BiFeO3 Porous Photoanode

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Triple Layer Heterojunction WO3/BiVO4/BiFeO3 Porous Photoanode for Efficient Photoelectrochemical Water Splitting Sadaf Khoomortezaei, Hossein Abdizadeh, and Mohammad Reza Golobostanfard ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b01041 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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ACS Applied Energy Materials

Triple Layer Heterojunction WO3/BiVO4/BiFeO3 Porous Photoanode for Efficient Photoelectrochemical Water Splitting Sadaf Khoomortezaei1, Hossein Abdizadeh*, 1, 2, Mohammad Reza Golobostanfard*, 1

1 School

of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box: 14395-553,Tehran, Iran

2

Center of Excellence for High Performance Materials, University of Tehran, Tehran, 14395-553, Iran

1 ACS Paragon Plus Environment

ACS Applied Energy Materials 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

Abstract

Storing sun light into chemical bonds for further use as a photoelectrochemical (PEC) water splitting is one of the most promising methods to fulfill human interminable energy desire. In this research, triple heterojunction WO3/BiVO4 (BVO)/BiFeO3 (BFO) porous photoanode is synthesized through a sol-gel method and its PEC performance is compared to those of WO3/BVO as well as pristine films. The triple layer heterojunction photoanode is comprised of three interdiffused layers of each component with interconnected pores, thickness of 340 nm, and absorption edge of slightly redshifted compared to pristine layers. Although WO3/BVO junction demonstrates enhanced PEC performance, introduction of BFO on WO3/BVO dramatically improves the overpotential and charge transfer resistance. Not only the appropriate band alignment between WO3 2 ACS Paragon Plus Environment

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and BVO, but also the self-polarization behavior as well as p-n junction formation by BFO top layer entirely improves the PEC performance of triple layer heterojunction photoanode. This photoanode also shows promising photostability and electron lifetime thanks to its enhanced charge separation compared to WO3/BVO and pristine components.

KEYWORDS: PEC water splitting, WO3, BiVO4, BiFeO3, Triple layer heterojunction, Solgel method, Self-polarization.

1. Introduction



Photoelectrochemical (PEC) water splitting has been considered as one of the most efficient methods to produce hydrogen fuel in a green and sustainable system.1 In response to the environmental crisis and rising global energy demand, employing interminable and renewable power sources has been progressed as a crucial approach. Thus, converting solar energy to hydrogen has gained overwhelming attention during the last two decades due to its clean, renewable, carbon-free, easily portable, and high energy density properties.1-3 Fujishima and Honda used TiO2 as photoanode for PEC water splitting under UV-light irradiation for the first time.4 Since then, metal oxide semiconductor materials have been identified as superior candidates for photoanode in this process, based on their appropriate PEC properties and appreciable stability.5 In general, photoelectrode properties including proper band gap to absorb visible light, efficient charge carrier separation to prevent recombination, adequate carrier lifetime, and thermodynamically proper valence band edge position that provides sufficient overpotential, have significant influence on PEC water splitting performance.


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Until now, highly efficient photoelectrode materials such as TiO2,6 ZnO,7 Fe2O3,8 BiVO4 (BVO),9 BiFeO3 (BFO),10 WO3,11,

12

CuWO4,13 Bi2WO6,14 and CZTSSe15 have been

intensively explored. In recent years, bismuth vanadate (BiVO4), an n-type semiconductor with monoclinic crystalline structure has been emerged as one of the most promising photoanodes for PEC water splitting application among numerous metal oxide semiconductors. It is known for its narrow band gap of 2.4 eV that permits superior capturing of visible light irradiation, suitable conduction band (CB) edge position which is very close to the thermodynamic H2 evolution potential (~0 eV vs. RHE), outstanding stability against photocorrosion, and cost effective fabrication. Nevertheless, BVO possesses a poor charge transfer properties which is mainly ascribed to immoderate photo-generated carrier recombination rate. Additionally, slow transfer of holes at the BVO/electrolyte interface leads to a slow oxygen evolution reaction (OER) kinetics which results in large overpotential and can be considered as another performance limiting factor.16, 17 Up until today, the reported photocurrent density of BVO photoanodes reaches up to 1.0 mA/cm2 at 1.23 V (vs. RHE) under AM 1.5G illumination which is far below its 5 ACS Paragon Plus Environment


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theoretical maximum value of 7.5 mA.cm-2.18 Hence, the overall PEC efficiency of individual BVO photocatalysts will be limited. Consequently, in order to take full advantage of each material and overcome their infirmities and drawbacks, a heterojunction electrode, which contains two or more various semiconductors such as TiO2/BVO,19

Co3O4/BVO,20

BVO/ZnO,21

BVO/WO3/SnO2,22

WO3/FeOOH23

and

CuWO4/WO324 are commonly used to address those aforementioned issues. In this context, WO3/BVO has been extensively investigated as a photoactive electrode for solar water splitting.25-32 For instance, it has been reported that the WO3/BVO bilayer films produced a photocurrent density of up to 3.3 mA.cm-2 under AM1.5 illumination at 100 mW.cm-2.33 Monoclinic WO3 is regarded as a favorable material due to its desirable absorbance of the visible portion of the solar spectrum (Eg = 2.5-2.8 eV), photocorrosion resistant, and a moderate carrier diffusion length of around 150 nm compared to BVO (∼ 75 nm), Fe2O3 (∼ 2–4 nm), and TiO2 (∼ 100 nm) which provide great electron transport properties for WO3.34, 35 Accordingly, as for their merits inclusive of good charge transport properties of


