensions in the two directions perpendicular to the interface were considered to be fixed, as they are influenced by the process of optimization of the bulk structure. The relaxation affects thus only to the direction perpendicular to the interface. Furthermore, a vacuum section of more than 10 Å was inserted in both materials, and the vacuum value was set to 0 eV. Here, the VB of NiOOH was higher than the CB of BiVO₄.

Figure 3 Positions of the Hartree potential, VB, and CB in the cases of BiVO₄ and NiOOH facing vacuum.

3.5 Overall Result

Figure 4 summarizes the results. It can be observed that in both cases the VB of BiVO₄ lies significantly lower than the VB of NiOOH. This indicates that holes generated in BiVO₄ (under conditions of irradiation) can be readily transferred to NiOOH, allowing the generation of O₂ in the latter. The two cases are distinctly different from each other in a crucial aspect. In the slab versus vacuum case, holes present in NiOOH can recombine with the electrons present in the CB of BiVO₄, while the recombination of charge carriers does not occur in the case of the combined slab. The electrons in the CB of BiVO₄ will then be transferred to the other electrode where they can potentially participate in the generation of H₂ or help in the generation of other reduction products.

Figure 4 Comparison of the results obtained by studying the combined slabs model and the slab vs. vacuum model.

3.6 Analysis of Charge Densities

Next, we analyzed the charge densities. Figure 5 presents the total charge densities in the left section of the figure. The results obtained by analyzing the valence space numbers presented at the beginning of Section 2 were considered. The differences in the densities are presented in the section to the right of the figure. The densities of each slab are subtracted from the values obtained from the combined slabs model. The atom and cell dimensions were the same as those observed in the case of the combined slabs. A transfer of charge from the NiOOH side to the BiVO₄ side was observed, and this agrees well with the results obtained using the combined slabs model, which predicted that transfer of charges occurred between these two sides. This result contradicts the result obtained from Bader analysis, which predicted that charge was transferred from BiVO₄ to NiOOH (0.011 e^–/Ų). The difference in the results can be attributed to the fact that the process of Bader analysis does not take into account the asymmetry (i.e., dipoles) of charge distribution. The asymmetry is represented by the – and + signs in the right part of Figure 5. This can explain the differences in the charge distribution.

Figure 5 Total charge densities and charge density differences when each partial slab is subtracted from the overall slab.

4. Conclusions

The best model is the combined slabs model. It can be used to predict the correct direction of charge transfer (the electrons in the CB of BiVO₄ migrate toward another electrode). It was observed that charge transfer could not be realized when the model considered the presence of a vacuum. An inhomogeneous spill of charge can also be realized in the latter case. Such a spill can potentially be avoided when the combined slabs model is considered.

Analysis of literature reports reveals that there are other issues that need to be addressed [26,27]. An embedding method was used in the first case, and this supported the the results obtained using the XPS X-ray Photoelectron Spectroscopy technique. Such choices are common when vacuum is taken into account. In the second case, however, a computational study was conducted using VASP and HSE06, and different results were obtained when adsorbing water (and OH^– or H⁺ ions) were considered, the results being confirmed by taking experimental flat band measurements. Here, it is assumed that liquid water remains in contact with NiOOH (it may also be in contact with BiVO₄). Hence, it needs to be verified whether the band alignment obtained using the combined slabs model is retained. This may potentially be the subject of further work.

Acknowledgments

Thanks are given to CSIC for allowing the use of its parallel computer trueno, in which all the DFT calculations here were carried out.

Author Contributions

Only the present author carried out all the calculations.

Funding

This research was funded by regional project S2018/NMT-4367 (FotoArt-CM) and national project PID2019-106315RB-100 (NHyMPha).

Competing Interests

The author declares that no competing interests exist.