Anisotropic Electronic Characteristics, Adsorption, and Stability of Low

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 5475−5484

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Anisotropic Electronic Characteristics, Adsorption, and Stability of Low-Index BiVO4 Surfaces for Photoelectrochemical Applications Jun Hu,†,‡ Wei Chen,‡ Xin Zhao,‡ Haibin Su,*,‡ and Zhong Chen*,‡ †

School of Chemical Engineering, Northwest University, Xi’an 710069, China School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

‡

ACS Appl. Mater. Interfaces 2018.10:5475-5484. Downloaded from pubs.acs.org by MIDWESTERN UNIV on 01/12/19. For personal use only.

S Supporting Information *

ABSTRACT: Many experimental results reveal different activities among different low-index surfaces of photocatalysts. The current investigation focuses on the theoretical understanding of the electronic characteristics, surface activity, and stability of different low-index surfaces of BiVO4 toward water splitting using first-principle calculations. The results indicate that BiVO4 has four types of low-index surfaces, namely, (010)T1, (010)T2, (110)T1, and (11̅ 1)T1. The different band edge potentials of the surfaces, resulting from the variation of the electrostatic potential, lead to a higher oxidation ability for (010)T1 and (010)T2 than for (110)T1 and (1̅11)T1 surfaces. The electrons prefer to accumulate on (010)T1 and (010)T2 surfaces, whereas holes like to accumulate on (110)T1 and (11̅ 1)T1 surfaces during a photocatalytic process. Moreover, investigation on the adsorbed intermediates during the water-splitting process indicates that the oxygen evolution reaction on BiVO4 surfaces is mainly dominated by the reaction OH* ↔ O* + H+ + e−, and (110)T1 and (11̅ 1)T1 surfaces are energetically more favorable as photoanodes for water splitting than (010)T1 and (010)T2. Furthermore, the BiVO4 surface as photoanodes tend to be unstable and can easily be corroded with or without the presence of an oxidative environment, however, there is an exception for the BiVO4 (010)T1 and (010)T2 surfaces, which are thermodynamically stable in the solution when there are no strong oxidative species. These results provide important insights into the anisotropy behaviors among low-index surfaces of BiVO4 for photocatalytic reactions. KEYWORDS: low-index surfaces, charge separation, surface energy, photocatalytic, Gibbs free energy

1. INTRODUCTION Water splitting using photoelectrochemical (PEC) catalysis is one of the most viable technologies for the conversion of sunlight into fuels and chemicals without any environment pollution. Among the investigated anode materials for the oxygen evolution reaction (OER), such as TiO2, α-Fe2O3, Co3O4, and so forth,1−3 monoclinic clinobisvanite bismuth scheelite (ms-BiVO4) has attracted huge attention because of its advantageous properties including reasonable band edge alignment, good optical absorption, abundance, and simple synthesis.4−9 In the past decades, huge efforts, including morphology regulation,10−14 elemental doping,15−18 heterogeneous coupling,19−22 and cocatalyst loading,23−26 have been made to improve the photoelectrocatalytic activity toward an optimized photocurrent density. However, the activity of ms-BiVO4 is still limited for practical applications. One of the main obstacles is the lack of fundamental understanding of complex surface effects. Recently, in situ experimental results of TiO2 have shown that many factors, for instance, facet orientations and crystal and electronic structures, play crucial roles in the surface properties.27−32 As a result, these factors should be considered carefully for the design and fabrication of highly efficient photoelectrocatalytic catalysts. A similar phenomenon was also © 2018 American Chemical Society

observed in ms-BiVO4; for example, experimental results indicate that both BiVO4 nanosheets and nanoplates with exposed (010) facets showed enhanced photocatalytic activity for degradation of rhodamine B and for OER.33,34 The photocatalytic activity toward water oxidation using the BiVO4 sample correlates very well with the extent of exposure of the (040) facet.35 Li et al. further confirmed the anisotropy properties of different facets on ms-BiVO4. They noticed that the reduction and oxidation cocatalysts are selectively deposited on the {010} and {110} facets, resulting in marked activity enhancement in both photocatalytic and photoelectrocatalytic OER.36,37 Considering the high stability of the (010) facet in BiVO4, it was widely employed as a representative model to explore the structural and catalytic activities in many published calculations.38 Yang et al. reported the anisotropic properties between (010) and (011) facets in the BiVO4 sample and found obvious difference in the geometric structure, optical properties, electronic structure, water adsorption, and the whole OER free-energy profiles.39 The calculations by Li et al. Received: October 8, 2017 Accepted: January 19, 2018 Published: January 19, 2018 5475

DOI: 10.1021/acsami.7b15243 ACS Appl. Mater. Interfaces 2018, 10, 5475−5484

Research Article

ACS Applied Materials & Interfaces

Figure 1. Unrelaxed and relaxed geometric structures of the low-index ms-BiVO4 surfaces; Bi (purple), V (gray), and O (red) atoms are shown in colored spheres, and the unit of bond length is Å.