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WO3 and good optical absorption properties of BVO, an interactive improvement can be anticipated when BVO and WO3 are combined. BFO is another material that has been implemented extensively in water splitting application as one of the new strategies to improve photocatalytic activity. Over the past few years, perovskite bismuth ferrite, with an excellent multiferroic properties, has received immense attention and has been studied widely in the photocatalytic field. BFO is characterized by a rhombohedrally distorted perovskite structure with the space group of R3c. It shows a unique combination of ferroelectric (Curie temperature: Tc ∼ 1100 K) and antiferromagnetic (N´eel temperature: TN ∼ 643 K) properties simultaneously.36, 37 The coexistence of such features in the BFO structure makes it appealing for applications in multifunctional sensors, optoelectronic devices, resistivity memory magnetic recording media, light-emitting diodes, ferroelectric solar cells, and spintronic devices. In addition, BFO has a good visible light response (Eg ~2.2 eV), chemical stability, and intrinsic electric self-polarization field (~90 μC.cm-2) which can enhance charge-carrier drift and separation.36 It has been reported that BVO photoanode passivated with BFO by chemical solution deposition method can efficiently increase photocurrent by about 4.4 times than 7 ACS Paragon Plus Environment


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the bare BVO photoanode.38 Another work has revealed that depositing an ultrathin BFO layer onto BVO semiconductor is profitable to enhance the photovoltaic effect and promoted photocurrent density from 60 to 140 μA.cm-2.39 In this research, a triple photoanode has been designed and fabricated with the n-type WO3 and BVO and p-type BFO semiconductors combined together to dramatically proliferate the solar water splitting capability. The band edges position of these three semiconductors are located in a very suitable configuration which extensively promote the charge separation and hence carrier lifetime. Moreover, since the WO3 can grow in porous structure, it can simply dictate its favorable structure to next layers and widely generates hierarchical porosity and surface area. Furthermore, formation of p-n junction at

the

BVO/BFO

can

enormously

enhance

the

charge

separation

and

photoelectrochemical water splitting performance. Therefore, since the solar water splitting based on triple WO3/BVO/BFO photoanode has never been reported and this electrode has a great potential for performance enhancement, the research on this triple layer heterojunction electrode should be crucially investigated. 2. Experimental Methods 8 ACS Paragon Plus Environment

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2.1. Materials The following chemicals were utilized in this work: sodium tungstate dehydrate (Na2WO4.2H2O, >99% purity Merck Co.), iron(III) nitrate nonahydrate (Fe(NO3)3.9H2O >99% purity, Merck Co.), bismuth nitrate pentahydrate (Bi(NO3)3.5H2O >99% purity, SigmaAldrich Co.), ammonium metavanadate (NH4VO3, >99% purity Merck Co.), hydrogen peroxide (H2O2 30%, >99% purity Merck Co.), nitric acid (HNO3 65%, >99% purity Merck Co.), ethanol (C2H5OH, >99% purity Merck Co.), polyethylene glycol (PEG, MW: 20000, >99% purity Merck Co.), glacial acetic acid (CH3COOH, >99% purity Merck Co.), citric acid (C6H8O7, >99% purity Merck Co.), polyvinyl alcohol (PVA, >99% purity Merck Co.), 2-methoxyethanol (C3H8O2, >99% purity Merck Co.), and diethanolamine (C4H11NO2, >99% purity Merck Co.). All materials were used as received without any purification. Deionized water (18.2 MΩ) was used in all experiments. Fluorine doped tin oxide transparent conductive glasses (15 Ω/square, Dyesol) were use as substrate.

2.2. Sol preparation and deposition of individual WO3, BVO, and BFO thin films Tungsten oxide thin film was fabricated using a sol-gel method.40 Briefly, 1 g of Na2WO4.2H2O dissolved in 10 mL DI water and 6 ml of HNO3 was added to the solution to acquire a yellow precipitation of H2WO4. After precipitants were washed several times with DI water, the tungstic acid was dissolved in 2 mL H2O2 and stirred for 2 h. In order to improve the adhesion and homogeneity of the film, 2 g of PEG was added to the mixture as a surfactant. At the end, 30 mL ethanol was added and the sol was stirred for another 2 h. After 24 h aging, the as obtained sol was deposited on the FTO substrates, using spin-coating at 3000 rpm for 30 s and dried at 100 °C for 10 min. Prior to deposition, the FTO glass was cleaned by 10 min sonication in a soap solution,



acetone, and finally isopropyl alcohol. The deposition process was repeated for 10 times. After coating, the films were annealed at 400 °C for 1 h. For preparing BVO film as reported previously,41 0.004 mol of citric acid was dissolved in 6 mL of diluted HNO3 (23.3 vol.%) and then, 0.002 mol of Bi(NO3)3.5H2O and NH4VO3 were added to the solution. After dissolution of the precursors, 0.04 g of polyvinyl alcohol and 0.25 mL of acetic acid was added to the above solution. A BVO layer was prepared by spin coating at 3000 rpm for 30 s and then the film was dried at 100 °C for 10 min. The deposition and drying processes were repeated 10 times and finally the films were annealed at 500 °C for 1 h. The procedure for BFO film preparation is based on previous report with some modification.42 To obtain 0.1 M sol of precursors, bismuth nitrate and iron nitrate as raw materials were dissolved separately in 10 ml of glacial acetic acid and 2-methoxyethanol, respectively (Bi:Fe = 1:1). Transparent bismuth nitrate sol was obtained by adding 0.1 mL of diethanolamine to the solution. Very smooth and dense surface can be also achieved by diethanolamine addition. Finally, the above solutions were mixed and stirred for 2 h. The depositions were carried out by spin-coating at 3000 rpm for 30 s. Each layer was dried at 100 °C for 10 min to remove volatile materials. The above spin-coating and drying procedures were repeated for 10 times. Finally, the thin films were calcined at 600 °C for 1 h.

