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SnO2 triple-layer photoanode with enhanced photoelectrochemical performance
Accepted Manuscript Solution-processed TiO2/BiVO4/SnO2 triple-layer photoanode with enhanced photoelectrochemical performance Sung Won Hwang, Jin Un Kim, Ji Hyun Baek, Shankara S. Kalanur, Hyun Suk Jung, Hyungtak Seo, In Sun Cho PII:
S0925-8388(19)30246-4
DOI:
https://doi.org/10.1016/j.jallcom.2019.01.251
Reference:
JALCOM 49289
To appear in:
Journal of Alloys and Compounds
Received Date: 11 September 2018 Revised Date:
16 January 2019
Accepted Date: 19 January 2019
Please cite this article as: S.W. Hwang, J.U. Kim, J.H. Baek, S.S. Kalanur, H.S. Jung, H. Seo, I.S. Cho, Solution-processed TiO2/BiVO4/SnO2 triple-layer photoanode with enhanced photoelectrochemical performance, Journal of Alloys and Compounds (2019), doi: https:// doi.org/10.1016/j.jallcom.2019.01.251. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Solution-processed TiO2/BiVO4/SnO2 Triple-layer Photoanode with Enhanced Photoelectrochemical Performance
Sung Won Hwang,a,b,1 Jin Un Kim,a,b,1 Ji Hyun Baek,c Shankara S. Kalanur,a Hyun Suk Jung,c Hyungtak Seo,a,b,* and In Sun Choa,b,*
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a
Department of Materials Science & Engineering, Ajou University, Suwon, 16499, South Korea Department of Energy Systems Research, Ajou University, Suwon, 16499, South Korea c School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon, 16419, South Korea
These authors contributed equally
*Corresponding Authors: Hyungtak Seo
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b
In Sun Cho
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Tel: +82-31-219-3532; Fax: +82-31-219-3532; Email: [email protected]
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Tel: +82-31-219-2468; Fax: +82-31-219-1613; Email: [email protected]
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ACCEPTED MANUSCRIPT Abstract The design of heterostructured multilayer oxide films for photoanodes enables the control of interfacial, charge transport/transfer and optical properties as well as stability, thus resulting in efficient photoelectrochemical (PEC) water splitting. Here, we report a triple-layered
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TiO2/BiVO4/SnO2 (T/B/S) photoanode that shows improved PEC water-oxidation performance and high visible transmittance at the wavelengths above 510 nm. The T/B/S photoanode was fabricated by a solution spin-coating method via a sequential deposition of the three layers. First, a bottom
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SnO2 layer with thickness ~200 nm was deposited on a F:SnO2 (FTO) substrate. Subsequently, a BiVO4 middle layer (~130 nm) and a TiO2 nanoparticle top layer (~100 nm) were deposited. The
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three distinct layers of TiO2, BiVO4, and SnO2 deposited on the FTO substrates were free of voids and cracks. Importantly, the bottom SnO2 layer caused an increase in the lateral grain size (up to ~600 nm) of the BiVO4 layer and formed a type-II heterojunction with the layer (similar to the BiVO4/WO3 case), thus efficiently improving charge separation and electron transport properties.
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Furthermore, the top TiO2 (anatase phase) layer formed a staggered conduction band structure with the BiVO4 layer and also protected the underlying layers against photocorrosion. The resultant T/B/S photoanode, which was devoid of any electrocatalyst, showed a higher photocurrent density
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of ~2.3 mA/cm2 and ~3.7 mA/cm2 at 1.23 V versus reversible hydrogen electrode for water oxidation and H2O2 oxidation, respectively, and a higher stability compared to those of BiVO4/SnO2
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and pristine BiVO4 photoanodes.
Keywords:
heterostructure,
BiVO4/SnO2,
TiO2
photoelectrochemical performance, photocurrent stability.