corresponding surfaces were constructed using the lattice parameter optimized for the ms-BiVO4 bulk with a vacuum region of 15 Å.41 During the calculations, self-consistent periodic DFT calculations were performed. The generalized gradient approximation method, in the form of the Perdew− Burke−Ernzerhof approximation, was used to calculate the exchange−correlation energy. The Broyden−Fletcher−Goldfarb−Shanno scheme was chosen as the minimization algorithm. DFT-D correction was used for dispersion corrections. Hubbard U-corrections to the d electrons of V (LDA + U, effective U(V) = 2.5 eV) and spin-polarized electrons were performed during the calculations.42 The energy cutoff is 380 eV, and the self-consistent field tolerance is 5.0 × 10−7 eV/atom. The optimization is finished when the energy, maximum force, maximum stress, and maximum displacement are smaller than 5.0 × 10−6 eV/atom, 0.01 eV/Å, 0.02 GPa, and 5.0 × 10−4 Å, respectively. The reciprocal space integrations were carried out at the gamma-points only with 2 × 2 supercells. The Fermi level is simply defined as the valence band maximum (VBM) for semiconductors and insulators in the CASTEP code and some other codes.43,44 The calculated crystallographic parameters and X-ray diffraction data of the optimized structure are listed in Table S1 and Figure S2 and are compared with experimental results.45 ms-BiVO4 has one conventional setting C2/c and several nonconventional settings such as I2/a, I2/b, and B2/b. During the calculation, C2/c, which has often been used by many investigators,46,47 was selected to carry out our DFT simulation. The adsorption energy (Eads) between the surface and adsorbed particles was computed by eq 1.

have laid a solid foundation to understand the anisotropic properties, including surface geometric/electronic structures, surface energy, work function, Bader charge, and oxygen vacancy formation energy on the (100), (010), (001), (101), (011), (110), and (111) surfaces.40 Another group of authors have also simulated the band structure of (010) and (011) surfaces to interpret possible charge separation between them.36 Although some researches on the fundamental properties of different facets in BiVO4 have been published, much more focus is needed on the low-index surfaces that have been actually prepared during the past experiment. Furthermore, the detailed mechanisms about adsorption on different surfaces and stability of different crystal facets have not been considered by the previous literatures. In this investigation, we report a comprehensive theoretical analysis on the effect of BiVO4 crystal surfaces on the photocatalytic OERs on BiVO4 based on density functional theory (DFT) calculations. The results are able to unveil insights into the surface anisotropy properties, including band structures, stability, and adsorption mechanism during the reaction process as well as the change of Gibbs free energy in the elementary steps of the catalysis.

2. COMPUTATIONAL DETAILS The CASTEP module of the Materials Studio software (Accelrys Inc.) was employed for the quantum chemistry calculations. Considering the symmetry, (001), (010), (100), (110), (101), (1̅01), (011), (111), and (1̅11) surfaces were chosen to be calculated (see the Supporting Information, Figure S1). It is well-known that every surface may have several terminations, and the termination must fulfill two conditions. The first is that the top and bottom surface should be the same; the second is that the atoms of the slab must be n times (n = 1, 2, 3, ...) that of unit cells. On the basis of these conditions, the

Eads = Emolecule + surface − Emolecule − Esurface

(1)

where Emolecule+surface is the total energy of the system, including the adsorption molecules and the BiVO4 facet; Emolecule is the energy of the adsorption molecules; and Esurface is the energy of 5476


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Figure 2. Calculated electronic characteristics of bulk BiVO4 and low-index surfaces. (a) Density of states (DOSs); (b) work function; (c) calculated surface energies and edges relative to the normal hydrogen electrode (NHE) potential at pH = 0 as well as the water redox potentials ϕ(O2/H2O) and ϕ(H+/H2); (d) schematic diagram of the possible charge separation during photocatalysis.

of Bi−O bonds. The unrelaxed and relaxed geometric structures of the selected low-index ms-BiVO4 surfaces are shown in Figure 1. From Figure 1, we can see that these low-index surfaces are terminated with Bi atoms and VO4 groups, which agree well with the previous calculation and crystal chemistry.40 For the unrelaxed slab of (010)T1, the surface Bi and V atoms are coplanar in the surface plane. The surface Bi atom has 6 coordination number, whereas the surface V atom possesses 4 coordination number. In the case of the relaxed slab, the surface Bi1 and O1 atoms move inward by 0.088 and 0.066 Å, respectively, whereas the surface V atom moves outward by only 0.037 Å. These results are consistent with the facts that the bond length of Bi1−O1 changed from 2.417 to 2.478 Å, whereas the bond length of V1−O6 changed from 1.746 to 1.671 Å. It is notable that the same phenomenon is also observed on the (010)T2 surface. For (110)T1 and (1̅11)T1 surfaces, the coordination number is 5 for the surface Bi atom, and 4 for the surface V atom. In the unrelaxed slab of ms-BiVO4 (110)T1, the outermost layer is indexed to the surface Bi1 atom, and the sublayer of the surface is ascribed to the VO4 group. In the relaxed slab, the surface Bi atom moves inward by a distance of 0.300 Å, and the surface V1 and O1 atoms move outward by 0.091 and 0.778 Å, respectively. This has led to the reduction of the bond lengths of Bi1−O2, Bi1−O3, Bi1−O4, Bi1−O5, V1−O5, and V1−O7 and increase of the bond lengths of Bi1−O1, V1− O6, and V1−O8, as shown in Figure 1. 3.2. Electronic Characteristics of Low-Index Surfaces. The calculated electronic characteristics of the selected lowindex surfaces are plotted in Figure 2. Detailed information about all low-index surfaces is given in Figure S5.