2.3. Preparation of WO3/BVO and WO3/BVO/BFO heterojunction photoanodes


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The triple layer heterojunction films were deposited by first applying the WO3 layer from the WO3 prepared sol. Then, the layer was calcined at 400 °C for 1 h. Next, the BVO layer was deposited on WO3 layer from the BVO prepared sol and calcined at 500 °C for 1 h. Finally, the BFO layer was spin coated on WO3/BVO layer and calcined at 600 °C for another 1 h. Each layer was deposited for 10 cycles. The schematic of this procedure is shown in Figure 1. For investigating the layer thickness, the deposition cycles of 5 and 15 for each layer was also considered.

Figure 1. Schematic of the preparation procedure of heterojunction photoanodes.

2.4. Characterization The crystalline phases of obtained powders were identified by X-ray diffraction (XRD, Philips X-pert pro, PW1730), with Cu Kα radiation (k = 1.5406 Å, 40 kV, 30 mA). UV-vis absorption spectra were examined to characterize the optical behavior properties of the as-prepared samples by PG instrument T80+. Morphology of the nanoparticles was observed with the field emission scanning electron microscope (FESEM, ZEISS-SIGMAVP) equipped with energy-



dispersive X-ray (EDS) microanalysis at an accelerating voltage of 20 kV. The optical properties of the samples were studied using PL method (PL, Varian Carry Elipson) with an exciting wavelength of 300 nm.

2.5. Photoelectrochemical measurements Photoelectrochemical characterizations were operated in a conventional three-electrode galvanostat/potentiostat system (Autolab PGSTAT302). As-prepared samples as the working electrodes, a platinum wire as counter electrode, and Ag/AgCl as reference electrode were used in an aqueous solution of Na2SO4 (0.5 mol.L-1) as the electrolyte. Xenon short arc lamp (OSRAM, HBM/OFR) with natural daylight continuous spectrum was used as the irradiation source. In order to accomplish the measurements at room temperature and in a visible region of solar spectrum, a glassy IR filter was placed in front of the lamp. In addition, by changing the input power, the intensity of lamp was adjusted at 1000 W.m-2 in the visible range. Electrical properties of the prepared samples have been studied using impedance spectroscopy with the same Autolab instrument and electrolyte. 3. Results and Discussion 3.1. WO3/BVO and WO3/BVO/BFO photoanodes To recognize the purity and crystalline structure of as synthesized samples, the XRD analysis was conducted for the WO3, BVO, BFO, WO3/BVO, and WO3/BVO/BFO photoanodes. Figure 2 shows the XRD results for pure WO3, BVO, and BFO photoanodes


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and WO3/BVO and WO3/BVO/BFO heterojunctions. As shown in Figure 2a, three characteristic peaks in WO3 XRD pattern at 23.2°, 23.6°, and 24.2° associated with (002), (020), and (200) crystal plane, respectively reveal the monoclinic crystal structure of WO3 (JCPDS No. 19-3532).43 As the monoclinic phase of WO3 is the most common and stable phase of tungsten oxide at room temperature which provides exceptional photocatalytic activity compared with the orthorhombic phase, the as-prepared photoanode expected to be advantageous.35, 44 BVO layer comprised monoclinic crystal structure (Figure 2a), based on the representative peaks at around 29.0° and 30.5° which could be indexed to (121) and (040) planes, respectively (JCPDS No. 75-2481).45 Since only the monoclinic sheelite structure of BVO yields greater PEC activity for oxygen evolution than that of the zircon or tetragonal scheelite BVO crystal structures,46 this electrode is hoped to exhibit excellent photocatalytic activity for water oxidation under visible light irradiation. As evidenced by the XRD pattern in Figure 2a, pure-phase of BFO is formed in the rhombohedral R3c phase (JCPDS No. 86-1518).10 The absence of any impurity peaks, suggesting the formation of secondary phases can be well controlled with mentioned heat treatment condition. According to Figure 2b, the diffraction peaks of the WO3/BVO heterojunction, corresponding to WO3 and BVO patterns, indicate successful formation of each layer with retaining crystal phases. Moreover, it is obvious that the XRD pattern of WO3/BVO/BFO triple layer heterojunction 13 ACS Paragon Plus Environment


exhibits characteristic peaks assigned to monoclinic WO3, monoclinic BVO, and rhombohedral BFO. The fitting of the XRD pattern of as-obtained WO3/BVO/BFO with those of WO3, BVO, and BFO confirms that the crystalline structure of each component is not affected during the preparation process and consecutive calcination.

Figure 2. XRD patterns of (a) bare WO3, BVO, and BFO photoanodes and (b) WO3/BVO and WO3/BVO/BFO heterojunctions.

