pH-Dependent Catalytic Reaction Pathway for Water Splitting at the

Mar 7, 2018 - We align the band edges of BiVO4 at the interface with liquid water by combining advanced electronic-structure calculations, molecular ...
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pH-Dependent Catalytic Reaction Pathway for Water Splitting at the BiVO4−Water Interface from the Band Alignment Francesco Ambrosio,* Julia Wiktor, and Alfredo Pasquarello

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Chaire de Simulation à l’Echelle Atomique (CSEA), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland S Supporting Information *

ABSTRACT: We align the band edges of BiVO4 at the interface with liquid water by combining advanced electronic-structure calculations, molecular dynamics simulations, and a computational hydrogen electrode. After accounting for spin−orbit coupling and thermal and nuclear quantum motions, we achieve good agreement with experiment, particularly with one-shot GW calculations and semiempirically tuned hybrid functionals. The pH-dependent mechanism of the water oxidation reaction is discussed in consideration of the pH at the point of zero charge, the pKa of adsorbed water molecules, and the redox levels of the rate-determining step of the reaction. The mechanism pertaining to acidic conditions is found to dominate over a large pH range. The kinetically more favorable oxidation of hydroxyl ions is favored only in highly alkaline conditions and could be hampered by corrosion processes. Advanced electronic-structure methods are shown to be instrumental in overcoming the erroneous physical picture achieved at the semilocal level of theory. than standard hybrid functionals.23−25 This puzzling observation has been recently explained as a consequence of error cancellation.26 In fact, by properly accounting for spin−orbit, thermal, and nuclear quantum effects, a band gap renormalization of ∼1 eV has been calculated.26,27 This indicates that these aspects should be taken under consideration when carrying out high-throughput searches to discover materials with the desired electronic properties. In this context, it is of interest to determine the effect of the sizable band gap renormalization on the band edges at the BiVO4 surface and at the BiVO4−water interface. Band gap renormalization due to thermal vibrations are generally moderate (usually below 0.1−0.2 eV) and comparable to other sources of error in the calculation.28 Therefore, it is fundamental to verify the robustness of the proposed benchmark against materials where this effect is substantial, such as BiVO4, which is currently considered as one of the most promising photoanodes. The band alignment at the semiconductor−water interface is usually employed in the screening of materials to assess whether the band edges are favorably aligned to the redox levels of hydrogen reduction and water oxidation.8,9 However, the oxidation of water to molecular oxygen is a multistep process; therefore, the

A

rtificial photocatalysis at the semiconductor−water interface may represent a viable solution for the clean production of fuel.1−6 The main hindrance to the success of this promising technology is represented by the slow kinetics of the multistep water oxidation reaction induced by the photogenerated holes in the semiconductor (2H2O + 4h+ → 2O2 + 4H+).7 Large efforts have been devoted to identify the ideal photocatalyst among the vast plethora of semiconducting materials.8−13 However, semiconductors catalyzing efficiently both the hydrogen reduction and the water oxidation reaction are currently not available. For this reason, the two half-reactions are carried out on separate p-type and n-type electrodes (photoanode and photocatode) through a device architecture known as Z-scheme.14 In this context, monoclinic bismuth vanadate m-BiVO4 is one of the most promising materials to be used as photoanode for the water oxidation reaction.15−19 However, the intrinsic photocatalytic properties of this material have not yet been clarified.20 BiVO4 has a band gap of 2.4−2.5 eV, which allows one to capture a large portion of the visible spectrum, and its band edges are favorably aligned with respect to the redox levels of the watersplitting reaction.21,22 However, the band gap of this material has long eluded a consistent theoretical description. In fact, electronic band gaps obtained with semilocal functionals and hybrid functionals with a reduced fraction of nonlocal Fock exchange have been found to be in much better agreement with experiment © 2018 American Chemical Society

