Crystallographic-Orientation-Dependent Charge Separation of BiVO4

Subscriber access provided by Washington University | Libraries

Letter

Crystallographic-Orientation-Dependent Charge Separation of BiVO4 for Solar Water Oxidation Deng Li, Yong Liu, Wenwen Shi, Chenyi Shao, Shengyang Wang, Chunmei Ding, Taifeng Liu, Fengtao Fan, Jingying Shi, and Can Li ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00153 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Crystallographic-Orientation-Dependent Charge Separation of BiVO4 for Solar Water Oxidation Deng Li,†‡ Yong Liu,†# Wenwen Shi,†‡ Chenyi Shao,†‡ Shengyang Wang,† Chunmei Ding,† Taifeng Liu,‖ Fengtao Fan,† Jingying Shi*† and Can Li*† †State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Zhongshan Road 457, Dalian, 116023, China ‡University of Chinese Academy of Sciences, Beijing, 100049, China. #College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, China ‖National & Local Joint Engineering Research Center for Applied Technology of Hybrid Nanomaterials, Collaborative Innovation Center of Nano Functional Materials and Applications, Henan University, Kaifeng, 475004, China

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Group website: http://canli.dicp.ac.cn/en/ * Email: [email protected]

ACS Paragon Plus Environment

1

ACS Energy Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 22

ABSTRACT: Charge separation plays a crucial role in determining the solar-to-hydrogen conversion efficiency for photoelectrochemical water splitting. Of the factors that affect charge separation, the anisotropic charge transport property of semiconductors shows great potential in promoting charge separation, while has received little attention. Herein, we report BiVO4 photoanodes with predominant [010] and [121] orientations and demonstrate a crystallographicorientation-dependent charge separation of BiVO4 for solar water oxidation. We found that [010]orientated BiVO4 photoanode generated a photocurrent 2.9 times that of [121]-orientated one, owing to the significant improved charge separation. An in-depth investigation of the surface band bending by open-circuit potential and film conductivity by contacting atomic force microscopy reveals that the higher electron mobility along [010] direction than that of [121] accounts for the improvement in charge separation. This work offers a fundamental insight of charge separation in anisotropic photoanodes for rational design of efficient photoanodes for solar energy conversion.

TOC GRAPHICS


2

Page 3 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Photoelectrochemical (PEC) water splitting that directly converts solar energy to hydrogen offers a promising way to produce clean energy for the future.1-4 Up to now, a key barrier for solar energy conversion via PEC approach is the lack of efficient photoanodes with high water oxidation activity.5, 6 One of the crucial factors determining the solar-to-hydrogen (STH) efficiency of a photoanode is the separation of photogenerated charges.7, 8 Understanding the charge separation process and developing new approaches to promote the charge separation efficiency are highly desirable in the research of PEC water splitting process. Fundamentally, over an excited photoanode, photogenerated holes move to the semiconductor/electrolyte interface to participate in water oxidation reaction, while photogenerated electrons transport to the conducting substrate and be pumped to the counter electrode for proton reduction reaction.2 If the holes and electrons are not sufficiently separated, recombination will occur, leading to a low charge separation efficiency. By far, most works to improve the charge separation efficiency focus on promoting the charge separation driving force by either tailoring the surface band bending property or building junctions.9-12 However, the charge separation occurs not only at the interface, but also in the bulk of semiconductor, where the electron transport is also crucial for charge separation, but has received little attention.13 Doping of heteroatoms and introducing vacancies have been regarded as effective ways to facilitate charge separation via improving the conductivity of semicondutors.14-16 Besides of the strategies of changing the physical properties, it is found recently that tailoring the crystallographic orientation of intrinsic semiconductor is also beneficial for enhanced charge separation over semiconductor photoelectrodes.17-19 Zheng’s group reported that a BiVO4 film with a preferred [001] orientation outperformed that with randomly orientated grains in PEC water oxidation.18 Lee’s group further demonstrated that the BiVO4 film with [010] orientation epitaxially grown on yttria-stabilized