The morphology of as fabricated samples was characterized by FE-SEM. Figure 3 shows the top-view and the corresponding cross sectional FE-SEM images of the WO3,


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BVO, BFO, WO3/BVO and WO3/BVO/BFO photoanodes. Figure 3a exhibits a uniform growth of WO3 thin film over the entire surface with full coverage on the FTO substrate. The pure WO3 photoanode shows a dense planar surface, which can be due to presence of PEG with long alcoxy-chain in precursor solution used as stabilizing agent. PEG can improve adherence and homogeneity of the final WO3 transparent film. Figures 3b and 3c show uniform microstructure of BVO and BFO on FTO surface, respectively. After deposition of BVO on WO3, a worm like morphology is formed as shown in Figure 3d. The as-obtained structure has been distributed uniformly through the entire porous thin film with pore size in the range of 60–180 nm. The morphology is possibly attributed to solvents evaporation and by-products decomposition during the calcination process.47 Thickness of the WO3 thin film is around 300 nm after 10 deposition cycles as displayed in Figure 3f. The thickness of the bare BVO and BFO layer on FTO substrate, formed with 10 cycles of spin coating is about 102 and 105 nm, respectively as shown in Figure 3g and 3h. The thickness of the WO3/BVO heterojunction photoanode increases to around 346 nm by BVO deposition (Figure 3i). The cross-sectional FE-SEM image confirms the fully coverage of BVO layers over the surface of the underlying WO3 layer. After 15 ACS Paragon Plus Environment


deposition of a thin layer of BFO on WO3/BVO heterojunction, a porous structure with high uniformity and an average pore diameter in the range of 100-220 nm is obtained (Figure 3j). It is absolutely clear that larger and more irregular pores are developed in triple layer heterojunction photoanode structure compared to WO3/BVO bilayer photoanode. According to Figure 3j, the thickness of the WO3/BVO/BFO triple layer heterojunction thin film is estimated to about 340 nm. It discloses that the porous structure dispenses uniformly across the thin film and the layers are in contact with each other without any significant cracks. It is suggested that the decrement of thickness after BFO deposition is likely caused by successive calcination and interdiffusion of BFO and BVO layers into the bottom layer. Figure 4 shows the EDS elemental mapping graphs of tin, tungsten, bismuth, vanadium, iron, and oxygen entities in cross sectional WO3/BVO and WO3/BVO/BFO heterojunctions. It is worth noting that although the distinct separation at the BVO/BFO interface cannot be detected due to very thin thickness of BFO, EDS elemental mapping of WO3/BVO/BFO cross sectional thin film can confirm the existence of bismuth, iron, and oxygen in the surface of the as-prepared photoelectrode. Tungsten and vanadium 16 ACS Paragon Plus Environment

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distributions also support the formation of WO3 and BVO layers at the bottom, though with high interdiffusion of the layers. The EDS line scan of the multilayer films is also shown in Figure S1 (Supporting information).




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Figure 3. Top view FE-SEM images of bare (a) WO3, (b) BVO, (c) BFO, (d) WO3/BVO, and (e) WO3/BVO/BFO photoanodes and cross sectional FE-SEM images of (f) WO3, (g) BVO, (h) BFO, (i) WO3/BVO, and (j) WO3/BVO/BFO photoanodes.

Figure 4. Cross section FESEM image and corresponding elemental mapping graphs of (a) WO3/BVO and (b) WO3/BVO/BFO photoanodes.



In order to determine the band gap energy and investigate the light absorption properties of the as obtained samples, the UV–Vis spectroscopy was performed. Figure 5a displays the absorbance curves of the samples. As shown in Figure 5a, WO3 shows strong light absorption from 430 nm, which exactly match to the band edge of monoclinic structure of WO3. It is clearly seen that the onset light absorbance of the as-synthesized WO3/BVO expands to around 590 nm, which is consistent with other reports showing the light absorption range of WO3 is greatly expanded when coupled with BVO.41, 48 This is due to the fact that BVO has a much broader absorption range with the absorption edge centered at about 530 nm. Although the double layer WO3/BVO sample shows higher absorption than triple layer WO3/BVO/BFO sample, the optical absorption edge of the WO3/BVO/BFO triple layer heterojunction demonstrates a slight redshift to a longer wavelength compared to that of WO3/BVO, which ensures appropriate and efficient visible light photoactivity of the photoanode. The higher absorption of double layer WO3/BVO sample is mainly due to the scattering effect in this sample caused by porous structure, which is slightly reduced in triple layer sample by introduction of BFO (confirmed by Figure 3). Furthermore, WO3/BVO/BFO shows the absorption characteristics of WO3, BVO, and 20 ACS Paragon Plus Environment

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BFO simultaneously. Figure 5b exhibits the photographs of the as-prepared samples. One observed evidence is the film color turning from transparent pale to yellow green and finally yellow with introduction of BVO and BFO on WO3, respectively. As depicted in Figure S2a and S2b (Supporting information), the band gap energies (Eg) are estimated by utilizing Tauc plot, assuming an indirect band gap for WO3 and direct band gaps for BVO and BFO.49 The values of Eg for pure WO3, BVO, and BFO are approximately 3.00, 2.42, and 2.14 eV, respectively which is in accordance with the previously reported band gap value for the synthesized crystal structure.50-52



Figure 5. (a) UV-Vis absorption spectra of WO3, BVO, BFO, WO3/BVO, and WO3/BVO/BFO photoanodes, (b) photograph images of the samples.