Received: January 22, 2018 Accepted: March 7, 2018 Published: March 7, 2018 829

DOI: 10.1021/acsenergylett.8b00104 ACS Energy Lett. 2018, 3, 829−834

Letter

Cite This: ACS Energy Lett. 2018, 3, 829−834

Letter

ACS Energy Letters

the same temperature.26 The shifts calculated for the valence and conduction band edge are listed in Table 1.

alignment of the valence band edge of the semiconductor to the redox levels pertaining to the proton-coupled electron transfers is even more important. In particular, the reaction is initiated through the dehydrogenation of H2O or the oxidation of OH− in acidic and alkaline conditions, respectively.7 Therefore, because both the band alignment and the coverage of the semiconductor surface are pH-dependent,28,29 the preference of one pathway over the other is not obvious. In this Letter, we determine the band edges of BiVO4 at its surface and at its interface with water. By comparison of the band alignment calculated at various levels of theory, one-shot GW calculations and semiempirically tuned hybrid functionals are found to provide the closest agreement with the experimental characterization, provided the band edges of the materials are corrected to account for the huge band gap renormalization of this material. By coupling the results achieved for the band alignment with the insights from the pH-dependent coverage of the BiVO4 surface and with the potentials of the water-splitting reaction, we then discuss the effectiveness of BiVO4 as a photoanode at varying pH conditions. The fundamental band gap of a semiconductor at room temperature (i.e., the temperature at which the photocatalytic device is operated) Etheory (T) is defined from theory as follows: g Egtheory (T ) = εctheory (T ) − εvtheory (T )

Table 1. Calculated Shifts for the Valence Band and Conduction Band Edge Due to Spin Orbit Coupling (SOC), Nuclear Quantum Effects (NQEs), and Thermal Vibrations at 300 K (T)a SOC NQE T total renorm. a

(1)

(2)

and εctheory (T ) = εctheory (0) + ΔεcSOC + ΔεcNQE + ΔεcT

Δεc

ΔEg

−0.15 −0.11 −0.25 −0.51

−0.13 −0.22 −0.70 −1.05

The band gap renormalization ΔEg26 is also reported.

In the Supporting Information, we verify that the band gap renormalization calculated for the bulk semiconductor is essentially unchanged with respect to the semiconductor in aqueous environment37 and that the shifts reported in Table 1 can therefore be adopted for the band alignment at the BiVO4−water interface.37 We model the (010) surface of BiVO4 using an orthorhombic supercell to achieve the interface with vacuum.37 Through the alignment of the electrostatic-potential across the interface, we align the band edges with respect to the vacuum level and calculate the ionization potential and the electron affinity of BiVO4. The neutral BiVO4−water interface, corresponding to the pH at the point of zero charge pHPZC, is composed of an orthorhombic supercell, which includes 56 water molecules corresponding to the experimental density of liquid water.37 Upon molecular dynamics simulations, interfacial water molecules are found to be molecularly adsorbed on the surface through weak bonds between the O atom of the molecule and the surface Bi atoms.37 All the interface calculations have been performed with the CP2K code,38 as described in refs 29 and 37. The band alignment at the BiVO4−water interface is achieved with respect to a computational standard hydrogen electrode μSHE, defined by the reduction of the hydronium ion to gaseous hydrogen.39,40 The band edges of the semiconductor and μSHE are aligned at the semiconductor−water interface through the alignment of the plane-averaged electrostatic potential.28,41,42 In particular, we define the potential shift of the semiconductor ΔVsc and of liquid water ΔVw as follows:

where εtheory (T) and εtheory (T) are the valence and conduction c v band edge of the semiconductor, respectively. εtheory (T) and c εtheory (T) read as follows: v εvtheory (T ) = εvtheory (0) + ΔεvSOC + ΔεvNQE + ΔεvT