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overlayer,
sol-gel
spin-coating,
ACCEPTED MANUSCRIPT Introduction Photoelectrochemical (PEC) water splitting that involves the direct conversion of sunlight into hydrogen fuel and its storage, is an attractive renewable energy technology because it is relatively simple (analogous to photosynthesis in nature) and produces clean hydrogen gas from
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earth’s most abundant energy source without pollutant byproducts.[1-4] To date, tremendous efforts have been devoted to the design and development of an efficient photoanode that possesses longterm stability and generates sufficiently high photocurrent density for commercially viable PEC
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devices.[5-8]
Bismuth vanadate (BiVO4) is one of the most attractive and promising photoanode because
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of its high visible-light absorption, suitable band-edge position for oxygen evolution reaction (OER), and modest stability in neutral electrolytes.[9-11] Nevertheless, BiVO4 still suffers from low electron mobility (~0.01 cm2/V•s), short hole collection-depth, and sluggish OER kinetics, thereby reducing the PEC performance (photocurrent generation and long-term stability) to values far below
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than those expected theoretically.[10, 12, 13]
Design of heterostructures enables control of optical, charge transfer/transport, interfacial properties, and stability of photoanodes.[11, 14-23] Recently, diverse heterostructures based on
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BiVO4, such as BiVO4/WO3,[24, 25] BiVO4/SnO2,[26-28] BiVO4/Sb:SnO2,[26] TiO2/BiVO4,[29-31] BiVO4/CuWO4,[32] Co3O4/BiVO4,[33] BiVO4/SnO2/WO3,[34] and BiVO4/WO3/SnO2,[35] have
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been demonstrated to form heterojunctions (i.e., band alignments), thereby improving PEC performance by enhancing charge separation, transport, and transfer efficiencies.[24, 36-38] For instance, BiVO4/WO3 has been extensively studied and diverse nanostructures have been synthesized due to its appropriate band alignment and band-gap energies.[11, 24, 36, 39] The BiVO4/SnO2 heterojunction was firstly reported by Chatchai et al.[40] The heterojunction showed efficient charge separation in BiVO4/SnO2 photoanode by transferring electrons from BiVO4 to SnO2 and inhibiting the transfer of holes from BiVO4 to surface of a F-doped tin oxide (FTO) 3
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substrate. More recently, Zhou et al.[26] and Chen et al.[41] have reported one-dimensional nanowire array-based BiVO4/SnO2 (or Sb:SnO2) heterojunction photoanodes that show improved light-trapping and charge-separation efficiencies for PEC water oxidation. Byun et al. have investigated the effect of the thickness of an SnO2 buffer layer on the BiVO4/SnO2 photoanode
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characteristics.[28] They showed the thickness of the SnO2 plays an important role in the determination of crystalline phase, grain size, film roughness, as well as the band-gap of BiVO4, which in turn determines the PEC performance. The combination of three layers: BiVO4, WO3, and
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SnO2 has been studied by Saito et al.[34] and Baek et al.[35]; the triple-layered photoanode showed a much better PEC performance than that of single and double-layered photoanodes. In addition,
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TiO2 (both the anatase and rutile phases), owing to its high photocatalytic activity and corrosion stability, has also been combined with BiVO4 to obtain heterostructures.[29, 31] In this study, we report a triple-layered TiO2/BiVO4/SnO2 (T/B/S) photoanode with high PEC performance, light absorption, and stability. The photoanodes were fabricated by a facile
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solution spin-coating route via sequential deposition of the three layers. First, a SnO2 nanoparticle film (~200 nm thick) was coated on a FTO substrate. Subsequently, a BiVO4 middle layer (~130 nm) and a TiO2 nanoparticle top layer (~100 nm thick) were deposited. The three distinct layers of
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TiO2, BiVO4, and SnO2 deposited on FTO substrates were free of voids and cracks. Importantly, we found that the bottom SnO2 layer causes an increase in the lateral grain size (up to ~600 nm) of the
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BiVO4 layer and forms a type-II heterojunction with the layer (i.e., staggered conduction and valence bands alignment in between SnO2 and BiVO4 layers), thereby resulting in improved charge separation and electron transport properties. Furthermore, the top TiO2 layer forms a staggered conduction band edge with BiVO4 and protects the underlying layers against photocorrosion. The resultant T/B/S photoanode, devoid of any electrocatalyst deposition, shows a photocurrent density of ~2.3 mA/cm2 and ~3.7 mA/cm2 at 1.23 V versus reversible hydrogen electrode (RHE) for water
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oxidation and H2O2 oxidation, respectively, and higher stability compared with that of BiVO4/SnO2 and pristine BiVO4 photoanodes.