3.2. Photoelectrochemical properties of photoanodes 3.2.1 Linear sweep voltammetry Figure 6 shows the comparison of linear sweep curves of WO3, BVO, BFO, WO3/BVO, and WO3/BVO/BFO photoanodes. A pronounced enhancement in the photocurrent with increasing applied potential is evident (Figure 6a). As shown in Figure 6c and 6d, both of the WO3/BVO and WO3/BVO/BFO photoanodes exhibit an almost negligible photocurrent density in dark condition, even under an applied potential of 1.23 V vs. RHE. Table 1 represents the overpotential and maximum photocurrent density of each photoanode. The potential at the intersection point of potential-axis and the tangent at maximum slope of photocurrent is considered as overpotential.38 The WO3/BVO heterojunction photoanode presents much higher solar water splitting performance with a maximum photocurrent density of 6.3 mA.cm−2 at 2.5 V vs. RHE in contrast to that of the individual component. From the Figure 6a, it can be confirmed that the presence of a WO3 layer make a


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significant improvement in BVO film performance, which is quite comparable with other articles reported values.30, 53 This sharply increase in photoresponse indicates that the WO3/BVO heterojunction effectively inhibits the recombination of electron/hole pairs generated in the WO3 and BVO films. The energy level of WO3 and BVO and charge transfer mechanism in WO3/BVO photoanode is illustrated in Figure 7.22, 38 The type II band alignment at the interface of WO3/BVO heterojunction facilitates the injection of CB electrons from BVO to WO3 and holes migration in the opposite direction from VB of WO3 to the VB of BVO. As a result, the electrons which were subsequently accumulated in the substrate, migrate to the photocathode (Pt) through the external circuit to participate in the hydrogen evolution reaction (HER). At the same time, the photogenerated holes in the photoelectrode and electrolyte interface lead to oxygen evolution reaction (OER) and completion of the water splitting process. This charge movement improves the separation of photogenerated carriers. Notably, the onset potential of water oxidation for WO3/BVO photoanode is 0.7 V vs. RHE, which is lower than those of 0.8 and 1.4 V vs. RHE for pristine WO3 and BVO photoanodes, respectively. Ultimately this favorable band



alignment can decrease the applied potential for water oxidation and enhance the photocurrent density simultaneously. It is found that the photocurrent is further increased by the applying of multiferroic BFO top layer at high enough voltages. The highest photocurrent density of 46.9 mA.cm−2 at 2.53 V vs. RHE is generated by the triple layer heterojunction photoanode. However, the photocurrent of WO3/BVO double layer sample is slightly higher than that of WO3/BVO/BFO sample at 1.23 V vs. RHE. The dark mode of LSV plot ranged from 0 to 2.2 V vs. RHE exhibits an almost negligible photocurrent density (Figure 6d). The higher photocurrent of WO3/BVO sample compared to that of WO3/BVO/BFO at 1.23 V vs. RHE can be due to the decreased porosity after BFO deposition. By comparison of Figure 3d and 3e, it can be said that the decreased porosity in triple layer heterojunction slightly reduces the surface area, which affects the reaction between electrolyte and photoanode. Furthermore, light absorbance of double layer heterojunction is higher than that of triple layer heterojunction because of scattering and light trapping which are come from morphology variation confirmed by Figure 5.


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However, the triple layer WO3/BVO/BFO represents far higher photocurrent at higher voltages, which can be described by p-n junction formation and self-polarization field of BFO. Figure 7 shows the band potential of semiconductors in triple layer heterojunction photoanode. According to the energy band theory, the Fermi level of p-type semiconductor locates near to its VB, whereas the Fermi level of n-type semiconductor locates near to its CB and the photogenerated electrons prefer to transfer from the photoelectrode with higher Fermi level to the lower one. The diffusion process of electron and hole carriers to the p-type BFO and n-type BVO semiconductors continues until an equilibrium state is attained and the Fermi levels of BVO and BFO align together. Consequently, coupling of p-type BFO and n-type BVO form a space charge region in the interface as a p-n junction. As a result, charge transfer forms internal built-in electric field (Eint) directing from the n-type BVO toward the p-type BFO in the space charge region. The inner electric field built at the p-n junction interface acts as a potential barrier that suppress the electron/hole pairs recombination and accelerates charge carrier transmission.38,

54

Therefore, the

photoelectrons can easily inject from the CB of BFO to CB of BVO and reversely the photogenerated holes move from the VB of BVO to the VB of BFO. It is believed that the



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p-n junction plays a crucial role in performance improvement of the photoanode without consumption of oxygen evolution catalyst (OEC). The formation of type II heterojunction and favorable band alignment at BVO/BFO interface also can positively influence on photogenerated carrier migration. More importantly, the occurrence of self-polarization due to oxygen vacancies is quite usual in BFO thin films. The self-polarization of the ferroelectric BFO is along [111] crystal orientation resulted from the Bi3+ lone pair (6 s2 orbital).55,

56

The contribution of self-

polarization effect (Pself) of ferroelectric BFO thin layer in BVO/BFO p-n heterojunction can constructively improve the electron-hole pairs separation and prohibit interfacial back recombination. Under the driving of BFO self-polarization field, not only photogenerated carrier distribution in the semiconductors but also width of space charge region at the interface can be altered. According to the other reports, the self-polarization effect becomes more serious with the thickness decrement of BFO films.39 Thus, the nanolayer of BFO ( 100 nm) in the WO3/BVO/BFO photoanode could play a profound role in creating a high selfpolarization effect and hinder the charge recombination process. It has been reported that if the external electric field direction is assigned as self-polarization direction of BFO, it can be proposed that the width of the depletion region increases owning to the 26 ACS Paragon Plus Environment

