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The Anisotropy Properties of Electronic, Adsorption and Stability of Low-index BiVO4 Surfaces for Photoelectrochemical Applications Jun Hu, Wei Chen, Xin Zhao, Haibin Su, and Zhong Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15243 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018
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The Anisotropy Properties of Electronic, Adsorption, and Stability of Low-index BiVO4 Surfaces for Photoelectrochemical Applications id Zhong Chen,*, ‡,○ id Jun Hu†,‡, Wei Chen‡, Xin Zhao‡, Haibin Su,*, ‡,○ †
School of Chemical Engineering, Northwest University, Xi’an, China 710069;
‡
School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
S Supporting Information ○ ABSTRACT： ： Many experimental results reveal different activities among different low-index surfaces of photocatalysts. The current investigation focuses on theoretical understandings of the electronic characteristics, surface activity and stability of different low-index surfaces of BiVO4 towards water splitting using first-principles calculations. The results indicate that BiVO4 has four types of low-index surfaces, namely, the (010)T1, (010)T2, (110)T1, and (111)T1. The different band edge potential of the surfaces, resulted from the variation of electrostatic potential, will lead to a higher oxidation ability for the (010)T1 and (010)T2 than (110)T1 and (111)T1 surfaces. The electrons prefer to accumulate on the (010)T1 and (010)T2 surfaces while holes like to accumulate on the (110)T1 and (111)T1 surfaces during a photocatalytic process. Moreover, investigation on the adsorbed intermediates during water splitting process indicates oxygen evolution reaction (OER) on BiVO4 surfaces are mainly dominated by the reaction of ∗ ↔ ∗ + + , and (110)T1 and ( 1 11)T1 surfaces are energetically more favorable as photoanode for water splitting than (010)T1 and (010)T2. Furthermore, BiVO4 surface as photoanodes tend to be unstable and can easily be corroded with or without the presence of oxidative environment, however, there is an exception for the BiVO4 (010)T1 and (010)T2 surfaces, which is thermodynamically stable in the solution when there are no strongly 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 oxygen evolution reaction, such as TiO2, α-Fe2O3 Co3O4 etc. 1-3 , monoclinic clinobisvanite bismuth scheelite (ms-BiVO4) has attracted
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huge attention due to its advantageous properties including reasonable band-edge alignment, good optical absorption, abundance, and simple synthesis 4-9. In past decades, huge efforts, including morphology regulation 10-14, elemental doping 15-18, heterogeneous coupling 19 - 22 and cocatalyst loading 23 - 26 , have been carried to improve the photoelectrocatalytic activity towards an optimized photocurrent density. However, the activity of ms-BiVO4 is still limited for practical applications. One of the main obstacles is due to the lack of fundamental understanding of complex surface effects. Recently, in-situ experimental results of TiO2 show that many factors, for instance, facet orientations, crystal and electronic structures, play crucial roles in the surface properties 27-32. As results, these factors should be considered carefully for the design and fabrication of highly efficient photoelectrocatalytic catalysts. Similar phenomenon was also 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 OER 33 , 34 . The photocatalytic activity towards water oxidation using 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 the 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 (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 BiVO4 sample, and found obvious difference in geometric structure, optical properties, electronic structure, water adsorption, and the whole OER free-energy profiles 39. The calculations by Li et al. have laid a solid foundation to understand the anisotropic properties, including surface geometric/electronic structures, surface energy, work function, bader charge, and oxygenvacancy 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 work is needed to focus 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 oxygen evolution reactions on BiVO4 based on DFT calculations. The results are able to unveil the insights of surface anisotropy properties, including band structures, stability, adsorption mechanism during the reaction process, as well as the change of Gibbs free energy in the elementary steps of the catalysis process. 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), (101), (011), (111) and (111) surfaces were chosen to be calculated (see Supporting Information, B ACS Paragon Plus Environment