the BiVO4 facet after optimization. In the definitions, the higher negative value of Eads indicates a more stable adsorption on the plane. All calculated energies reported herein include the zeropoint energy (ZPE) correction.48 The surface energy (γ) can be calculated by the following eq 2 γ=

1 (Esurface − nE bulk ) 2A

(2)

where Ebulk is the total energy per unit cell of the bulk, n is the number of unit cells that the slab model contains, and A is the surface area of the slab model. On the basis of the calculated results shown in Figure S3, at least four Bi layers were selected during the simulation to reduce the errors.

3. RESULTS AND DISCUSSION 3.1. Geometric Structure of Low-Index Surfaces. Among the above surface terminations, we found only the surfaces terminated by the Bi−O bond have low surface energies. More information about determinations of the lowindex surfaces can be found in Table S2 and Figure S4 in the Supporting Information. The low-index surfaces can be divided into four categories. The first one is (010)T1, which terminates with two Bi−O bonds with a length of 2.459 Å; the second one is (010)T2, which terminates with two Bi−O bonds with a length of 2.522 Å; the third one is composed of the (110)T1 surface, which breaks into three Bi−O bonds with bond lengths of 2.459, 2.440, and 2.417 Å. The last one is the (1̅11)T1 surface, which breaks into three Bi−O bonds with lengths of 2.522, 2.440, and 2.417 Å. Therefore, the stable surface is VO4 tetrahedron with six- or five-coordinate Bi atoms. There is a difference among the stable surfaces in the number and length 5477


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Figure 3. Adsorption structures and energies of OHads, Oads, and OOHads involved in a water-splitting process on low-index surfaces. Bi (purple), V (gray), and O (red) atoms are shown in colored spheres, and the unit of bond length is Å. The “@” signs stand for the adsorption state on the facet.

potential imposes a remarkable influence on the position of the band structure. For the PEC catalysis, the holes will transfer from the bulk to the surface, whereas the electrons will flow from the anode to the cathode. The surface is used to accumulate the holes, and the relationship of the VBM between the bulk and the surface plays an important role in charge separation. As indicated in Figure 2c, the VBM of (110)T1 and (1̅11)T1 are lower than that of the bulk; this will enhance the hole separation from the bulk to the surface. For photocatalysis, holes and electrons will both transfer to the surface to react with the electrolyte solution. Therefore, the surface is used to accumulate not only the holes but also the electrons. The positions of CBM and VBM between two surfaces will be important for the charge separation. When the heterogeneous contacts between these two facets are constructed, the charge separation can be easily promoted, which is verified by Li et al.36 Details about this phenomenon are described in Figure 2d. It can be seen that the differences of the CBM and the VBM between (010)T1 and (110)T1 are 1.35 and 1.12 eV. This implies that the spatial charge separation between different surfaces will be obvious under light irradiation. The photogenerated electrons on the (110)T1 surface can transfer to the (010)T1 surface to be the reduction site, whereas the photogenerated holes will migrate to (110)T1 from the (010)T1 surface. The potential of hole scavengers ([PtCl6]2−/[PtCl4]2−, [PtCl4]2−/Pt, [IrCl6]3−/Ir, Ag+/Ag, and [AuCl4]−/Au) and electron acceptors ([IrCl6]2−/[IrCl6]3−, MnO2/Mn2+, O2/H2O, and Mn2O3/Mn2+) in solution during photodepositions can be obtained from the electrochemical series. The CBM of (010)T1 is higher (more negative in potential) than the reduction potential of [PtCl6]2−/[PtCl4]2−, [PtCl4]2−/Pt, [IrCl6]3−/Ir, Ag+/Ag, and [AuCl4]−/Au, whereas the VBM of semiconductors are lower (more positive in potential) than the oxidation potential of [IrCl6]2−/[IrCl6]3−, MnO2/Mn2+, and O2/H2O. Accordingly, the particles of Ag, Au, and Pt ought to be selectively deposited on the (010)T1 and (010)T1 surfaces, and metal oxides should be loaded onto