3


Page 4 of 22

zirconia substrate showed a higher charge separation efficiency than that with [001] orientation on SrTiO3.19 However, the different substrates that were adopted to grow specific direction orientated BiVO4 films may lead to distinct conclusions. Till now, the effect of crystallographic orientation on the PEC activity was seldom studied due to the limitations of the epitaxial growth of BiVO4 photoanodes with controlled crystallographic orientations. Among the metal oxide semiconductors for solar water splitting, monoclinic BiVO4 is the most promising candidate considering its relatively narrow band gap (~2.4 eV), low onset potential and good stability.20 Density functional theory (DFT) calculations had indicated that electrons and holes showed different mobilities along different crystallographic axes of BiVO4.21 Moreover, the spatial charge separation nature of decahedral BiVO4 crystals due to the anisotropic charge transport demonstrates a great potential of tailoring the crystal property to improve the charge separation efficiency in a significant way.21, 22 Herein, we fabricated BiVO4 photoanodes comprised of microcrystals in parallel and perpendicular arrays on the FTO substrate via a precursor recrystallization method. The corresponding crystallographic orientations were inferred as [121] and [010] directions respectively. The PEC performance, charge separation and injection, as well as the surface band bending property and electron mobility of the two types of photoanodes were investigated. It is shown that the electron mobility along [010] is faster than that along [121] direction, contributing to a higher charge separation efficiency and hence a better PEC performance of the [010]orientated BiVO4 photoanode. BiVO4 films comprised of microplate crystal arrays with tunable crystallographic orientation were fabricated via a precursor recrystallization approach.23 Typically, the precursor films firstly grew on the FTO substrate with a BiVO4 seed layer by a chemical bath at controlled temperatures.


4

Page 5 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

The as-prepared precursor films were then subjected to a hydrothermal treatment, followed by a calcination process. By controlling the precursor growth temperature, the morphology and crystallographic orientation of the precursor films were changed as evidenced by the XRD patterns and scanning electron microscopy (SEM) images (Figure S1). At 5 oC, the precursor film exhibited a morphology of microplates with a spread distribution on the substrate and the precursor was indexed as tetragonal zircon BiVO4 with a preferred orientation of [312] direction. While, at 40 oC, the precursor film of aggregated particles with a preferred orientation of [200] direction was prepared instead (Figure S1). After hydrothermal treatment, two types of BiVO4 photoanodes consisting of microplate crystals perpendicular and parallel to the substrate were obtained (Figure 1). The studies on the evolution of the morphology and crystal orientation revealed that the precursors of tetragonal zircon BiVO4 were dissolved and recrystallize to grow monoclinic sheelite BiVO4 crystals during the hydrothermal process (Figure S2-S5, see details in ESI).23 Induced by the crystallographic orientation and the growth sites of the precursor films, the microplate crystals were grown as perpendicular or parallel arrays on the substrate.


5


Page 6 of 22

Figure 1. The schematic illustrations of (a) perpendicular array and (b) parallel array of BiVO4 photoanodes. Top and cross-sectional views (inset) of (c) [121]-BiVO4 and (d) [010]-BiVO4. (e) XRD patterns of [121]-BiVO4 and [010]-BiVO4. (f) The texture coefficient of [121]-BiVO4 and [010]-BiVO4 photoanodes. Figure 1a and 1b schematically illustrate the two types of BiVO4 photoanodes. The corresponding top and cross-sectional view SEM images clearly demonstrate that decahedral


6

Page 7 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

BiVO4 microplates were well grown as either perpendicular (Figure 1c) or parallel arrays (Figure 1d) on the FTO substrate. The cross-sectional views further suggest the film thicknesses as ca. 3.0 and 2.2 μm respectively. XRD patterns of two samples are both indexed as monoclinic sheelite BiVO4 (Figure 1e).24 Accordingly, the exposed facets of the decahedral BiVO4 crystals are inferred as {010} and {011} facets.24,

25

Following the crystal theories, the orientations of

perpendicular and parallel decahedral BiVO4 crystal array can be inferred as [121] direction and [010] direction respectively, which is also evidenced by the XRD patterns (Figure 1e).24 The sharp and intense peak of (121) plane for the perpendicular array and that of (040) plane for parallel array correspond to the preferred [121] and [010] orientations respectively. The degrees of the orientation preference were further quantified by the texture coefficients (P) of (121) and (040) planes derived from the XRD data (Figure 1f).26 The calculated P values of the (121) plane for perpendicular array and (040) plane for parallel array are 1.24 and 3.86 respectively, which are both larger than a standard value of 1, confirming the predominance of [121] and [010] orientations. For short, the perpendicular and parallel arrays are denoted as [121]-BiVO4 and [010]-BiVO4 respectively. Regardless of the different crystal arrays, they exhibit a same band gap of 2.36 eV and similar light absorption intensity (UV-Vis spectra in Figure S6). The flat band potentials and electron densities derived from Mott-Schottky (MS) plots are also similar to each other (Figure S7, Table S1), indicating same band structures for [121]-BiVO4 and [010]-BiVO4. In this respect, two types of BiVO4 photoanodes with predominant [121] and [010] crystallographic orientations were fabricated for further investigation.