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enhancement of ferroelectric polarization, which promotes carrier separation and enhances current density. Conversely, if the external electric field direction is assigned as reverse direction of BFO self-polarization, the width of the depletion region decreases, which fastens the carrier recombination and reduces the current density.38, 57, 58 Based on this argument, it seems that the positive bias voltage beyond 1.7 V vs. RHE (cross of two LSV curves) applied to triple layer heterojunction, lead to ferroelectric polarization enhancement of BFO on PEC performance and significantly increase photocurrent density. The difference between photocurrent of WO3/BVO and WO3/BVO/BFO samples in higher voltage under dark condition also can well confirm the self-polarization effect of BFO layer. Applying high bias voltage during LSV test can well align BFO random domains which contribute to the Eint, by which the photogenerated electron-hole pairs are separated and the charge recombination is prohibited and hence give rise to photocurrent increment.59 As a further confirmation of self-polarization effect of BFO layer the LSV curves of BVO/BFO and WO3/BFO photoanodes were also illustrated in Figure S3. The photocurrent density significantly increases in BVO/BFO and WO3/BFO samples, which 27 ACS Paragon Plus Environment


indicates that BFO can enhance PEC performance by influence of the built-in potential at p-n junction. Cathodic shift of the onset potential from 0.7 to 0.5 V vs. RHE can be also observed by deposition of the BFO layer on WO3/BVO photoelectrode, which can demonstrate the OER kinetics and catalytic activity improvement by p-n junction formation. In such beneficial condition, the holes accumulated in the BFO-electrolyte interface participate in OER to oxidize water into the oxygen and the electrons accumulated in n-type WO3 reach conductive FTO surface and transfer to the counter electrode (Pt electrode) to produce hydrogen through HER. Therefore, the applicable band alignment of WO3/BVO/BFO and forming the double-heterojunction with influence of the built-in potential at p-n junction and self-polarization effect of BFO cause amazing enhancement in photocatalytic activity of triple layer heterojunction photoanode. In order to investigate transfer and recombination processes of photogenerated electron hole pairs, PL emission spectroscopy is carried out. Figure 6f shows the comparison of PL spectra with 300 nm excitation wavelength for WO3, BVO, BFO, WO3/BVO, and WO3/BVO/BFO photoanodes at room temperature. The PL spectra of 28 ACS Paragon Plus Environment

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WO3, BVO, and BFO are observed at around 430, 540, and 570 nm, respectively. These results show good agreement with UV-Vis results analysis (Figure 5a). The WO3 represents very weak luminescence due to its indirect band gap, while the BVO and BFO bare films demonstrate strong emission. It can be clearly observed that WO3/BVO heterojunction exhibits diminished PL intensity in comparison to pure BVO and WO3, implying reduced photogenerated charge carrier recombination. The results clearly show that the charge carrier recombination is inhibited greatly by further deposition of thin layer of BFO, which reflects an enhancement of charge separation ability and the photocatalytic reaction enhancement accordingly. This can be ascribed to suitable band alignment of semiconductors which has been previously mentioned. Figure 6e displays the photocurrent-potential curves for WO3/BVO/BFO triple layer heterojunction for 5 (WVF5), 10 (WVF10), and 15 (WVF15) cycles of deposition for each layer to optimize coating cycles of WO3, BVO, and BFO films. It is absolutely obvious that decreasing and increasing the thickness of the layers reduces the photocurrent density of WO3/BVO/BFO photoanode. WVF5 and WVF15 produce a photocurrent of 9.25 and



16.19 mA.cm-2, respectively. This poor performance ralates to increased transport limitations in a thicker film and decreased absorbance in thinner film.

Figure 6. (a) Current - potential plots for WO3, BVO, BFO, WO3/BVO, and WO3/BVO/BFO photoanodes with scan rate of 100 mV.s-1 under AM 1.5 G irradiation in an aqueous solution of 0.5 mol.L-1 Na2SO4. (b) enlarged image for clearly determining the different curves


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ranged from 0 to 2.03 V vs. RHE. Current - potential plots under light illumination and dark mode for (c) WO3/BVO and (d) WO3/BVO/BFO photoanodes. (e) Current - potential plots for WO3/BVO/BFO photoanode with the coating cycles of 5 and 15. (f) PL emission spectra of WO3, BVO, BFO, WO3/BVO, and WO3/BVO/BFO photoanodes.

Figure 7. The schematic energy diagram and possible photogenerated electron-hole pathways of WO3, BVO, and BFO (a) before and (b) after contact with each other.

Table 1. Over potential and maximum photocurrent density of each photoanode under AM 1.5 G irradiation in an aqueous solution of 0.5 mol.L-1 Na2SO4



Photoanode

Over potential (V vs. RHE)

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Maximum photocurrent at 2.53 V vs. RHE (mA.cm-2)

WO3

0.8

6.5

BVO

1.4

4.6

BFO

1.6

17.5

WO3/BVO

0.7

6.3

WO3/BVO/BFO

0.5

46.9

3.2.2 Chronoamperometry Figure 8 shows the photocurrent generated with respect to time under chopped light illumination mode with light ON/OFF cycles for pure WO3, BVO, BFO, bilayer WO3/BVO, and triple layer WO3/BVO/BFO heterojunction. All of the photoanodes display excellent photoswitching performance against turning light ON/OFF with rapid response and recovery times. This good sensitivity to the irradiation confirms that they can be used as an effective visible-light responsive photoanode. As shown in Figure 8e, WO3/BVO/BFO triple layer heterojunction photoanode produces stable and reproducible photocurrent density of 760 µA.cm-2 over several ON/OFF cycles