Experimental
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Preparation of TiO2/BiVO4/SnO2 (T/B/S) triple-layer photoanode The T/B/S photoanodes were fabricated by a solution spin-coating method. To prepare the SnO2 coating solution (0.2 M), we dissolved 0.701 g of tin (IV) chloride pentahydrate (SnCl4·5H2O,
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98%, Samchun Chemicals) in 10 mL of 2-methoxyethanol (2-ME, ≥99.3%, Alfa Aesar) with ultrasonication for 30 min [42]. The BiVO4 coating solution (0.1 M) was prepared by dissolving
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0.485 g of bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O, ≥98%, Sigma-Aldrich) and 0.265 g of vanadyl acetylacetonate (98%, Sigma-Aldrich) in a solvent mixture of 2ME/acetic acid (99%, Duksan)/2,4-pentanedione (99%, Alfa Aesar) = 8:2:2 mL with ultrasonication (30 min) and stirring (12 h).[43] Similarly, we prepared the TiO2 coating solution (0.1 M) by dissolving 0.340 g of
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titanium (IV) butoxide (Ti(OCH2CH2CH2CH3)4, 97%, Sigma-Aldrich) in a solvent mixture of isopropyl alcohol (99.5%, Daejung)/deionized (DI) water/nitric acid (64-66%, Duksan) = 10:0.9:0.12 mL with ultrasonication (60 min) and stirring (24 h).[44] The coating solutions were
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sequentially spin-coated onto FTO substrates (Pilkington, TEC-8) at 3000 rpm for 40 s. After each layer was coated, we performed intermediate annealing on a hot-plate at 350 °C for 10 min; the
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final annealing was performed at 500 °C for 1 h. The average film thickness per coating cycle was predetermined (~15-20 nm/cycle), and the optimal thickness for each film was determined from the photocurrent-potential (J-V) curves (Fig. S1). Material characterization The morphology and thickness of the films were examined using a field-emission scanning electron microscope (FE-SEM, S-4800, Hitachi). Transmission electron microscopy (TEM) images 5
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of the samples were obtained using a focused ion beam (FIB) system (JIB-4601F, Jeol) and crosssectional TEM and scanning TEM (STEM) images as well as energy dispersive X-ray spectroscopy (EDS) data of the photoanodes were obtained using a high-resolution TEM (JEM-2100F, Jeol). The crystal structure/phase of the photoanodes were examined using X-ray diffraction (XRD, Ultima III,
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Rigaku). The transmittance (T%) and reflectance (R%) spectra of the photoanodes were measured using a UV-Vis spectrometer (Cary 5000, Agilent Technologies). The absorption (A) was calculated from T and R (i.e., A = 100 − %T – %R).
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PEC measurements
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The PEC and electrochemical properties of all photoanodes were measured with a potentiostat (SP-200, Biologics) under a simulated solar light (AM1.5G, 100 mW/cm2) illumination using a solar simulator (Model 94011A, Newport). A standard three-electrode cell with an Ag/AgCl reference electrode (3M KCl, V° = 0.210 V), the photoanode (working electrode), and a Platinum wire counter electrode were employed. Before all measurements, we calibrated the intensity of solar
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simulator using a reference silicon diode (Mode 91150V, Newport). The active area of the working electrode was 0.18 cm2, as defined by a mask. We used 0.5 M phosphate buffer solution (PBS, pH
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7) or a 0.5 M phosphate buffer solution with H2O2 as a electrolyte. For J-V measurements, a potential scan rate of 50 mV/s was used and the potentials vs. Ag/AgCl were recorded with
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correction using the Nernst equation: VRHE = VAg/AgCl + (0.059 × pH) + 0.210 where VAg/AgCl is the applied bias potential and 0.210 corresponds to the reference potential of Ag/AgCl electrode with respect to RHE. Electrochemical impedance spectroscopy (EIS) spectra were recorded in three electrode configuration. The sinusoidal voltage-amplitude of 10 mV was used, and the frequency range recorded was from 7 MHz to 1 Hz. The incident photon to current conversion efficiency (IPCEs) was measured at 1.23 V vs. RHE using an IPCE system (Dongwoo Optron Co., Ltd). A 250 W tungsten-halogen lamp equipped with a monochromator (MonoRa150i, Dongwoo Optron Co., Ltd) was used to generate a monochromatic light. The charge transport/transfer efficiencies were 6
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determined by using a H2O2 under simulated solar-light irradiation.[45]
Results and Discussion Fabrication of TiO2/BiVO4/SnO2 (T/B/S) triple-layer photoanode As described in the experimental section, the T/B/S photoanode was prepared using the
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solution spin-coating method (Fig. 1a). A dense SnO2 layer with a thickness ~200 nm was deposited on a FTO substrate using a SnO2 coating solution (0.2 M, 9 cycles, annealed at 500 °C/1 h). Subsequently, a BiVO4 layer (0.1 M, 9 cycles, annealed at 500 °C/1 h) with thickness ~130 nm
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and a TiO2 nanoparticle layer with thickness ~100 nm (0.1 M, 6 cycles) were obtained by spincoating the respective coating solutions. Anatase TiO2 was formed after annealing at 500 °C for 1 h.