Δεv −0.02 0.11 0.45 0.54

(3)

where εtheory (0) and εtheory (0) are the valence and conduction v c band edges calculated at 0 K at a level of theory not including spin−orbit coupling and aligned with respect to the average SOC electrostatic potential of the bulk semiconductor. ΔεSOC v , Δεc , NQE T T ΔεNQE , Δε , Δε , and Δε are the shifts of the band edges due v c v c to spin−orbit coupling (SOC), nuclear quantum effects (NQEs), and thermal effects, respectively. In this Letter, we calculate the band edges at 0 K at various levels of theory. First, we consider computationally cheap schemes such as the semilocal PBE functional;30 the standard hybrid functional PBE0;31 and the simplest GW scheme,32 i.e., the oneshot G0W0(0) method with starting wave functions achieved at the PBE level. We also employ the G0W0(0.25) method, in which the starting point is achieved at the PBE0 level, and the semiempirical hybrid functional PBE0(α), in which the fraction of Fock exchange is set to reproduce the experimental gap. Finally, we also consider the self-consistent QSGW̃ method, which includes vertex corrections in the screened interaction33 and has been found to provide the band gap of BiVO4 in excellent agreement with experiment.26 All these calculations26 have been performed with the freely available ABINIT code.34−36 To achieve the individual shifts of the valence and conduction band edges, we follow the scheme proposed in ref 26 for the band gap renormalization. In particular, SOC effects are evaluated from fully relativistic calculations, thermal effects from molecular dynamics simulations with classical nuclei at room temperature, and NQEs from path-integral molecular dynamics simulations at

ΔVsc = Vsc(bulk) − Vsc(int)

(4)

ΔVw = Vw(bulk) − Vw(int)

(5)

where Vsc(int) and Vw(int) are the average electrostatic potentials in the bulklike region of the interface for the semiconductor and for liquid water, respectively. ΔVsc(int) and ΔVsc(int) are calculated from a 10 ps molecular dynamics simulation of the BiVO4(010)−water interface, which is sufficient to achieve converged values. A schematic representation of the alignment scheme used in this work for the BiVO4(010)−water interface is given in Figure 1. The calculation of the band alignment at the BiVO4(010)− water interface is performed through the neutral interface model, which corresponds to the pH value at the point of zero charge (pHPZC).28 This is defined as the pH for which the concentration of adsorbed protons is equal to that of adsorbed hydroxyl ions and hence no net charge is found at the surface.29,37 The position of the valence band edge of a semiconductor at pHPZC with respect to μSHE (at pH 0 by definition) reads as follows: εvSHE(PZC) = εvtheory (T ) − μSHE − ΔVsc + ΔVw 830

(6)

DOI: 10.1021/acsenergylett.8b00104 ACS Energy Lett. 2018, 3, 829−834

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ACS Energy Letters

Figure 1. Schematic representation of the band alignment at the BiVO4−water interface. In the left panel, the conduction and valence band edges (ϵc and ϵv in red) of the semiconductor are shown with respect to the electrostatic potential (gray) and its average Vsc(bulk). Similarly, on the right, the SHE level (green) is given with respect to the electrostatic potential (gray) and its average Vw(bulk) in liquid water. The alignment between the electrostatic potentials Vsc(int) and Vw(int) is illustrated in the middle panel.

Consequently, the conduction band edge ϵSHE c (PZC) is obtained as follows: theory εcSHE(PZC) = ϵSHE (T ) v (PZC) + Eg

(7)

Following recent experimental work,43 we assume Nernstian behavior for the band edges of BiVO4 in aqueous environment. Hence, the conduction band edge measured at a given pH, ϵSHE c (pH), can be shifted to its value at pHPZC as follows: SHE ϵSHE c (PZC) = ϵc (pH) + (0.059 eV) · (pH PZC − pH)

(8)