As illustrated in Figure 2, we found that the band gap of bulk ms-BiVO4 is around 2.15 eV, which is consistent with previous calculations.49−52 For all of those surfaces, it can be observed that the VBM is composed of Bi 6s and O 2p resonance peaks, whereas the conduction band minimum (CBM) is composed of V 3d and O 2p resonance peaks. The same is true for the bulk (see the Supporting Information, Figure S6). However, there is still a small difference for the DOSs and band gap of different surfaces. It is found that the DOSs of VBM and CBM are greatly increased for all of those surfaces when compared with the bulk (as shown in Figure S6a), which are mainly due to the unsaturated bond on the surface. It should be pointed out that the band gap of (1̅11)T1 is 2.46 eV, which is in accordance with the experimental results with the value of 2.40−2.50 eV.53 The VBM and CBM of bulk BiVO4 are 2.33 and 0.18 eV, which is also in agreement with previous results.54 The surface energies for (010)T1 and (110)T1 are determined to be 0.427 and 0.460 J/m2, respectively, which are very close to previous calculations.38,40 The electrostatic potential of the low-index surfaces is dramatically changed, as shown in Figure 2b. The average potential for (010)T1, (010)T2, (110)T1, and (11̅ 1)T1 is −7.00, −7.02, −5.88, and −5.79 eV, respectively. Such results reveal that the electrons prefer to accumulate on the (010)T1 and (010)T2 surfaces rather than on (110)T1 and (1̅11)T1 surfaces. On the contrary, holes like to accumulate on the (110)T1 and (1̅11)T1 surfaces rather than on (010)T1 and (010)T2 surfaces. Although the cleavage bonds are different for (010)T1 and (010)T2 surfaces, the bond length, surface energies, and band structure only have a small difference after geometric optimization. This indicates that it is reasonable to select any one of the (010) surfaces without considering the effect of different terminations, for the investigation of the structure and catalytic activities of BiVO4. To better understand the ability of different surfaces for PEC and photocatalytic applications toward selective photodeposition and water splitting, the band edge energies of different facets are investigated. Figure 2c indicates that the electrostatic 5478


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ACS Applied Materials & Interfaces the surface of (110)T1 and (11̅ 1)T1. This can well explain the simultaneous reduction reaction with photogenerated electrons and the oxidation reaction with photogenerated holes on different surfaces. Recently, researchers obtained direct evidence that holes are indeed easily accumulated on the (110) surface, using spatially resolved surface photovoltage spectroscopy, which supports the above conclusion.55,56 Furthermore, the experiment showed that improved photooxidation performance of ms-BiVO4 particles is correlated with the presence of (010) facets.33−35 This result seems to be contradictory to our calculation that (110) is preferred for photo-oxidation reactions. However, as indicated by Tan et al., the improved photoactivity of (010)-dominant ms-BiVO4 particles is actually because of the enhanced electron accumulation on the (010) surfaces.57 Because electron transport is the limiting step for this particle system, the exposed (010) facets lead to the reported activity improvement because of the improvement in electron injection. The greater extent of electron trapping in (110)-dominant ms-BiVO4 also deteriorated its photoactivity by inducing an electron−hole pair recombination. 3.3. Adsorption Properties of Low-Index Surfaces. It is well-known that chemical adsorption on the BiVO4 surface plays an important role in the activity evolution by PEC water oxidation reactions and/or photocatalytic reactions.58 Figure 3 displays the adsorption structures, energies, and bond lengths of H2Oads, OHads, and Oads involved in the water-splitting process on low-index surfaces. Only the top site of the Bi adatom is considered before the minimum energy of the system because the Bi adatom on the BiVO4 surface is the active site for catalysis.59 More detailed results are listed in Table S3. Figure 3 illustrates the adsorption of several key species on the low-index surfaces of BiVO4, which indicates that the stable absorption position for the above species is nearly directly above Bi sites (with a small tilt). The calculation results also verify that the bond length of O−H in adsorbents is equal to that in water. Surprisingly, the bond length of Bi−O has been obviously changed. Comparing OHads on different surfaces, we found that the bond lengths of Bi−O on (010)T1 and (010)T2 are shorter than those on (110)T1, and (1̅11)T1. The Bi atoms of (010)T1 and (010)T2 move outward after adsorption of OHads, whereas the position of the Bi atom on (110)T1 and (1̅11)T1 almost remain unchanged. In addition, the adsorption of Oads on (110)T1 and (1̅11)T1 is more stable than that on (010)T1 and (010)T2. This is mainly attributed to the different surface cleavage where Bi sites of (110)T1 and (1̅11)T1 are 5fold coordinated, whereas Bi sites of (010)T1 and (010)T2 are 6-fold coordinated. Because of the existence of unpaired electrons on a single O atom, the 5-fold coordinated Bi sites exhibit stronger interaction than the Bi sites with 6-fold coordination. Furthermore, the adsorption energy does not correspond exactly to the bond lengths of Bi−O. The phenomenon shows that the above species has interaction not only with the top Bi atom but also with the adjacent atoms on the surface, which can be verified by the corresponding partial DOSs of the (010)T2 surface, as shown in Figure S7. 3.4. Surface Activity. The adsorption on the BiVO4 surface plays an important role in the PEC water oxidation reactions and catalysis mechanism. The oxidation process (OER) occurs at the anode at the standard oxidation potential of 1.23 V (vs NHE). Because of the four-electron-transfer process during the OER, the minimum free energy required to split two molecules

of water under equilibrium and standard conditions are estimated as follows: ΔGO2 = 4 × 1.23 = 4.92 eV