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Figure 1b shows a SEM image (cross-sectional) and a photograph of the fabricated T/B/S triplelayer photoanode. As observed, the layers of SnO2 (200 nm), BiVO4 (130 nm), and TiO2 (100 nm) are in intimate contact and free of voids and cracks. Notably, the BiVO4 layer exhibits larger grains (~300 nm in lateral size) compared to those of the SnO2 and TiO2 layers (~30 nm) (See Fig. S2).
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Additionally, the T/B/S photoanode is transparent to visible-light and exhibited bright-yellow color (inset of Fig. 1b). Figure 1c shows the XRD pattern of the T/B/S photoanode. All the peaks matched with those of rutile SnO2 (JCPDS: #41-1445), monoclinic BiVO4 (JCPDS: #75-1867), and
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anatase TiO2 (JCPDS: #21-1272), indicating the formation of three layers, i.e., SnO2, BiVO4, and TiO2 layers. Figure 1d shows the schematic band alignment of SnO2, BiVO4, and TiO2, as
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estimated from the Mott-Schottky measurement and band-gap values[35] (see Fig. S3). The SnO2 and BiVO4 layers form a type-II heterojunction, similar to the WO3 and BiVO4 case, promoting electron/hole separation and transfer between the layers.[40] In the case of the top TiO2 layer, it passivates the surface defects of the BiVO4 layer and protects it against photocorrosion.[29] Figure 2 shows the effect of the bottom SnO2 layer on the lateral grain size of the BiVO4 layer. Compared to the BiVO4 layer deposited on bare FTO substrate (lateral grain size ~110 nm), the BiVO4 deposited on the SnO2 nanoparticle film exhibits three times larger grains (~290 nm). 7
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This result indicates that the underlying SnO2 layer causes a decrease in the surface roughness as well as an increase in the surface energy of the substrate (See Fig. S4), leading to high wettability of BiVO4 droplets the contact, and eventually facilitating the grain growth of BiVO4.[28] The detailed microstructure and elemental distributions of the fabricated T/B/S photoanode
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were investigated using TEM and EDS, respectively (Fig. 3). As shown in the TEM image (Fig. 3a), the three distinct layers, which are in intimate contact, can be identified. The total thickness of the layers is ~450 nm, which is in agreement with the value obtained from the cross-sectional SEM
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image (Fig. 1b). In addition, the bottom SnO2 and top TiO2 layers are composed of nanoparticles smaller than those of the middle BiVO4 layer. To confirm the crystal structure and crystallinity,
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high-resolution TEM analysis was performed on each layer at spots A (SnO2), B (BiVO4), and C (TiO2) (Fig. 3b). The images show highly crystalline, interconnected SnO2 nanoparticles of size ~5– 30 nm. The high-resolution TEM image confirms the crystal structure of the SnO2 nanoparticles to be tetragonal rutile. The middle BiVO4 layer is found to be dense; the HR-TEM image confirms the
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formation of a highly crystalline BiVO4 layer with monoclinic scheelite structure. In addition, the HR-TEM image corresponding to spot C confirms the crystal structure of the top TiO2 layer to be anatase. Moreover, the STEM image (Fig. 3c) clearly shows the three distinct layers, and the EDS
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elemental maps confirm uniform elemental distribution within the layers. Notably, the good contact/ adhesion among the three layers without voids helps to inhibit interfacial recombination.