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Figure S1). It is well known that every surfaces 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 slab must n times (n=1, 2, 3,…) that of unit cells. Based on those conditions, the 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, selfconsistent periodic Density Functional Theory (DFT) calculations were performed. The Generalized Gradient Approximation (GGA) method, in the form of the Perdew-BurkeErnzerhof (PBE) approximation, was used to calculate the exchange-correlation energy. The Broyden-Fletcher-Goldfarb-Shanno (BFGS) 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 were performed during the calculations 42. The energy cutoff is 380 eV and the SCF 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 Gamma point only k-points samplings were used 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 XRD 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 ( ) between surface and adsorbed particles was computed by eq. 1. = − −
(1)
where is the total energy of the system, including the adsorption molecules and the BiVO4 facet; is the energy of adsorption molecules; is the energy of BiVO4 facet after optimization. In the definitions, the higher negative value of indicates a more stable adsorption on the plane. All the calculated energies reported herein include Zero-Point Energy (ZPE) correction 48. The surface energy () can be calculated by the following eq. (2):
= ( !"#$ − % &' ) (2) where &' is the total energy per unit cell of the bulk, n is the number of unit cells that the slab model contains, and * is the surface area of the slab model. Based on the calculated results shown in Figure S3, at least four Bi layers were selected during the simulation in order to reduce the errors. 3. RESULTS AND DISCUSSION. 3.A. Geometric Structure of Low-index Surfaces. Among the above surface terminations, we found only the surfaces terminated by Bi-O bond have low surfaces energies. More information about determinations of the low-index surfaces can be found in Table S2 and Figure S4 in Supporting Information. The low-index surfaces can be divided into four categories. The first one is (010)T1, which terminates with two Bi-O bond with length of 2.459 Å; The second one is (010)T2, which terminates with two Bi-O bond with length of 2.522 Å; the third one is C ACS Paragon Plus Environment
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composed of (110)T1 surface, which breaks three Bi-O bond with bond lengths of 2.459 Å, 2.440 Å, 2.417 Å. The last one is (111)T1 surfaces, which breaks three Bi-O bond with lengths of 2.522 Å, 2.440 Å, 2.417 Å. Therefore, the stable surface are VO4 tetrahedron with six or five coordinate Bi atom. There is a difference among the stable surfaces in the number and length of Bi-O bonds. The unrelaxed and relaxed geometric structure of the selected low-index ms-BiVO4 surfaces are shown in Figure 1.
Figure 1. The unrelaxed and relaxed geometric structure 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.
From Figure 1, we can see those low-index surfaces are terminated with Bi atoms and VO4 groups, which agree well with previous calculation and crystal chemistry 40. For unrelaxed slab of (010)T1, the surface Bi and V atoms are coplanar in the surface plane. The surface Bi atom has six coordination, while surface V atom possesses four coordination. In the case of relaxed slab, the surface Bi1 and O1 atom move inward by 0.088 Å and 0.066 Å, respectively, while 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 Å to 2.478 Å, while the bond length of V1-O6 changed from 1.746 Å to 1.671 Å. It is notable that the same phenomenon is also observed on (010)T2 surface. For (110)T1 and (111)T1 surfaces, the coordination number is five for the surface Bi atom, and four for the surface V atom. In the unrelaxed slab of ms-BiVO4 (110)T1, the outermost layer is indexed to surface Bi1 atom and the sublayer of the surface is ascribed to D ACS Paragon Plus Environment
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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 atom outward by 0.091 Å 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.B. Electronic Characteristics of Low-index Surfaces. The calculated electronic characteristics of the selected low-index surfaces are plotted in Figure 2. Detail information about all low-index surfaces is given in Figure S5.
Figure 2. The calculated electronic characteristics of bulk BiVO4, and low-index surfaces. (a) Density of States (DOS); (b) work function; (c) Calculated surface energies and edges relative to the 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 photo-catalysts.