The experimental range for the pHPZC of the BiVO4(010)−water interface is 2.5−3.5.44,45 In particular, the value achieved from the most recent experimental characterization (3.5 in ref 45) is close to the one recently calculated (3.46) within a grand-canonical formulation of adsorbates at the semiconductor−water interface.29 Therefore, to make contact with the band-alignment at the BiVO4−water interface measured for an electrode immersed in a solution at pH 7,22 we use eq 8 with a value of pHPZC equal to 3.46.29,46 We now discuss the band alignment at the BiVO4(010)−vacuum and BiVO4(010)−water interfaces. The values of the IP and the EA achieved at various levels of theory are illustrated in Figure 2a, while those for εSHE and εSHE are in Figure 2b. The results are also v c collected in Table 2. For each level of theory, we also report the mean absolute error (MAE) from the individual errors on IP, EA, SHE εSHE v , and εc . We notice that the trends among electronicstructure methods observed in Figure 2a,b are very similar, indicating a clear correlation between the description of the band edges in the two systems. We first focus on the semilocal PBE functional, which clearly fails in describing the position of the band edges of BiVO4. In particular, the errors are as large as ∼0.7 eV for the position of the conduction band edge. It is noteworthy that if the corrections to the band edges listed in Table 1 are neglected, the results at the PBE level misleadingly appear to match the experimental characterization closely (cf. Figure 2). The standard PBE0 functional does not provide a satisfactory agreement with experiment, with a MAE of 0.31 eV. The G0W0(0) and G0W0(0.25) results are found to be closer to experiment, with average errors of only 0.10 and 0.22 eV, respectively. The QSGW̃ gives the fundamental band gap of BiVO4 in excellent agreement with experiment26 but does not improve the band alignment with respect to less computationally demanding methods, in line with previous observations for other semiconductors.28 Finally, when the fraction of Fock exchange, α, is set to reproduce the experimental band gap, an overall good agreement with experiment is reached for all the considered quantities, thus confirming the effectiveness of the

Figure 2. Band alignment of BiVO4 at the interface with (a) vacuum and (b) liquid water, as calculated at various levels of theory. IP and EA are referred to the vacuum level; band edges at the semiconductor−water interface are given at pH 7 and are referred to the SHE. Dashed lines indicate the band-alignment achieved at the PBE level without considering SOC, thermal, and nuclear quantum effects. Experimental values from refs 21 and 22 are reported as dotted lines for comparison. SHE Table 2. Calculated Etheory (T), IP, EA, εSHE at Various g v , and εc a Levels of Theory

method

Etheory (T) g

IP

EA

εSHE v

εSHE c

MAE

PBE PBE0 G0W0(0) G0W0(0.25) QSG W̃

1.48 2.87 2.47 2.92 2.58 2.50 2.48−2.50

6.92 7.82 7.21 7.52 7.67 7.5 7.27

5.44 4.95 4.74 4.60 5.09 5.00 4.79

1.96 2.86 2.25 2.56 2.71 2.50 2.40

0.48 −0.01 −0.22 −0.36 0.13 0.00 −0.10

0.52 0.37 0.10 0.22 0.31 0.16

PBE0(α) exptl a SHE εv

and εSHE are given for pH 7. Experimental values from refs 21 c and 22 are included for comparison. Mean absolute errors (MAEs) are provided for each level of theory. All values are given in electronvolts.

PBE0(α) method for the alignment of the band edges of semiconductors at the interfaces with vacuum and liquid water.28,42 It should be noted that α has to be set to 19% in order to 831