(3)

The NHE potential is defined as the following half reaction H2(g) ↔ Hsol + + eM

(4)

If the NHE potential is considered to be zero, we will obtain 1 μ (5) 2 H2 With the process of a single electron transfer, water splitting can be divided into four reaction paths, as shown in R1 to R4.60,61 μH+ + μe = sur

R1: H 2O + * ↔ OH* + H+ + e−

(6)

R2: OH* ↔ O* + H+ + e−

(7) +

R3: O* + H 2O ↔ OOH* + H + e

−

(8)

R4: OOH* ↔ O2 + H+ + e−

(9)

where * represents a surface site. The free energy of each adsorbate can be calculated as below DFT θ ΔGOH = EOH * − Esurface − μ H O + 2

1 θ ̂ * μ − GOH 2 H2

θ θ ̂ ΔGO = EODFT * − Esurface − μ H O + μ H − GO * 2

2

(10) (11)

DFT θ θ ̂ ΔGOOH = EOOH * − Esurface − 2μ H O + 1.5μ H − GOOH * 2

2

(12)

where Ĝ i includes contributions from the vibrational energy (ZPE and internal energy) and entropy of the adsorbate at 300 K. This correction is calculated using the harmonic approximation for every adsorbate and surface studied, with typical values of 0.35, 0.05, and 0.40 for OH*, O*, and OOH*, respectively. The following equations summarize the shift of standard free energy during each elementary step θ ΔG R1 = ΔGOH

(13)

θ ΔG R2 = ΔGO − ΔGOH

(14)

θ ΔG R3 = ΔGOOH − ΔGO

(15)

θ ΔG R4 = ΔGOθ 2(g) − ΔGOOH

(16)

On the basis of the equilibrium condition ∑jvjμj = 0, we can obtain the surface potential relative to the solution if the NHE potential is considered to be zero. ηsur − ηsol = ΔG Rθi /e

(17)

As a result, the theoretical overpotential obtained from the Gibbs free energy difference at each step can be expressed as the following equation η = max(ΔG Rθi)/e − 1.23

(18)

ΔGθR1

ΔGθR2

This means that the overpotential is zero if = = ΔGθR3 = ΔGθR4 = 1.23 eV, and this would define the ideal catalyst. Gibbs free energy indicates the direction of a chemical reaction from the perspective of thermodynamics. The 5479


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Figure 4. (a) Free-energy profiles of the OER on different BiVO4 surfaces; the black line is the ideal catalyst. The unit of Gibbs free energy is eV; (b) OER theoretical overpotential (η) volcano, where each leg is labeled by the step limiting OER performance when the scaling relation between ΔGOH and ΔGOOH is ΔGOOH = ΔGOH + 2.8.

calculated Gibbs free energy during the OER process on different BiVO4 surfaces are shown in Figure 4a (more details in Table S4). The four surfaces have different abilities for water splitting. The first step is the reaction process that the adsorbed H2O moiety is dissociated at the Bi sites with the interaction of the photogenerated surface hole. A surface hole (h+) reacts with adsorbed H2O to release a proton and create an OHads radical. Then, the OHads will release another H adatom to generate O adatom. This process will consume more energy because of the high positive value of ΔG. After that, the generated O adatom is extremely electrophilic and immediately obtains an electron by bonding to adjacent H2O. The last step is O−O separation from the surface with the help of a potential bias. Comparing the Gibbs free energy on different surfaces, the reaction process for (110)T1 and (1̅11)T1 surfaces are closer to the ideal catalysis process than those for (010)T1 and (010)T2 surfaces, indicating that (110)T1 and (1̅11)T1 surfaces are energetically more favorable as the photoanode for water splitting. The relationship between ΔGO − ΔGOH and ΔGOH can be applied to determine the limiting potential, as shown in Figure 4b. Top, bottom, left, and right regions stand for the limiting potential determined by R1, R4, R3, and R2, respectively. This map clearly indicates that the OER on BiVO4 surfaces is mainly determined by R2, no matter on which low-index surfaces the reactions are taking place. Furthermore, the theoretical overpotentials for (010)T1, (010)T2, (110)T1, and (1̅11)T1 surfaces are 1.89, 1.80, 1.56, and 1.54 eV, respectively, which agrees with the experimental results of BiVO4 (nearly 1.85 eV as shown in Figure S8). During the PEC catalysis, a higher applied potential would result in an increase in band bending of the depletion layer, which promotes the electron hole separation on the surface and suppresses the surface recombination. Thus, the water oxidation efficiency by the hole will be enhanced. 3.5. Stability of Low-Index Surfaces. Pourbaix diagrams can be used to assess the stability against chemical decomposition. The calculated Pourbaix diagrams of BiVO4 is shown in Figure 5 based on the method of Toma et al.62 To verify the calculation, experiments were also carried out for the open potential in different solutions. The experimentally measured open potentials are all located in the yellow region. Therefore, it is confirmed that BiVO4 is stable in aqueous solution in a certain range of pH solutions. Resistance against photocorrsion (degradation or decomposition) is an important index to evaluate the potential of a semiconductor material to be used as a photoelectrode toward