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 valence band maximum (VBM) is composed of Bi 6s and O 2p resonance peaks, while the conduction band minimum (CBM) is composed of V 3d and O 2p resonance peaks. The same is true for the bulk (see Supporting Information, Figure S6). However, there is still a small difference for the DOS and band gap of different surfaces. It is found that the DOS of VBM and CBM are greatly increased for all of those surfaces when compared with bulk (as shown in Figure S6(a)), which are mainly due to the unsaturated bond on the surface. It should be pointed E ACS Paragon Plus Environment
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out that the band gap of (111)T1 is 2.46 eV, which is in accordance to the experimental results with the value of 2.40-2.50 eV 53. The VBM and CBM of bulk BiVO4 are 2.33 eV and 0.18 eV, which also agreement with previous results 54. The surface energies for (010)T1, (110)T1 are determined to be 0.427 J/m2 and 0.460 J/m2, which are very close to the previous calculations 38,40. The electrostatic potential of the low-index surfaces are dramatically changed, as shown in Figure 2(b). The average potential for (010)T1, (010)T2, (110)T1 and (111)T1 are -7.00 eV,-7.02 eV,-5.88 eV and -5.79 eV, respectively. Such results reveal the electrons prefer to accumulate on the (010)T1 and (010)T2 surfaces rather than (110)T1 and (111)T1 surfaces. On the contrary, holes like to accumulate on the (110)T1 and (111)T1 surfaces rather than (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 structure and catalytic activities of BiVO4. To better understand the ability of different surfaces for PEC and photocatalytic applications towards selective photo-deposition and water splitting, the band edge energies of different facets are investigated. Figure 2(c) indicates the electrostatic potential imposes remarkable influence on the position of band structure. For the PEC catalysis, the holes will transfer from the bulk to surface while the electrons will flow from the anode to cathode. The surface is used to accumulate the holes and the relationship of VBM between bulk and surface plays an important role for the charge separation. As indicted in Figure 2(c), the VBM of (110)T1 and (111)T1 are lower than those of the bulk, this will enhance hole separation from bulk to surface. For photocatalysis, holes and electrons will both transfer to surface to react with the electrolyte solution. Therefore the surface is not only used to accumulate the holes but also 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 can be described in Figure 2(d). It can be seen that the differences of conduction band minimum (CBM) and valence band maximum (VBM) between (010)T1 and (110)T1 are 1.35 eV and 1.12 eV. This implies the spatial charge separation between different surfaces will be obvious under light irradiation. The photogenerated electrons on (110)T1 surface can transfer to (010)T1 surface to be the reduction site, while the photogenerated holes will migrate to (110)T1 from (010)T1 surface. The potential about hole scavenger ([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 photo-depositions can be obtained from 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, while 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 the surface of the (110)T1 and (111)T1. This can well explain the simultaneous reduction reaction with photo-generated electrons and oxidation reaction with photogenerated holes on F ACS Paragon Plus Environment
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different surfaces. Recently, researches 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, experiment showed that improved photooxidation performance of ms-BiVO4 particles is correlated with presence of the (010) facets 33-35. This result seems to be contradictory to our calculation that the (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 due to the enhanced electron accumulation on the (010) surfaces 57. Since electron transport is the limiting step for this particle system, the exposed (010) facets leads 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 electronhole pair recombination. 3.C. 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 length of H2Oads, OHads, Oads involved in the water splitting process on low-index surfaces. Only top site of Bi adatom is considered before the minimum energy of the system because Bi adatom on BiVO4 surface is active site for catalysis 59. More detailed results are listed in Table S3.
Figure 3. The 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 adsorption state on the facet. G ACS Paragon Plus Environment
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Figure 3 illustrates detailed information about the adsorption of several key species on lowindex surfaces of BiVO4, which indicates that the stable absorption position for the above species are 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 the OHads on difference surfaces, we found the bond length of Bi-O on (010)T1, and (010)T2 are shorter than those on (110)T1, and (111)T1. Bi atom of (010)T1, and (010)T2 moves outward after adsorption of OHads while the position of Bi atom on (110)T1 and (1 11)T1 almost remain unchanged. In addition, the adsorption of Oads on (110)T1 and (1 11)T1 are more stable than those on (010)T1 and (010)T2. This is mainly attributed to the different surface cleavage, where Bi sites of (110)T1 and (111)T1 are 5-fold coordinated, while Bi sites of (010)T1 and (010)T2 are 6-fold coordinated. Because of the existence of unpaired electrons on single O atom, the 5-fold coordinated Bi sites exhibits 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 above species not only have interaction with the top Bi atom, but also have interaction with adjacent atoms on the surface, which can be verified by the corresponding partial density of states (PDOS) of (010)T2 surface, as shown in Figure S7. 3.D. Surface Activity. The adsorption on the BiVO4 surface plays an important role in the PEC water oxidation reactions and catalysis mechanism. The oxidation process (oxygen evolution reaction, OER) occurs at the anode at standard oxidation potential of 1.23 V (vs. NHE). Because of the four-electron-transfer process during OER, the minimum free energy required to split two molecules of water at equilibrium and standard conditions are estimated as follows: (3)
∆,-. = 4 × 1.23 = 4.92 eV The NHE potential is defined as the following half reaction (7) ↔ 89 + : If the NHE potential is considered to be zero, we will obtain:
(4) (5)
;? = ;