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under visible-light illumination for 200 s. This value is extremely greater than that of WO3/BVO (1.1 µA.cm-2) in Figure 8d, which is consistent with the LSV results in Figure 6a. Figure 8e displays the stability of WO3/BVO bilayer and WO3/BVO/BFO triple layer heterojunction photoelectrodes. For WO3/BVO/BFO photoanode, only 42% attenuation of photocurrent density can be observed in the given time period (3000 s), indicating the long term photostability of triple layer heterojunction as photoanode for PEC activity. However, for WO3/BVO photoanode, photocurrent density reduces continuously during irradiation time and decays by almost 90% after 3000 s, which is 2.25 times greater than that of WO3/BVO/BFO. The fast performance decay demonstrates poor stability of WO3/BVO compared to WO3/BVO/BFO which may be attributed to the presence of the serious charge recombination and photocorrosion during OER. Long-term stability indicates that BFO layer can increase photocurrent density and enhance durability of photoelectrode due to its high chemical stability under illumination in aqueous environments.36 Furthermore, BFO deposition is a viable technique to protect BVO from photocorrosion under long-term operation and can be considered as a superior OEC. This 33 ACS Paragon Plus Environment


is apparent by different morphology of WO3/BVO and WO3/BVO/BFO photoanodes before and after PEC water splitting operation (Figure S4). WO3/BVO/BFO photoanode morphology can well retain after process in contrast with WO3/BVO, which confirms high chemical stability of BFO in Na2SO4 aqueous solution. It is noteworthy that the p-type behavior of BFO layer in Figure 6c, is clearly observed in enlarged image of chronoampereometric measurements performed on WO3/BVO/BFO photoanode (Figure S5). In BFO, small amounts of Fe2+ ions and oxygen vacancies exist. Incidentally, BFO shows p-type conductivity, which can be caused by the substitution of a small amount of Fe2+ ions in Fe3+ positions (acceptor doping of Fe3+ by Fe2+).60


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Figure 8. The chopped current–time curves of (a) WO3, (b) BVO, (c) BFO, (d) WO3/BVO, (e) WO3/BVO/BFO photoanodes at 1 V vs. RHE in an aqueous solution of 0.5 mol.L-1 Na2SO4, and (f) stability of WO3/BVO, and WO3/BVO/BFO photoanodes at 1.5 V vs. RHE in an aqueous solution of 0.5 mol.L-1 Na2SO4.

3.2.3 Lifetime of the photogenerated carriers 35 ACS Paragon Plus Environment


Slight decrease in the photocurrent density at the beginning of the illumination at each time interval for all samples is observable in Figure 9a. This decay has been employed to examine the charge recombination behavior of a semiconductor and photogenerated electron transport through layers. The photocurrent density decrement during light illumination to the photoanodes can be attributed to two competitive processes: photogeneration of electron-hole pairs and their recombination. The following kinetic equation controls the recombination process:

𝐷 = exp (

―𝑡

𝜏)

where D as a normalized parameter defined as: 𝐷 = (𝐼𝑡 ― 𝐼𝑓) (𝐼𝑖 ― 𝐼𝑓) here, It denotes the photocurrent at a time t, Ii is the initial photocurrent, and If is the final (steady-state) photocurrent, and τ is transient time constant.61 It is impossible to find out the recombination mechanisms from these observations but a possible way to describe the photocurrent change over time may be as following: photogenerated holes may accumulate on the photoanode surface and the process of recombination with photogenerated electrons occurs before reacting with electrolyte and 36 ACS Paragon Plus Environment

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participating in OER. Another possibility is presence of electron acceptors such as O2 in electrolyte solution that can prevent photogenerated electrons reach to the conductive substrate (FTO) and take part in HER.62 Hence, the lifetime achieved by this method is transport lifetime (the electron lifetime to reach the back contact). Figure 9a shows the normalized plots of lnD vs. t. These investigations reveal that the electron transport lifetime is about 1.6, 2.6, and 5.2 s for the pure WO3, BVO, and BFO photoanodes, respectively; while, this time is determined about 3.6 and 6.2 s for WO3/BVO and WO3/BVO/BFO photoanodes, respectively. These results elucidate that the improvement in photoelectrochemical performance and the significant enhancement in water splitting reaction on WO3/BVO and WO3/BVO/BFO photoanodes compared to their component, is owing to slower recombination process in heterojunction photoanodes. In addition, BFO deposition can develop charge separation of WO3/BVO photoanode by removing the surface state defects. Furthermore, the BFO may act as hole scavenger layer, which dramatically reduce the OER (the half reaction with lower kinetic that control the whole reaction), leading to high kinetic of HER. The reduction in recombination rate can be also explained by recombination centers at the grain boundaries. It has been well known that 37 ACS Paragon Plus Environment


charge separation is weakened by the existence of many grain boundaries among semiconductor particles as trapping center, which obstruct electron migration prior to reaching the FTO surface.63 Based on extra heat treatment applied on triple layer heterojunction sample, the grain boundaries may reduce in WO3/BVO/BFO compared to WO3/BVO film, which enhances the charge transfer. 3.2.4 Electrochemical impedance spectroscopy To evaluate the electrical properties and the kinetics of the charge transfer process at the photoelectrode/electrolyte interface of the as obtained photoanodes, electrochemical impedance spectroscopy (EIS) measurements were carried out. Figure 9b and 9c demonstrate the Nyquist diagrams of the WO3, BVO, BFO, WO3/BVO, and WO3/BVO/BFO photoanodes in the frequency range of 0.5 Hz - 100 kHz in 0.5 M Na2SO4 at open circuit potential under AM 1.5 G illumination and dark condition, respectively. The diameter of semicircular portion in the Nyquist plots is related to charge-transfer resistance (Rct) at the photoelectrode/electrolyte interface and the value of Rct is proportional to the charge recombination rate. The obtained Nyquist plot is simulated with the equivalent circuit shown in the inset of Figure 9b, where Rs indicates the sheet 38 ACS Paragon Plus Environment