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Furthermore, the larger BiVO4 grains help to decrease bulk recombination of charge carriers.[46,
Optical properties
Figure 4a is a photograph showing the fabricated T/B/S, B/S, and BiVO4 (BVO) photoanodes. The T/B/S photoanode is bright yellow, while B/S and BVO photoanodes are greenish-yellow. The color variation is attributed to the optical interference in the layers, which 8
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have different thicknesses and refractive indices (Fig. S5), and this affects the optical properties of the fabricated photoanodes. We have analyzed the light absorption, reflectance, and transmittance spectra of the photoanodes. Figures 4b and 4c show transmittance and reflectance spectra, respectively, of the three photoanodes. Notably, the T/B/S photoanode shows higher transmittance
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and lower reflectance than those of B/S and BVO photoanodes at wavelengths above ~510 nm, which is beneficial for the construction of tandem PEC cells.[35] Additionally, the T/B/S photoanode exhibits a slightly higher light absorption than that of the other two photoanodes in the
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wavelength range 350–480 nm (Fig. 4d). The integrated light absorption value of the T/B/S photoanode is ~76%, which is higher than that of B/S (68%) and BVO (65%) photoanodes. As
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shown in Fig. S6, estimated band gap values of T/B/S photoanode was ~2.4 eV, which is comparable to the band gap value of pure BiVO4.
Photoelectrochemical performances
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We have examined the PEC performance of the T/B/S photoanode for water oxidation and compared them with those of B/S, BVO, and BiVO4/TiO2/SnO2 (B/T/S) photoanodes. Schematic structures of the four photoanodes are shown in Fig. 5a. Fig. 5b presents the photocurrent-potential
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(J-V) curves (the characteristic measurement for a PEC device) of the four photoanodes. The J-V curves of individual materials (i.e., BiVO4, TiO2, and SnO2 films) are shown in Fig. S7. Compared
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to the BVO (~0.07 mA/cm2 at 1.23 V vs. RHE), the B/S photoanode exhibits a photocurrent density (~2.1 mA/cm2 at 1.23 V vs. RHE) that is 30 times higher. As shown in Fig. 4c, there is little difference in the light absorption (i.e., charge generation) between samples. Therefore, the higher value is attributed to the formation of a heterojunction (type-II) between the BiVO4 and SnO2 layers of the B/S photoanode.[40] The electrons/holes generated from the BVO layer can be separated effectively by the internal electric field; subsequently, the electrons are transferred to the conduction band of SnO2 and holes are blocked by the SnO2 layer, thereby improving the charge separation and 9
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transport efficiencies.[28, 40] Additionally, the larger grain size of the BiVO4 layer in the B/S photoanode (see Fig. 2) causes a reduction in bulk recombination, which in turn leads to superior charge transport properties.[46] However, the T/B/S photoanode exhibits a slightly higher photocurrent density (~2.3 mA/cm2 at 1.23 V vs. RHE) than that of the B/S photoanode. As shown
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in Fig. S8, the IPCEs of T/B/S, B/S, and BVO photoanodes exhibit a similar trend as the J-V results. The integrated photocurrent-density values (shown in parenthesis) also correspond with those from the J-V results. On the other hand, for the B/T/S photoanode, with a TiO2 layer between
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the BiVO4 and SnO2 layers, the photocurrent density decreases drastically (~0.32 mA/cm2), indicating that the TiO2 layer should be deposited on the BiVO4 layer to obtain higher photocurrent
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density. The two-electrode J-V curve of the BVO photoanode was measured to estimate the performance in real application. As shown in Fig. S9, there was little differences in between 2- and 3-electrode measurements.