7


Page 8 of 22

Figure 2. (a) The photocurrent density-potential curves of [010]-BiVO4 and [121]-BiVO4 without (solid line) and with (dashed line) a hole scavenger (e.g. Na2SO3). (b) The charge injection and separation efficiency ratios of [010]-BiVO4 to [121]-BiVO4. (c) The charge recombination rate constant (Kr) and (d) the transient photocurrent of [010]-BiVO4 and [121]-BiVO4. Figure 2a depicts the photocurrent density-potential (j-E) curves of [010]-BiVO4 and [121]BiVO4 in 1 M potassium borate (KBi, pH = 9) with the presence or absence of a hole scavenge (e.g. Na2SO3). It is shown that [010]-BiVO4 produced a higher photocurrent density for either water (jwater) or sulfite (jsulfite) oxidation than [121]-BiVO4, indicating a better PEC performance of [010]-BiVO4 than [121]-BiVO4. At 1.23 VRHE, the jwater of [010]-BiVO4 (0.61 mA/cm2) is about 2.9 times that of [121]-BiVO4 (0.21 mA/cm2). The difference in the PEC performance was further confirmed by measuring their incident photon-to-current conversion efficiencies (IPCEs) (Figure


8

Page 9 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

S8). The maximum IPCEs of the [010]-BiVO4 and [121]-BiVO4 were 21.4 and 7.5 % respectively, which is in good agreement with the results of the jwater. In view of the light absorption difference (Figure S6a), the absorbed photon-to-current conversion efficiencies (APCE) of the [010]-BiVO4 were evaluated as 27.9 %, which is about 3.0 times as high as that of [121]-BiVO4 (9.3 %). Furthermore, considering the electrochemical surface area (ECSA) of [010]-BiVO4 is 0.64 times that of [121]-BiVO4 (Figure S9), the former actually exhibits more than 3 times higher PEC activity in the case of normalization by ECSA. Since the APCE of a photoanode is determined by the interfacial charge injection (ηinj) and charge separation efficiencies (ηsep) (APCE = ηsep × ηinj), the higher APCE, in other words, the better PEC performance of [010]-BiVO4 than [121]-BiVO4 can be attributed to the higher ηsep and ηinj.27 Given that ηinj of the photoanode in a hole scavenger of Na2SO3 equals 100%, the ηsep and ηinj can be evaluated by ηinj = jwater/jsulfite and ηsep = jsulfite/jabs respectively, where jabs is the maximum photocurrent density that a photoanode can achieve and determined by the light absorption (Figure S6b). At 1.23 VRHE, the ηinj of [010]-BiVO4 and [121]-BiVO4 were 37.1 % and 27.6 % respectively, and the ηsep were 28.3 % and 12.7 % (Figure S10). It is clearly shown that the [010]-BiVO4 exhibits higher ηinj and ηsep than [121]-BiVO4. Meanwhile, the difference in ηsep between these two samples is greater than that in ηinj. To further visualize the difference in ηinj and ηsep, the ratios of ηsep and ηinj ([010]-BiVO4/[121]-BiVO4) were plotted against the applied potentials (Figure 2b). The ratio of ηsep was seen to locate in the range of 1.5 – 2.3, while that of ηinj in the range of 0.6 – 1.3. Obviously, the ratio of ηsep is much larger than that of ηinj, indicating the higher jwater of [010]BiVO4 as compared to [121]-BiVO4 is mainly attributed to the significant improvement in charge separation than in charge injection.