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resistance of the charge collector (FTO), CPE indicates the constant phase element, and Rct indicates the charge transfer resistance at the electrode/electrolyte interface. Relative electronic parameters derived from EIS data of different samples were summarized in Table S1. As shown in Figure 9b, the semicircular radius lowers in the order of WO3>BVO>BFO>WO3/BVO>WO3/BVO/BFO under light illumination. The charge transfer resistance value of bare BVO is exceedingly reduced after decorated on WO3. It can be said that higher electrical conductivity of WO3 layer than that of BVO can possibly assist photogenerated electron transfer from the BVO layer to the charge collector. Obviously, the WO3/BVO/BFO triple layer heterojunction photoelectrode presents the lowest charge transfer resistance, implying that the WO3/BVO/BFO photoanode can decline the photoelectron and holes recombination and accelerate the electron mobility. The charge transfer resistance of the WO3/BVO/BFO EIS Nyquist plot is smaller than that of WO3/BVO, which means the higher mobility in WO3/BVO/BFO photoanode. Therefore, it can be said that BFO layer can ease the charge transfer and improve produced photocurrent. Also, electron-hole pairs recombination reduced owing to suitable band alignment



of semiconductors in triple heterojunction structure with internal electric field contribution of p-n junction. Comparison of Figure 9b and 9c clearly states the influence of light illuminating onto the PEC cell on impedance spectrum. Figure 9c illustrates that the charge-transfer resistance of the as-prepared samples increase strikingly with no light illumination. Consequently, very few charges can pass across the interface between the photoelectrode and electrolyte under dark. In other words, as synthesized photoanodes scarcely emit the dark currents. In fact, dark currents of the WO3/BVO and WO3/BVO/BFO heterojunctions are approximately zero as shown in Figures 6c and 6d, because of infinite Rct.


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Figure 9. (a) Normalized plots of the Ln D vs. time dependence for WO3, BVO, BFO, WO3/BVO, and WO3/BVO/BFO photoanodes, EIS of photoanodes in an aqueous solution of 0.5 mol.L-1 Na2SO4 at open circuit potential (b) under AM1.5 G illumination and (c) under dark.

Conclusions



A triple layer heterojunction WO3/BVO/BFO porous photoanode on FTO glass substrate is successfully deposited by using a sol-gel spin-coating technique. The triple heterojunction WO3/BVO/BFO photoanode shows impressive PEC performance (improve stability and onset potential) compared to the bilayer WO3/BVO photoanode as well as WO3, BVO, and BFO bare films. This remarkable phenomenon mainly comes from appropriate band alignment between WO3/BVO/BFO layers, which dramatically improves electron/hole pairs separation confirmed by PL spectra. Moreover, charge carrier separation promotes considerably in WO3/BVO/BFO photoanode due to the contribution of inner electric field built at the p-n junction interface of BVO/BFO and high selfpolarization effect of thin BFO layer. Furthermore, the resultant triple layer heterojunction can efficiently enhance electron transport lifetime to 6.2 s relative to bilayer heterojunction and pristine photoanodes. The charge recombination resistance of the BVO layer is entirely increased by applying this layer on WO3, while it is astonishingly promoted by introduction of BFO top layer. This concept of porous photoanode containing heterojunction of two appropriate semiconductors and a p-type ferroelectric top layer


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semiconductor exhibits dramatic improvement in PEC water splitting and can be prosperously applied in storing sunlight energy in chemical bonds. Acknowledgements

The authors thank Iran National Science Foundation for supporting this research under Grant No. 94002821. ASSOCIATED CONTENT

Supporting Informaion

Supporting information included: Figure S1-S5 and Table S1: cross sectional EDS line scan of WO3/BVO and WO3/BVO/BFO photoanodes; Calculated direct band gap extrapolation according to UV-Vis absorption spectra and Tauc function for BVO and BFO photoanodes, and calculated indirect band gap of WO3 photoanode; Current - potential plots for BVO/BFO and WO3/BFO photoanodes under AM 1.5 G irradiation in an aqueous solution of 0.5 mol. L-1 Na2SO4; FE-SEM image of WO3/BVO/BFO and WO3/BVO photoanodes after PEC water splitting stability test; Enlarged image for clearly determining the chopped current–time curves of WO3/BVO and WO3/BVO/BFO



photoanodes; Relative electronic parameters derived from EIS data by fitting with the equivalent circuit. AUTHOR INFORMATION

Corresponding Author

** E-mail address: [email protected] (H. Abdizadeh).

*E-mail address: [email protected] (M.R. Golobostanfard).

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Sadaf Khoomortezaei, Hossein Abdizadeh, and Mohammad Reza Golobostanfard contributed equally in this work.

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TOC graphic


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119x34mm (300 x 300 DPI)

ACS Paragon Plus Environment


80x95mm (300 x 300 DPI)


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Figure 3 119x204mm (300 x 300 DPI)



Figure 4 119x119mm (300 x 300 DPI)


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Figure 5 80x93mm (300 x 300 DPI)



Figure 6 160x148mm (300 x 300 DPI)


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Figure 9 80x139mm (300 x 300 DPI)




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