To understand the effects of the top TiO2 and bottom SnO2 layers, the charge transfer
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(ηtransfer) and transport efficiencies (ηtransport) of the four photoanodes have been estimated using a hole scavenger method[44, 45] (Figs. 5c and 5d). As shown in Fig. 5c, T/B/S and B/S photoanodes exhibit similar ηtransfer (~62%), which is much higher than those of B/T/S (~33%) and BVO (~5%)
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photoanodes. Similarly, T/B/S and B/S photoanodes exhibit ~2–4 times higher ηtransport (~72%) than those of B/T/S (~27%) and BVO photoanodes (~18%). These results confirm that the deposition of
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the bottom SnO2 layer in T/B/S and B/S photoanodes greatly improves both ηtransfer and ηtransport. The advantageous effect is ascribed to the formation of a type-II heterojunction between SnO2 and BiVO4 layers (similar to the case of BiVO4/WO3). In addition, the large grains in the BiVO4 layer improve charge separation/transport efficiencies and reduce bulk recombination. On the other hand, deposition of the TiO2 layer on top of BiVO4/SnO2 effectively improves the electron transport, resulting from the cascade alignment of the conduction-band edges (Fig. 1d & Fig. S10).[29] To further understand the superior PEC performance of T/B/S and B/S photoanodes, we have 10
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performed electrochemical impedance spectroscopy on the samples under 1 sun illumination. Figure 5e shows the Nyquist plots of the four photoanodes. The inset is a schematic equivalent circuit model, which was used to fit the EIS spectra; R1 is the series resistance, R2 is the bulk resistance, R3 is the charge transfer resistance, C1 is the chemical capacitance (= cµL, L: film
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thickness) of the photoanode, and C2 is Helmholtz layer capacitance[48, 49]. Table S1 summarized the extracted R1, R2, and R3 values; the data show that T/B/S and B/S photoanodes have much lower R2 and R3 values than those of B/T/S and BVO photoanodes. In addition, R2 and R3 of the
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T/B/S photoanode are slightly lower than those of the B/S photoanode. These results indicate that T/B/S and B/S photoanodes have superior charge transport and transfer properties, in accordance
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with the results of the ηtransfer and ηtransport measurements.
The PEC performance of the T/B/S photoanode has been investigated for H2O2 and Na2SO3 oxidation reactions, as well. Figure 6 shows the J-V curves of T/B/S, B/S, and BVO photoanodes. Both H2O2 and Na2SO3 act as hole scavengers during oxidation and the kinetics of
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peroxide and sulfite oxidation are thermodynamically much faster than that of water oxidation. Because of the fast kinetics of the reactions, even the BVO layer exhibits slightly high photocurrent density (~1.3 mA/cm2 at 1.23 V vs. RHE) for both H2O2 and Na2SO3 oxidations. Importantly, T/B/S
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and B/S photoanodes exhibit significantly high photocurrent density. In particular, the T/B/S photoanode shows a photocurrent density of ~3.7 mA/cm2 at 1.23 V vs. RHE for H2O2 oxidation.
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As a result, the T/B/S photoanode shows enhanced photocurrent density for water, H2O2, and sulfite oxidation reactions compared to the B/S and BVO photoanodes, which is ascribed to the improved charge transport and transfer efficiencies resulting from the intimate contact, type-II heterojunction, and larger grains of BiVO4. We have also evaluated the photocurrent stability of T/B/S and B/S photoanodes under simulated solar-light irradiation (Fig. 7). As shown in Fig. 7a, the T/B/S photoanode exhibited a more stable photocurrent density than that of the B/S photoanode for 5 h. This result indicates that 11
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the top TiO2 layer plays a crucial role in protecting the BiVO4 against photocorrosion/degradation. The SEM images obtained after the stability test (Figs. 7b & 7c) show that the portion (or thickness) of the remaining BiVO4 layer in the T/B/S photoanode is much larger compared to that in the B/S photoanode. This result clearly indicates that the top TiO2 layer helps in stagnating the degradation
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of the BiVO4 layer. However, the BiVO4 layer in the T/B/S photoanode has undergone some degradation during the stability test, indicating that the coverage of the TiO2 layer is not perfect (Fig. S11). Therefore, some other TiO2 deposition method (e.g., atomic layer deposition, ALD)
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and/or electrocatalyst coating, needs to be explored to achieve higher stability for the T/B/S photoanode. Lastly, the T/B/S photoanode also showed a high faradaic efficiency over 90% for the
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water-oxidation (Fig. S12).