9


Page 10 of 22

The charge separation properties of [010]-BiVO4 and [121]-BiVO4 were further investigated by evaluating the charge recombination kinetics using photo-electrochemical impedance spectra (PEIS) (Figure S11).28 As shown in Figure 2c, [010]-BiVO4 exhibited a much slower recombination rate constant (Kr) than [121]-BiVO4, which agrees well with the higher ηsep of [010]BiVO4. Thus, with a higher ηsep and a slower charge recombination rate, more holes are certain to survive on the surface of [010]-BiVO4 under irradiation, which can be directly quantified by the transient photocurrent of charge extraction process.29 Figure 2d shows that [010]-BiVO4 exhibited a higher transient photocurrent than [121]-BiVO4 and the integrated amount of the survived charges of [010]-BiVO4 is about 1.9 times that of [121]-BiVO4, which further supports the higher ηsep of [010]-BiVO4. These results clearly demonstrate that the charge separation of [010]-BiVO4 is more efficient than that of [121]-BiVO4. In PEC process, excited electrons and holes are generated in the region within the penetration depth of light.30, 31 The excited electrons and holes in the space charge layer (thinner than the penetration depth) can be separated by the driving force generated from band bending at the semiconductor/electrolyte interface, while most of the excited charges generated beyond the space charge layer will recombine.30-32 The separated holes move directly towards the surface, while the separated electrons need to transport through the bulk of semiconductor to the substrate. The charge separation in the space charge layer is decided by the intensity of band bending of the semiconductor, while the electron transport is related more to the conductivity of the film.31 Note that the effective charge separation means the successful collection of holes at the surface and electrons at the conducting substrate. For a typical undoped BiVO4, the thickness of space charge layer is in the scale of hundred nanometers, which is much smaller than the size scale of our samples.33, 34 Therefore, the charge separation in our BiVO4 is influenced by not only the intensity


10

Page 11 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

of band bending but also the conductivity of the semiconductor. To clarify the origin of the difference in ηsep of [121]-BiVO4 and [010]-BiVO4, the above two factors were examined.

Figure 3. (a) The OCPs of [121]-BiVO4 and [010]-BiVO4 under dark and irradiation. The schematic illustration of the band bendings of (b) [010]-BiVO4 and (c) [121]-BiVO4. To begin with, we first evaluated the intensity of surface band bending of the semiconductors. Considering the different configuration of the crystal arrays, the exposing facet ratios of two samples differ from each other, that is more {011} facets are exposed for [010]-BiVO4, while more {010} facets for [121]-BiVO4. As reported, the {011} facets of decahedral BiVO4 crystal show a more intense band bending than {010} facets.35 Thus, [010]-BiVO4 with a relatively larger ratio of {011} facets is expected to show a higher intensity of overall band bending than [121]-BiVO4, which can be evidenced by the open-circuit potentials (OCPs) under dark and irradiation.36 Note that the irradiation intensity was adjust to be high enough to produce a flat band condition of the photoanodes. As shown in Figure 3a, the OCPs of [010]-BiVO4 (0.343 ± 0.004 VRHE) and [121]BiVO4 (0.347 ± 0.003 VRHE) under irradiation are similar, indicating similar flat band potentials of two photoanodes. It is consist with the result of the MS plots (Table S1). Under dark condition, the OCP of [010]-BiVO4 (0.551 ± 0.006 VRHE) is larger than that of [121]-BiVO4 (0.497 ± 0.005 VRHE). The larger difference of OCPs under dark and irradiation of [010]-BiVO4 corresponds to a


11


Page 12 of 22

more intense band bending of [010]-BiVO4, which can be attributed to the different Fermi level pinning effect caused by the surface states on different facets.36, 37 The surface states were further evidenced by the derivative from the Mott-Schottky plots (Figure S7b).38, 39 The emerged peaks in Figure S7b indicate the existing of the intra band surface states. Specially, the broader peak for [121]-BiVO4 as compared to [010]-BiVO4 suggests a higher density of surface states and thus a greater extent of Fermi level pinning over [121]-BiVO4. The band bending conditions of [010]BiVO4 and [121]-BiVO4 based on the OCPs are schematically illustrated in Figure 3b and 3c. It is clear that [010]-BiVO4 demonstrates a more intense band bending than [121]-BiVO4, contributing to an improved ηsep.

Figure 4. The crystal lattice and the theoretically calculated electron mobility along (a) [121] and (b) [010] direction of monoclinic BiVO4. Violet, gray and red balls represent Bi, V and O atoms.