Figure 5. Pourbaix diagram of 50−50 at % Bi−V system in aqueous solution, assuming Bi and V ion concentrations at 10−6 mol·kg−1. The yellow shaded area stands for the stable region for BiVO4, and the green star sign stands for the tested open potential in NaH2PO4 solution with different pH values. Other regions are labelled for stable phase(s) of ABi3+ + VO4−; BBi3+ + VO2+; CBi3+ + VO2+; D Bi(s) + VO2+; EBiO+ + VO2+; FBiO+ + VO4−; GBiO+ + V3O5(s); HBiO+ + V2O3(s); JBi(s) + V(s); KBi(s) + V2O3(s); LBi 4 O 7 (s) + VO 4 − ; MBi 6 O 6 (OH) 3 3+ + V 2 O 3 (s); N V 3 (Bi 3 O 8 ) 2 (s) + V 2 O 3 (s); OBi 2 O 3 (s) + V 2 O 3 (s); P V3(Bi3O8)2(s) + HVO42−; QBi2O3(s) + HVO42; RBi4O7(s) + HVO42−; SBi2O3(s) + VO43−; TBi4O7(s) + VO43−; and UBi(s) + VO43−. Open potentials were obtained after immersing into the solution for 3 h by using a three-electrode configuration in a PCI4/300 potentiostat with PHE200 software (Gamry Electronic Instruments, Inc.), with the as-prepared BiVO4 as the working electrode, Pt mesh as the counter electrode, and Ag/AgCl as the reference electrode.

water splitting under light illumination. During our research, we noticed that photogenerated electrons (e−) has the potential to take part in reactions for self-reduction rather than the reactions for water splitting, which can be summarized as follows63 2BiVO4 + 3H 2 ↔ 2Bi + V2O5 + 3H 2O

(19)

2BiVO4 + H 2 ↔ Bi 2O3 + 2VO2 + H 2O

(20)

2BiVO4 + 5H 2 ↔ Bi 2O3 + 2V + 5H 2O

(21)

BiVO4 + 4H 2 ↔ Bi + V + 4H 2O

(22) +

Similarly, the photogenerated holes (h ) can also participate in the self-oxidization reaction instead of water splitting. The possible reaction paths are as follows64,65 5480


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Figure 6. pH value dependence of the reduction and oxidation potentials of BiVO4 surfaces at 298.15 K and 100 000 Pa (1 bar). (a) (010)T1 and (b) (11̅ 1)T1 with the corresponding reactions labeled near the lines. Red lines stand for oxidation reactions and blue lines for reduction reactions. The dependence of the water redox potentials and the band edges are also plotted. The dependence of the semiconductor redox potentials ϕre and ϕox on the pH value is determined by the specific reactions. When the reactions do not involve H+ or OH−, their ΔG is fixed relative to the pH change, and ϕre and ϕox shift together with ϕ (H+/H2) according to the Nernstian relation. When the full reactions involve H+ or OH−, ϕox is fixed with respect to the pH change. Considering the similarity with (010)T1 and (1̅11)T1, (010)T2 and (110)T1 are not plotted here. Detailed information can be found in Tables S5, S6, and Figure S9.

aqueous solution from a thermodynamic perspective when there are no strongly oxidative conditions (R23−R26), but for BiVO4 (1̅11)T1, the oxidation potentials of R26 is smaller than ϕ(O2/H2O), indicating that the surface will be corroded than in water splitting. Moreover, R27−R28 are mainly referred to strongly oxidative conditions such as surface-accumulated holes under photoexcitation or applied high bias potential and production of reactive oxygen species (OH− and O−) during the water-splitting process. Figure 6 shows that BiVO4 photoanodes are unstable and will be easily corroded in this condition, which is consistent with the experimental results where dissolution of BiVO4 in water is promoted by illumination as well as anodic biasing to 1.23 V versus reversible hydrogen electrode.68,69 Furthermore, our results also indicate that BiVO4 will remain stable (Table S6) if no strongly oxidative conditions are imposed on BiVO4. This can well explain the previous experimental results, where integration of a catalyst on the surface can suppress photocorrosion and enhance the long-term stability of BiVO4 when there is no exposure to strongly oxidative conditions,70−73 even for the case of porous and ion-permeable catalysts, such as CoPi and FeOOH, which do not physically isolate the semiconductor from the electrolyte environment. As discussed above, the bias potential greatly affects the catalytic activity and stability of a PEC process. On the one hand, a higher applied potential increases the activity because of the enhanced electron hole separation. On the other hand, the higher applied potential would induce a stronger oxidative condition on the surface. This will make the BiVO4 photoanode unstable and more easily corroded. The competitive reactions of self-oxidation and water oxidation will determine the stability under higher applied potentials.