Conclusion
In summary, we prepared a triple-layered TiO2/BiVO4/SnO2 (T/B/S) planar-type photoanode
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that showed enhanced visible-light transmittance and PEC performance. The T/B/S was fabricated by a solution spin-coating route. A bottom SnO2 layer (~200 nm thick) was first coated on FTO substrates. Subsequently, a middle BiVO4 layer (~130 nm) and a top TiO2 nanoparticle layer (~100
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nm) were deposited. From the TEM and EDS results, we confirmed the formation of three distinct, highly crystalline layers in intimate contact and free of voids and cracks. Additionally, we found
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that the bottom SnO2 layer not only caused an increase in the lateral grain size of the BiVO4 layer but also formed a type-II heterojunction with the layer, which led to a great improvement in charge separation and electron transport properties. Furthermore, the top TiO2 layer protected the BiVO4 layer against photocorrosion. The optimized T/B/S photoanode exhibited high photocurrent densities (~2.3 mA/cm2 and 3.7 mA/cm2 at 1.23 V vs. RHE for water oxidation and H2O2 oxidation, respectively). The T/B/S photoanode also exhibited relatively stable photocurrent density for 5 h under simulated solar light irradiation compared to those of BiVO4/SnO2 and BiVO4. We believe 12
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that the design and development of heterostructures with multi-layered films for oxide-based photoanodes is a facile and effective route to achieve a simultaneous enhancement in charge
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transport/transfer efficiencies, thus allowing the fabrication of efficient PEC water-splitting device.
Acknowledgments
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This research was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF2015R1C1A1A01053785).
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Figure 1. Fabrication and characterization of TiO2/BiVO4/SnO2 (T/B/S) triple-layer photoanode. (a) Schematic of fabrication. (b) Cross-sectional SEM image and photograph of the T/B/S photoanode. (c) XRD pattern of the T/B/S photoanode. (d) Band alignment of the three layers.
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Figure 2. Effect of the bottom SnO2 layer on the lateral grain size of the BiVO4 layer. Top-view SEM images and statistical grain size distribution of (a) the BiVO4 layer directly deposited on FTO and (b) the BiVO4 layer deposited on the SnO2 layer.
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Figure 3. TEM characterization of the TiO2/BiVO4/SnO2 triple-layer photoanode. (a) The cross-sectional TEM image and (b) Magnified TEM images corresponding to the spots A, B, and C in (a). Insets are high-resolution TEM images (scale bar = 2 nm). (c) Dark-field STEM image and EDS elemental maps.
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Figure 4. Optical properties of the photoanodes. (a) A photograph showing T/B/S, B/S, and BVO photoanodes with variation in color and visible transparency. (b) Transmittance, (c) Reflectance and (d) Light absorption spectra (= 100 –%R–%T). The corresponding integrated light absorption values are given in parentheses in (d).
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Figure 5. Water oxidation: PEC characterization of T/B/S, B/S, BVO, and B/T/S photoanodes. (a) A schematic layer structure of the photoanodes. (b) Typical photocurrent-potential (J-V) curves. (c) Charge transfer efficiency. (d) Charge transport efficiency. (e) Electrochemical impedance spectra (Nyquist plots); inset is the schematic equivalent circuit model.
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Figure 6. J-V curves of T/B/S, B/S, and BVO photoanodes. (a) H2O2 oxidation and (b) Na2SO3 oxidation reactions performed under AM1.5G (100 mW/cm2) simulated solar light illumination.
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Figure 7. (a) Result of the photocurrent stability test performed on T/B/S and B/S photoanodes for 4 h under simulated solar light illumination (measured at 1 V vs. Pt). The inset is a photograph of the wrapped with epoxy resin. Top-view SEM images of (b) B/S photoanode and (c) T/B/S photoanode obtained after J-t curve measurement for 2 and 4 h, respectively.