12

Page 13 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

(c) Current-potential (I-V) curves of [010]-BiVO4 and [121]-BiVO4 by C-AFM under dark and light condition. Another factor of the conductivity of BiVO4 photoanodes with different crystallographic orientations were investigated by DFT calculations and current-voltage (I-V) measurements. Figure 4a illustrates the atomic arrangements for the crystal lattices along [010] and [121] directions. The electron transport along crystal orientations was assumed to follow a small polaron hopping mechanism which describes the electron migration as gradual changes in the disorder lattice configurations between two adjacent polaron configurations.21 More hopping sites along [010] than [121] favor the electron transport, leading to a higher electron mobility along the former orientation. The calculated electron mobilities along [010] orientation (3.264 × 10-6 cm2/(V·s)) is indeed higher than that along [121] (1.813 × 10-6 cm2/(V·s)). The conductivities were further experimentally evaluated by the I-V curves of a solid devices comprised of the BiVO4 films (Figure S12) and by C-AFM technique (Figure S13). It is shown that at a constant potential (e.g. 0.75 V), the currents of [010]-BiVO4 are higher than those of [121]-BiVO4 under both dark and light, indicating a superior electron transport in [010]-BiVO4. Furthermore, under irradiation, the currents of both [010]-BiVO4 and [121]-BiVO4 are higher than those under dark condition, which displays a typical photoconductivity property of BiVO4 semiconductor. Typical I-V curves of the photoanodes by C-AFM under dark and light are depicted in Figure 4b. By substituting the electron densities and the film thicknesses, the conductivities (σ) and electron mobilities (μ) of [010]-BiVO4 and [121]-BiVO4 were calculated and summarized in Table 1 (see ESI for calculation details). The conductivity of [010]-BiVO4 is 1.6 times that of [121]-BiVO4. The electron mobilities of [010]-BiVO4 and [121]-BiVO4 were determined to be 7.9 × 10-5 and 4.5 × 10-5 cm2/(V·s)) respectively. Evidently, the former shows a higher electron mobility than the latter,


13


Page 14 of 22

which is consist with the DFT calculation results despite the different orders of magnitude of mobility values. Therefore, the origin of the higher conductivity of [010]-BiVO4 can be ascribed to the higher electron mobility along [010] orientation than along [121] orientation. Table 1. The calculated conductivities and electron mobilities of [121]-BiVO4 and [010]-BiVO4 by C-AFM I (pA) at μ σ (s/cm) 0.75 V (cm2/(V·s)) [121]-BiVO4

0.272 1.4 × 10-4 4.5 × 10-5

[010]-BiVO4

0.578 2.2 × 10-4 7.9 × 10-5

To sum up, a more intense band bending due to the more exposed {011} facets and a higher conductivity due to the higher electron mobility along [010] orientation jointly contribute to the higher ηsep of [010]-BiVO4 as compared with [121]-BiVO4. However, which factor is more dominating in affecting ηsep remains unknown. To figure out this problem, we further loaded a water oxidation cocatalysts of CoBi on the as-investigated photoanodes (CoBi/[010]-BiVO4 and CoBi/[121]-BiVO4) and studied the PEC activities of the cocatalyst decorated photoanodes (Figure S14 and S15). Loading cocatalyst on the photoanode is generally able to passivate the surface states to relax the Fermi level pining of the photoanodes.40 In this regard, the difference in the surface band bendings of [010]-BiVO4 and [121]-BiVO4 was expected to decrease. Therefore, we can focus on the electron transports of [121] and [010] orientated photoanodes.


14

Page 15 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Figure 5. (a) The photocurrent density-potential curves of CoBi/[121]-BiVO4 and CoBi/[010]BiVO4 without (solid line) and with (dashed line) a hole scavenger (e.g. Na2SO3). (b) The charge injection and separation efficiency ratios of CoBi/[121]-BiVO4 and CoBi/[010]-BiVO4. As shown in Figure S16, the OCPs of CoBi/[010]-BiVO4 and CoBi/[121]-BiVO4 were both increased, corresponding a strengthened band bending of each photoanode as compared with naked one. Meanwhile, as expected, the difference in OCPs between two samples was decreased, indicating similar intensities of band bending for CoBi/[010]-BiVO4 and CoBi/[121]-BiVO4. Figure 5a depicts the photocurrent-potential curves of CoBi/[010]-BiVO4 and CoBi/[121]-BiVO4 in 1 M KBi with the presence or absence of Na2SO3. The ratios of ηsep and ηinj for CoBi/[010]BiVO4 and CoBi/[121]-BiVO4 are likewise plotted against the bias potentials (Figure 5b, Figure S17). CoBi/[010]-BiVO4 and CoBi/[121]-BiVO4 both produced a higher jwater than the corresponding pristine photoanode respectively, which can be ascribed to the strengthened band bending and the accelerated water oxidation kinetics after CoBi decoration. Interestingly, the jwater of CoBi/[010]-BiVO4 is higher than that of CoBi/[121]-BiVO4 and the ratios of ηsep is also larger than that of ηinj, which is the same case of [010]-BiVO4 and [121]-BiVO4. Notably, the ratio of ηsep of the CoBi decorated photoanodes remains larger than 1.9 times. Given that the band bendings of two samples are similar, the improvement in ηsep of CoBi/[010]-BiVO4 as compared to