DOI: 10.1021/acsenergylett.8b00104 ACS Energy Lett. 2018, 3, 829−834

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the PBE0(α) level as a function of the pH of the electrode cell. For the redox levels associated with the dehydrogenation of the water molecule and the oxidation of the hydroxyl ion, we take those calculated for the bulk liquid.39 Figure 3 shows that the valence band of BiVO4 is favorably aligned with respect to the redox levels associated with the first step of the water oxidation reaction. In particular, in acidic conditions the dominant mechanism is the dehydrogenation of adsorbed water molecules, the level of which lies at only 0.08 eV from the valence band edge of BiVO4. When the pH increases toward values for which both water molecules and hydroxyl ions are adsorbed on the surface, the acidic mechanism still overrules the alkaline one, as an energy difference larger than 0.5 eV is found between the valence band edge of BiVO4 in this pH range and the non-Nernstian OH−/OH* redox level. This alignment is indeed kinetically less favorable for nonadiabatic charge transfer, the rate of which shows an inverse exponential dependence on the energy difference between initial and final state.49 When considering pH > 9.46, we note that the energy separation between the valence band of the semiconductor and OH−/OH* redox level becomes small only in highly alkaline conditions (i.e., 0.22 eV at pH 14). However, in such a strong basic aqueous environment, corrosion phenomena can undermine the stability of the surface.47,48 Therefore, we find that the alignment between the oxidation potential and the valence band edge of BiVO4 is unsatisfactory, as only extreme basic conditions are likely to strongly favor the more desirable OH−/OH* process. We note that when the band alignment is performed at the semilocal level, as commonly achieved in previous studies, the physical picture is drastically affected. In fact, while the valence band edge is still below the water oxidation redox level (1.23 eV vs SHE at pH 0), it is now found at higher energies with respect to the H2O/OH* and the OH−/OH* levels (cf. Figure 3). Therefore, our results imply that high-throughput screening procedures based on the position of the band edge with respect to water oxidation overpotentials cannot be performed satisfactorily at the semilocal level. The proposed physical picture may be altered by surface defect levels, which are thought to be responsible for recombination processes and therefore for the poor performance of BiVO4 in the absence of overlayers.18,22,50−52 However, the passivation of surface defects through the deposition of small clusters proposed in ref 20 is not sufficient to explain the promising results achieved for the water oxidation at the BiVO4 surface in neutral pH conditions, because the energy gap between the OH−/OH* redox level and the valence band of the semiconductor would be sizable. Therefore, further efforts need to be deployed to understand the possible phenomena occurring at the interface. For instance, in view of the recent detection of hole polarons in BiVO4,53 it should be noted that localized states above the valence band could dramatically affect the picture of Figure 3 and may indeed play a key role in the water oxidation process. In fact, hole polarons lying above the valence band edge of BiVO454−56 may favor the reaction mechanism in alkaline conditions at lower pH values, thus being possibly beneficial to the efficiency of heterogeneous water splitting at the surface of this material. Indeed, recent literature points to a hole polaron level at 1.2 eV from the valence band at the BiVO4 surface.54 However, an accurate determination of the hole polaron level at BiVO4 interface has not yet been achieved. Furthermore, the redox level pertaining to the individual proton-coupled electron-transfer reactions of the water-splitting process may change from those

reproduce the experimental band gap of 2.5 eV. This value is remarkably higher than the value of 5% reported in ref 25, which has been obtained ignoring the sizable band gap renormalization induced by SOC, thermal, and nuclear quantum effects. We next analyze the effect of the pH on the alignment of redox levels at the BiVO4(010)−water interface by combining the calculated band alignment with the acid−base chemistry of the BiVO4(010) surface. We consider the first step of the water oxidation reaction, which generally represents the limiting step of the process, because of its high overpotential. This reaction occurs through the dehydrogenation of H2O and the oxidation of OH− in acidic and alkaline conditions, respectively.7 While the redox level of the former shows a Nernstian dependence on the pH of the system, that of the latter does not, as no proton transfer is implied in the reaction. The preference of one mechanism over the other for heterogeneous water splitting depends upon the pH-dependent composition of the semiconductor−water interface. In turn, this is determined by the acidity of the surface sites.29,37 It has been demonstrated that when water molecules are adsorbed on surface Bi sites at the BiVO4(010)−water interface, their acidity is noticeably enhanced as the respective pKa is reduced from 15.74 of the bulk liquid to 8.46.29 Therefore, for the BiVO4(010)−water interface, we can distinguish three regions of pH: (i) pH < 7.46, where the water oxidation reaction is initiated mainly through the dehydrogenation of adsorbed water molecules, which are the dominant species at the surface as hydroxyl ions are found to adsorb on less than 10% of the Bi surface sites; (ii) pH > 9.46, where the adsorption of hydroxyl ions prevails at the surface, thus favoring the kinetically more favorable mechanism occurring under alkaline conditions;7 (iii) 7.46 < pH < 8.46, where adsorbed water molecules and hydroxyl ions coexist in similar ratios, thus allowing for both mechanisms to be operative at the interface. It should be noted that, according to previous measurements and theoretical Pourbaix diagrams, BiVO4 is stable in aqueous solution and in the absence of applied voltage in a pH range between 4 and 12.47,48 Using eq 8, it is possible to align the redox levels at the BiVO4(010)−water interface at different conditions of pH. In Figure 3, we report the band alignment at the BiVO4(010)−water interface achieved at