4BiVO4 + 12H+ + 12Cl− ↔ 4BiCl3 + 2V2O5 + 6H 2 + 3O2

(23)

2BiVO4 + 12H+ + 12Cl− ↔ 2BiCl3 + 2VOCl3 + 6H 2 + 3O2

(24)

2BiVO4 + H 2O ↔ 2BiVO5 + H 2

(25)

8BiVO4 + H 2O ↔ 2Bi4O7 + 4V2O5 + H 2

(26)

2BiVO4 + O2 ↔ 2BiVO5

(27)

8BiVO4 + O2 ↔ 2Bi4O7 + 4V2O5

(28)

The above reactions define the thermodynamic reduction potential of BiVO4 (ϕre) and the oxidation potential of BiVO4 (ϕox) for the hole and electron, respectively, in which ϕre and ϕox can be determined by the Gibbs free energy changes of eqs 19−28. ϕre − ϕ(H+/H 2) = −⎡⎣∑ G(products) −

∑ G(reactants)⎤⎦/nEF

(29)

ϕox − ϕ(H+/H 2) = ⎡⎣∑ G(products) −

∑ G(reactants)⎤⎦/nEF

(30)

where G(products) and G(reactants) are the Gibbs free energy of products and reactants, most of which can be obtained from the handbook.66,67 n is the number of holes or electrons generated during the reactions. On the basis of the study by Chen and Wang, whether the semiconductor has the ability to resist photocorrosion largely depends on the alignment of ϕox relative to ϕ(O2/H2O) for the photoanode and ϕre relative to ϕ(H+/H2) for the photocathode. If ϕox (ϕre) is smaller (higher) than ϕ(O2/H2O) [ϕ(H+/H2)], the semiconductor will be corroded.63 For the reduction potential ϕre, all surfaces have higher ϕre than ϕ(H+/ H2) and thus will be easily corroded under light illumination from a thermodynamic perspective although the (1̅11)T1 surface has the ability to generate H2 [CBM lower than ϕ(H+/ H2)]. For the oxidation potential, BiVO4 (010)T1 is stable in

4. CONCLUSIONS In this paper, a comprehensive theoretical analysis is presented on the roles of crystal facets on PEC OER using the BiVO4 anode. The results show that the surfaces terminated by the Bi−O bond have low surface energies. These low-index surfaces can be divided into four categories, (010)T1, (010)T2, (110)T1, and (1̅11)T1. These surfaces were selected to calculate the electronic characteristics, surface activity, and stability. The results show that the variation in the electrostatic 5481