15


Page 16 of 22

CoBi/[121]-BiVO4 cannot attribute to the band bending difference. Therefore, the faster electron transport along the [010] orientation than along the [121] orientation is thought to be the dominated reason for the 1.9 times higher ηsep of the CoBi/[010]-BiVO4 than CoBi/[121]-BiVO4. Eventually, these results demonstrate a crystallographic-orientation dependence of charge separation for BiVO4 photoanode in the PEC process. In summary, by controlling the growth of the precursor film, we successfully fabricated BiVO4 photoanodes with predominant [010] and [121] crystallographic orientations. A much higher PEC performance was obtained for the [010] orientated BiVO4 photoanode as compared with the [121] orientated one. The enhanced performance in the [010] orientated BiVO4 originates from the higher charge separation efficiency. A further comparison of the PEC activities of CoBi decorated BiVO4 photoanodes with different orientation infers that the faster electron transport along [010] than [121] crystal axis contributes to the improvement in charge separation. It is thus demonstrated that the crystallographic orientation of the photoanode which exhibits a higher electron mobility is preferred to improve the PEC performance. These findings can be useful for the rational design of photoelectrodes with a high charge separation efficiency for higher PEC performance.

ASSOCIATED CONTENT Experimental details; additional photoelectrocatalytic data; composition and structural characterization by XRD, UV-Vis, SEM, MS plots, and calculations of film conductivities and electron mobilities. AUTHOR INFORMATION *Email: [email protected] Group website: http://canli.dicp.ac.cn


16

Page 17 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

*Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (No. 21573230 and 21761142018), the National Key R&D Program of China (No. 2017YFA0204804) and the Pioneer Initiative (B) Projection of CAS (No. XDB17030200).

REFERENCES (1)

Hisatomi, T.; Kubota, J.; Domen, K. Recent Advances in Semiconductors for

Photocatalytic and Photoelectrochemical Water Splitting. Chem. Soc. Rev. 2014, 43, 7520-7535. (2)

Sivula, K.; van de Krol, R. Semiconducting Materials for Photoelectrochemical Energy

Conversion. Nature Reviews Materials 2016, 1, 15010. (3)

Li, D.; Shi, J.; Li, C. Transition-Metal-Based Electrocatalysts as Cocatalysts for

Photoelectrochemical Water Splitting: A Mini Review. Small 2018, 14, 1704179. (4)

Bard, A. J.; Fox, M. A. Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and

Oxygen. Acc. Chem. Res. 1995, 28, 141-145. (5)

Ding, C.; Shi, J.; Wang, Z.; Li, C. Photoelectrocatalytic Water Splitting: Significance of

Cocatalysts, Electrolyte, and Interfaces. ACS Catal. 2017, 7, 675-688. (6)

Sheng, C.; Wei, L.; Yanfa, Y.; Thomas, H.; Ishiang, S.; Dunwei, W.; Zetian, M. Roadmap

on Solar Water Splitting: Current Status and Future Prospects. Nano Futures 2017, 1, 022001.


17


(7)

Page 18 of 22

Zhao, X.; Luo, W.; Feng, J.; Li, M.; Li, Z.; Yu, T.; Zou, Z. Quantitative Analysis and

Visualized Evidence for High Charge Separation Efficiency in a Solid-Liquid Bulk Heterojunction. Adv. Energy Mater. 2014, 4, 1301785. (8)

Zandi, O.; Schon, A. R.; Hajibabaei, H.; Hamann, T. W. Enhanced Charge Separation and

Collection in High-Performance Electrodeposited Hematite Films. Chem. Mater. 2016, 28, 765771. (9)

Zhang, K.; Ravishankar, S.; Ma, M.; Veerappan, G.; Bisquert, J.; Fabregat-Santiago, F.;

Park, J. H. Overcoming Charge Collection Limitation at Solid/Liquid Interface by a Controllable Crystal Deficient Overlayer. Adv. Energy Mater. 2017, 7, 1600923. (10)