Figure 3. εSHE v , as calculated at the PBE (dotted line) and at the PBE0(α) level, given as a function of pH. The redox levels associated with the dehydrogenation of the water molecule (red dashed) and the oxidation of the hydroxyl ion (blue dotted) are reported as calculated in ref 39. Above the figure, a schematic representation of the pH-dependent change in surface coverage (from molecularly adsorbed water molecules to hydroxyl ions) is represented. 832

DOI: 10.1021/acsenergylett.8b00104 ACS Energy Lett. 2018, 3, 829−834

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ACS Energy Letters

(4) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar water splitting cells. Chem. Rev. 2010, 110, 6446−6473. (5) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z. Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway. Science 2015, 347, 970−974. (6) Luo, J.; Im, J.-H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N.-G.; Tilley, S. D.; Fan, H. J.; Grätzel, M. Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts. Science 2014, 345, 1593−1596. (7) Koper, M. T. M. Theory of multiple proton-electron transfer reactions and its implications for electrocatalysis. Chem. Sci. 2013, 4, 2710−2723. (8) Castelli, I. E.; Olsen, T.; Datta, S.; Landis, D. D.; Dahl, S.; Thygesen, K. S.; Jacobsen, K. W. Computational screening of perovskite metal oxides for optimal solar light capture. Energy Environ. Sci. 2012, 5, 5814−5819. (9) Castelli, I. E.; Landis, D. D.; Thygesen, K. S.; Dahl, S.; Chorkendorff, I.; Jaramillo, T. F.; Jacobsen, K. W. New cubic perovskites for one- and two-photon water splitting using the computational materials repository. Energy Environ. Sci. 2012, 5, 9034−9043. (10) Zhuang, H. L.; Hennig, R. G. Single-layer group-III monochalcogenide photocatalysts for water splitting. Chem. Mater. 2013, 25, 3232−3238. (11) Wu, Y.; Chan, M. K. Y.; Ceder, G. Prediction of semiconductor band edge positions in aqueous environments from first principles. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 235301. (12) Wu, Y.; Lazic, P.; Hautier, G.; Persson, K.; Ceder, G. First principles high throughput screening of oxynitrides for water-splitting photocatalysts. Energy Environ. Sci. 2013, 6, 157−168. (13) Katz, J. E.; Gingrich, T. R.; Santori, E. A.; Lewis, N. S. Combinatorial synthesis and high-throughput photopotential and photocurrent screening of mixed-metal oxides for photoelectrochemical water splitting. Energy Environ. Sci. 2009, 2, 103−112. (14) Maeda, K. Z-scheme water splitting using two different semiconductor photocatalysts. ACS Catal. 2013, 3, 1486−1503. (15) Kudo, A.; Omori, K.; Kato, H. A novel aqueous process for preparation of crystal form-controlled and highly crystalline BiVO4 powder from layered vanadates at room temperature and its photocatalytic and photophysical properties. J. Am. Chem. Soc. 1999, 121, 11459−11467. (16) Yu, J.; Kudo, A. Effects of structural variation on the photocatalytic performance of hydrothermally synthesized BiVO4. Adv. Funct. Mater. 2006, 16, 2163−2169. (17) Luo, H.; Mueller, A. H.; McCleskey, T. M.; Burrell, A. K.; Bauer, E.; Jia, Q. Structural and photoelectrochemical properties of BiVO4 thin films. J. Phys. Chem. C 2008, 112, 6099−6102. (18) Park, Y.; McDonald, K. J.; Choi, K.-S. Progress in bismuth vanadate photoanodes for use in solar water oxidation. Chem. Soc. Rev. 2013, 42, 2321−2337. (19) Starr, D. E.; Favaro, M.; Abdi, F. F.; Bluhm, H.; Crumlin, E. J.; van de Krol, R. Combined soft and hard X-ray ambient pressure photoelectron spectroscopy studies of semiconductor/electrolyte interfaces. J. Electron Spectrosc. Relat. Phenom. 2017, 221, 106−115. (20) Zachaus, C.; Abdi, F. F.; Peter, L. M.; van de Krol, R. Photocurrent of BiVO4 is limited by surface recombination, not surface catalysis. Chem. Sci. 2017, 8, 3712−3719. (21) Cooper, J. K.; Gul, S.; Toma, F. M.; Chen, L.; Glans, P.-A.; Guo, J.; Ager, J. W.; Yano, J.; Sharp, I. D. Electronic structure of monoclinic BiVO4. Chem. Mater. 2014, 26, 5365−5373. (22) Kim, T. W.; Choi, K.-S. Nanoporous BiVO4 photoanodes with dual-layer oxygen evolution catalysts for solar water splitting. Science 2014, 343, 990−994. (23) Walsh, A.; Yan, Y.; Huda, M. N.; Al-Jassim, M. M.; Wei, S.-H. Band edge electronic structure of BiVO4: elucidating the role of the Bi s and V d orbitals. Chem. Mater. 2009, 21, 547−551.