Research Article


(6) Chakthranont, P.; Seitz, L. C.; Jaramillo, T. F. Mapping Photoelectrochemical Current Distribution at Nanoscale Dimensions on Morphologically Controlled BiVO4. J. Phys. Chem. Lett. 2015, 6, 3702−3707. (7) Hu, J.; Zhao, X.; Chen, W.; Su, H.; Chen, Z. Theoretical Insight into the Mechanism of Photoelectrochemical Oxygen Evolution Reaction on BiVO4 Anode with Oxygen Vacancy. J. Phys. Chem. C 2017, 121, 18702−18709. (8) Chae, S. Y.; Lee, C. S.; Jung, H.; Joo, O.-S.; Min, B. K.; Kim, D. Y.; Hwang, Y. J. Insight into Charge Separation in WO3/BiVO4 Heterojunction for Solar Water Splitting. ACS Appl. Mater. Interfaces 2017, 9, 19780−19790. (9) Jung, H.; Chae, S. Y.; Shin, C.; Min, B. K.; Joo, O.-S.; Hwang, Y. J. Effect of the Si/TiO2/BiVO4 Heterojunction on the Onset Potential of Photocurrents for Solar Water Oxidation. ACS Appl. Mater. Interfaces 2015, 7, 5788−5796. (10) Shi, X.; Choi, I. Y.; Zhang, K.; Kwon, J.; Kim, D. Y.; Lee, J. K.; Oh, S. H.; Kim, J. K.; Park, J. H. Efficient Photoelectrochemical Hydrogen Production From Bismuth Vanadate-Decorated Tungsten Trioxide Helix Nanostructures. Nat. Commun. 2014, 5, 4775. (11) Chen, L.; Toma, F. M.; Cooper, J. K.; Lyon, A.; Lin, Y.; Sharp, I. D.; Ager, J. W. Mo-Doped BiVO4 Photoanodes Synthesized by Reactive Sputtering. ChemSusChem 2015, 8, 1066−1071. (12) Yu, J.; Kudo, A. Effects of Structural Variation on the Photocatalytic Performance of Hydrothermally Synthesized BiVO4. Adv. Funct. Mater. 2006, 16, 2163−2169. (13) Kim, T. W.; Choi, K.-S. Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343, 990−994. (14) Jeong, S. Y.; Choi, K. S.; Shin, H.-M.; Kim, T. L.; Song, J.; Yoon, S.; Jang, H. W.; Yoon, M.-H.; Jeon, C.; Lee, J.; Lee, S. Enhanced Photocatalytic Performance Depending on Morphology of Bismuth Vanadate Thin Film Synthesized by Pulsed Laser Deposition. ACS Appl. Mater. Interfaces 2017, 9, 505−512. (15) Abdi, F. F.; Han, L.; Smets, A. H. M.; Zeman, M.; Dam, B.; van de Krol, R. Efficient Solar Water Splitting by Enhanced Charge Separation in a Bismuth Vanadate-Silicon Tandem Photoelectrode. Nat. Commun. 2013, 4, 2195. (16) Zhou, B.; Zhao, X.; Liu, H.; Qu, J.; Huang, C. P. Visible-light Sensitive Cobalt-doped BiVO4 (Co-BiVO4) Photocatalytic Composites for the Degradation of Methylene Blue Dye in Dilute Aqueous Solutions. Appl. Catal., B 2010, 99, 214−221. (17) Jo, W. J.; Jang, J.-W.; Kong, K.-J.; Kang, H. J.; Kim, J. Y.; Jun, H.; Parmar, K. P. S.; Lee, J. S. Phosphate Doping into Monoclinic BiVO4 for Enhanced Photoelectrochemical Water Oxidation Activity. Angew. Chem., Int. Ed. 2012, 51, 3147−3151. (18) Luo, W.; Yang, Z.; Li, Z.; Zhang, J.; Liu, J.; Zhao, Z.; Wang, Z.; Yan, S.; Yu, T.; Zou, Z. Solar Hydrogen Generation from Seawater with a Modified BiVO4 Photoanode. Energy Environ. Sci. 2011, 4, 4046−4051. (19) Liang, Y.; Tsubota, T.; Mooij, L. P. A.; van de Krol, R. Highly Improved Quantum Efficiencies for Thin Film BiVO4 Photoanodes. J. Phys. Chem. C 2011, 115, 17594−17598. (20) Kong, H. J.; Won, D. H.; Kim, J.; Woo, S. I. Sulfur-Doped gC3N4/BiVO4 Composite Photocatalyst for Water Oxidation under Visible Light. Chem. Mater. 2016, 28, 1318−1324. (21) Rao, P. M.; Cai, L.; Liu, C.; Cho, I. S.; Lee, C. H.; Weisse, J. M.; Yang, P.; Zheng, X. Simultaneously Efficient Light Absorption and Charge Separation in WO3/BiVO4 Core/shell Nanowire Photoanode for Photoelectrochemical Water Oxidation. Nano Lett. 2014, 14, 1099−1105. (22) Cooper, J. K.; Scott, S. B.; Ling, Y.; Yang, J.; Hao, S.; Li, Y.; Toma, F. M.; Stutzmann, M.; Lakshmi, K. V.; Sharp, I. D. Role of Hydrogen in Defining the n-Type Character of BiVO4 Photoanodes. Chem. Mater. 2016, 28, 5761−5771. (23) Pilli, S. K.; Furtak, T. E.; Brown, L. D.; Deutsch, T. G.; Turner, J. A.; Herring, A. M. Cobalt-phosphate (Co-Pi) Catalyst Modified Modoped BiVO4 Photoelectrodes for Solar Water Oxidation. Energy Environ. Sci. 2011, 4, 5028−5034.

potential of the low-index surfaces causes (010)T1 and (010)T2 to have higher oxidation ability than (110)T1 and (1̅11)T1 surfaces, as a photoanode. Electrons prefer to accumulate on the (010)T1 and (010)T2 surfaces, whereas holes like to accumulate on the (110)T1 and (11̅ 1)T1 surfaces during a photocatalytic process. Furthermore, the analysis of adsorbed intermediates during the water-splitting process indicates that the OER on BiVO4 surfaces is mainly determined by H adatom release and O adatom generation from OHads (110)T1, and (1̅11)T1 surfaces are energetically more favorable as the photoanode for water splitting than (010)T1 and (010)T2 surfaces. In terms of stability, all BiVO4 surfaces are found unstable when used as a photoanode except (010)T1 and (010)T2 surfaces, which are stable from a thermodynamic perspective when there are no strong oxidative conditions (R23−R26). In all, BiVO4 (010)T1 and (010)T2 surfaces have stronger oxidation ability, higher stability, and higher overpotential than (110)T1 and (1̅11)T1 surfaces. These results provide important insights into the roles of low-index surfaces on BiVO4 for photocatalytic reactions.

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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/acsami.7b15243. Simulation verification, more information about lowindex surfaces, DOSs, and stability (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.S.). *E-mail: [email protected] (Z.C.). ORCID

Haibin Su: 0000-0001-9760-6567 Zhong Chen: 0000-0001-7518-1414 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Ministry of Education of Singapore (RG15/16), the National Natural Science Foundation of China (no. 21676216), and the China Postdoctoral Science Foundation (nos. 2014M550507; 2015T81046) are greatly acknowledged.

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REFERENCES

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