Li, A.; Wang, Z.; Yin, H.; Wang, S.; Yan, P.; Huang, B.; Wang, X.; Li, R.; Zong, X.; Han,

H., et al. Understanding the Anatase-Rutile Phase Junction in Charge Separation and Transfer in a TiO2 Electrode for Photoelectrochemical Water Splitting. Chem. Sci. 2016, 7, 6076-6082. (11)

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, 10991105. (12)

Abdi, F. F.; Han, L.; Smets, A. H.; 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. (13)

Steier, L.; Herraiz‐Cardona, I.; Gimenez, S.; Fabregat‐Santiago, F.; Bisquert, J.; Tilley,

S. D.; Grätzel, M. Understanding the Role of Underlayers and Overlayers in Thin Film Hematite Photoanodes. Adv. Funct. Mater. 2014, 24, 7681-7688.


18

Page 19 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

(14)

Wang, Z.; Mao, X.; Chen, P.; Xiao, M.; Monny, S. A.; Wang, S.; Konarova, M.; Du, A.;

Wang, L. Understanding the Roles of Oxygen Vacancies in Hematite-Based Photoelectrochemical Processes. Angew. Chem. Int. Ed. 2019, 58, 1030-1034. (15)

Kalanur, S. S.; Yoo, I.-H.; Eom, K.; Seo, H. Enhancement of Photoelectrochemical Water

Splitting Response of WO3 by Means of Bi Doping. J. Catal. 2018, 357, 127-137. (16)

Zhao, J.; Olide, E.; Osterloh, F. E. Enhancing Majority Carrier Transport in WO3 Water

Oxidation Photoanode Via Electrochemical Doping. J. Electrochem. Soc. 2015, 162, H65-H71. (17)

Mashiko, H.; Yoshimatsu, K.; Oshima, T.; Ohtomo, A. Fabrication and Characterization

of Semiconductor Photoelectrodes with Orientation-Controlled α-Fe2O3 Thin Films. J. Phys. Chem. C 2016, 120, 2747-2752. (18)

Han, H. S.; Shin, S.; Kim, D. H.; Park, I. J.; Kim, J. S.; Huang, P. S.; Lee, J. K.; Cho, I. S.;

Zheng, X. L. Boosting the Solar Water Oxidation Performance of a BiVO4 Photoanode by Crystallographic Orientation Control. Energy Environ. Sci. 2018, 11, 1299-1306. (19)

Song, J.; Seo, M. J.; Lee, T. H.; Jo, Y.-R.; Lee, J.; Kim, T. L.; Kim, S.-Y.; Kim, S.-M.;

Jeong, S. Y.; An, H., et al. Tailoring Crystallographic Orientations to Substantially Enhance Charge Separation Efficiency in Anisotropic BiVO4 Photoanodes. ACS Catal. 2018, 8, 5952-5962. (20)

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. (21)

Liu, T.; Zhou, X.; Dupuis, M.; Li, C. The Nature of Photogenerated Charge Separation

among Different Crystal Facets of BiVO4 Studied by Density Functional Theory. Phys. Chem. Chem. Phys. 2015, 17, 23503-23510.


19


(22)

Page 20 of 22

Li, R.; Zhang, F.; Wang, D.; Yang, J.; Li, M.; Zhu, J.; Zhou, X.; Han, H.; Li, C. Spatial

Separation of Photogenerated Electrons and Holes among {010} and {110} Crystal Facets of BiVO4. Nat. Commun. 2013, 4, 1432. (23)

Zhao, Y.; Li, R.; Mu, L.; Li, C. Significance of Crystal Morphology Controlling in

Semiconductor-Based Photocatalysis: A Case Study on BiVO4 Photocatalyst. Cryst. Growth Des. 2017, 17, 2923-2928. (24)

Kim, C. W.; Son, Y. S.; Kang, M. J.; Kim, D. Y.; Kang, Y. S. (040)-Crystal Facet

Engineering of BiVO4 Plate Photoanodes for Solar Fuel Production. Adv. Energy Mater. 2016, 6, 1501754. (25)

Tan, H. L.; Wen, X.; Amal, R.; Ng, Y. H. BiVO4 {010} and {110} Relative Exposure

Extent: Governing Factor of Surface Charge Population and Photocatalytic Activity. J. Phys. Chem. Lett. 2016, 7, 1400-1405. (26)

Ren, X.; Dang, W.; Ma, Q.; Zhu, X.; Zi, W.; Jia, L.; Liu, B.; Zhang, X.; Xiao, F.; Yang,