calculated in the bulk liquid because of the catalytic effect of the semiconductor surface. In summary, we have aligned the band edges of BiVO4 at the interface with liquid water. We have shown that the sizable band gap renormalization of this material has to be considered to achieve reliable electronic-structure properties. In particular, one-shot GW calculations and semiempirically tuned hybrid functionals provide the closest match with the experimental characterization. By combining the calculated band alignment with the acid−base chemistry of the material and the redox levels associated with the first step of the water oxidation reaction, we discussed the mechanism of the reaction as a function of pH. More specifically, we found that the mechanism of water splitting in acidic conditions dominates over a large range of pH values while the kinetically more favorable oxidation of hydroxyl ions prevails only in highly alkaline conditions, where corrosion phenomena would take over. The present work highlights that a correct description of the band gap and band edges of the semiconductor, which can be achieved only with sophisticated electronic-structure methods, is instrumental to understand through which underlying mechanisms the water oxidation reaction occurs.

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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/acsenergylett.8b00104. Computational details, an analysis of the comparison between the band gap renormalization in the bulk material and at the semiconductor−water interface, a description of the theory used to calculate the pH at the point of zero charge and the pKa values of the individual adsorption sites at the BiVO4(010)−water interface, and additional refs 57−63 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: Francesco.Ambrosio@epfl.ch. Phone: +41 21 6933423. Fax: +41 21 693 5419. ORCID

Francesco Ambrosio: 0000-0002-6388-9586 Julia Wiktor: 0000-0003-3395-1104 Alfredo Pasquarello: 0000-0002-9142-2799 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been performed in the context of the National Center of Competence in Research (NCCR): Materials’ Revolution: Computational Design and Discovery of Novel Materials (MARVEL) of the Swiss National Science Foundation. We used computational resources of the Swiss National Supercomputing Center (CSCS) and of the Scientific IT Application Support (SCITAS) of EPFL.

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REFERENCES

(1) Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37−38. (2) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Efficient photochemical water splitting by a chemically modified n-TiO2. Science 2002, 297, 2243−2245. (3) Kudo, A.; Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 2009, 38, 253−278. 833

DOI: 10.1021/acsenergylett.8b00104 ACS Energy Lett. 2018, 3, 829−834

Letter

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DOI: 10.1021/acsenergylett.8b00104 ACS Energy Lett. 2018, 3, 829−834