H., et al. Superior Texture-Controlled ZnO Thin Film Using Electrochemical Deposition. Sol. Energy 2016, 125, 192-197. (27)

Chen, Z.; Jaramillo, T. F.; Deutsch, T. G.; Kleiman-Shwarsctein, A.; Forman, A. J.;

Gaillard, N.; Garland, R.; Takanabe, K.; Heske, C.; Sunkara, M., et al. Accelerating Materials Development for Photoelectrochemical Hydrogen Production: Standards for Methods, Definitions, and Reporting Protocols. J. Mater. Res. 2011, 25, 3-16. (28)

Upul Wijayantha, K. G.; Saremi-Yarahmadi, S.; Peter, L. M. Kinetics of Oxygen Evolution

at α-Fe2O3 Photoanodes: A Study by Photoelectrochemical Impedance Spectroscopy. Phys. Chem. Chem. Phys. 2011, 13, 5264-5270.


20

Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

(29)

Barnes, P. R. F.; Miettunen, K.; Li, X.; Anderson, A. Y.; Bessho, T.; Gratzel, M.; O'Regan,

B. C. Interpretation of Optoelectronic Transient and Charge Extraction Measurements in DyeSensitized Solar Cells. Adv. Mater. 2013, 25, 1881-1922. (30)

Gerischer, H. The Impact of Semiconductors on the Concepts of Electrochemistry.

Electrochim. Acta 1990, 35, 1677-1699. (31)

Gärtner, W. W. Depletion-Layer Photoeffects in Semiconductors. Phys. Rev. 1959, 116,

84-87. (32)

Linsebigler, A. L.; Lu, G.; Yates Jr, J. T. Photocatalysis on TiO2 Surfaces: Principles,

Mechanisms, and Selected Results. Chem. Rev. 1995, 95, 735-758. (33)

Zhang, L.; Ye, X.; Boloor, M.; Poletayev, A.; Melosh, N. A.; Chueh, W. C. Significantly

Enhanced Photocurrent for Water Oxidation in Monolithic Mo:BiVO4/SnO2/Si by Thermally Increasing the Minority Carrier Diffusion Length. Energy Environ. Sci. 2016, 9, 2044-2052. (34)

Ma, Y.; Pendlebury, S. R.; Reynal, A.; Le Formal, F.; Durrant, J. R. Dynamics of

Photogenerated Holes in Undoped BiVO4 Photoanodes for Solar Water Oxidation. Chem. Sci. 2014, 5, 2964-2973. (35)

Zhu, J.; Fan, F.; Chen, R.; An, H.; Feng, Z.; Li, C. Direct Imaging of Highly Anisotropic

Photogenerated Charge Separations on Different Facets of a Single BiVO4 Photocatalyst. Angew. Chem. Int. Ed. 2015, 54, 9111-9114. (36)

Du, C.; Yang, X.; Mayer, M. T.; Hoyt, H.; Xie, J.; McMahon, G.; Bischoping, G.; Wang,

D. Hematite-Based Water Splitting with Low Turn-on Voltages. Angew. Chem. Int. Ed. 2013, 52, 12692-12695.


21


(37)

Page 22 of 22

Li, W.; Yang, K. R.; Yao, X.; He, Y.; Dong, Q.; Brudvig, G. W.; Batista, V. S.; Wang, D.

Facet-Dependent Kinetics and Energetics of Hematite for Solar Water Oxidation Reactions. ACS Appl. Mater. Interfaces 2019, 11, 5616-5622. (38)

Tomkiewicz, M. The Potential Distribution at the TiO2 Aqueous Electrolyte Interface. J.

Electrochem. Soc. 1979, 126, 1505-1510. (39)

Yao, T.; Chen, R.; Li, J.; Han, J.; Qin, W.; Wang, H.; Shi, J.; Fan, F.; Li, C. Manipulating

the Interfacial Energetics of N-Type Silicon Photoanode for Efficient Water Oxidation. J. Am. Chem. Soc. 2016, 138, 13664-13672. (40)

Hajibabaei, H.; Schon, A. R.; Hamann, T. W. Interface Control of Photoelectrochemical

Water Oxidation Performance with Ni1-xFexOy Modified Hematite Photoanodes. Chem. Mater. 2017, 29, 6674-6683.


22

Crystallographic-Orientation-Dependent Charge Separation of BiVO4

Recommend Documents