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References
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[1] M. Grätzel, Photoelectrochemical cells, nature, 414 (2001) 338. [2] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Solar water splitting cells, Chemical reviews, 110 (2010) 6446-6473. [3] A.J. Bard, M.A. Fox, Artificial photosynthesis: solar splitting of water to hydrogen and oxygen, Accounts of Chemical Research, 28 (1995) 141-145. [4] Y. Tachibana, L. Vayssieres, J.R. Durrant, Artificial photosynthesis for solar water-splitting, Nature Photonics, 6 (2012) 511. [5] C. Jiang, S.J. Moniz, A. Wang, T. Zhang, J. Tang, Photoelectrochemical devices for solar water splitting– materials and challenges, Chemical Society Reviews, 46 (2017) 4645-4660. [6] K. Sivula, R. Van De Krol, Semiconducting materials for photoelectrochemical energy conversion, Nature Reviews Materials, 1 (2016) 15010. [7] M.D. Bhatt, J.S. Lee, Recent theoretical progress in the development of photoanode materials for solar water splitting photoelectrochemical cells, Journal of Materials Chemistry A, 3 (2015) 10632-10659. [8] H.M. Chen, C.K. Chen, R.-S. Liu, L. Zhang, J. Zhang, D.P. Wilkinson, Nano-architecture and material designs for water splitting photoelectrodes, Chemical Society Reviews, 41 (2012) 5654-5671. [9] T.W. Kim, K.-S. Choi, Nanoporous BiVO4 photoanodes with dual-layer oxygen evolution catalysts for solar water splitting, Science, (2014) 1245026. [10] Y. Park, K.J. McDonald, K.-S. Choi, Progress in bismuth vanadate photoanodes for use in solar water oxidation, Chemical Society Reviews, 42 (2013) 2321-2337. [11] J.H. Kim, J.S. Lee, BiVO4-based heterostructured photocatalysts for solar water splitting: a review, Energy and Environment Focus, 3 (2014) 339-353. [12] A.J.E. Rettie, H.C. Lee, L.G. Marshall, J.-F. Lin, C. Capan, J. Lindemuth, J.S. McCloy, J. Zhou, A.J. Bard, C.B. Mullins, Combined Charge Carrier Transport and Photoelectrochemical Characterization of BiVO4 Single Crystals: Intrinsic Behavior of a Complex Metal Oxide, Journal of the American Chemical Society, 135 (2013) 11389-11396. [13] F.F. Abdi, T.J. Savenije, M.M. May, B. Dam, R. van de Krol, The Origin of Slow Carrier Transport in BiVO4 Thin Film Photoanodes: A Time-Resolved Microwave Conductivity Study, The Journal of Physical Chemistry Letters, 4 (2013) 2752-2757. [14] C.X. Kronawitter, L. Vayssieres, S. Shen, L. Guo, D.A. Wheeler, J.Z. Zhang, B.R. Antoun, S.S. Mao, A perspective on solar-driven water splitting with all-oxide hetero-nanostructures, Energy & Environmental Science, 4 (2011) 3889-3899. [15] J. Yin, F. Di, J. Guo, K. Zhang, W. Xu, Y. Wang, S. Shi, N. Chai, C. Chu, J. Wei, W. Li, X. Shao, X. Pu, D. Zhang, X. Ren, J. Wang, J. Zhao, X. Zhang, X. Wei, F. Wang, H. Zhou, Tuning Ni-Foam into NiOOH/FeOOH Heterostructures toward Superior Water Oxidation Catalyst via Three-Step Strategy, ACS Omega, 3 (2018) 11009-11017. [16] J. Huang, X. Deng, H. Wan, F. Chen, Y. Lin, X. Xu, R. Ma, T. Sasaki, Liquid Phase Exfoliation of MoS2 Assisted by Formamide Solvothermal Treatment and Enhanced Electrocatalytic Activity Based on (H3Mo12O40P/MoS2)n Multilayer Structure, ACS Sustainable Chemistry & Engineering, 6 (2018) 5227-5237. [17] E. Ahn, B.-S. Kim, Multidimensional Thin Film Hybrid Electrodes with MoS2 Multilayer for Electrocatalytic Hydrogen Evolution Reaction, ACS Applied Materials & Interfaces, 9 (2017) 8688-8695. [18] H. Gao, H. Yang, J. Xu, S. Zhang, J. Li, Strongly Coupled g-C3N4 Nanosheets-Co3O4 Quantum Dots as 2D/0D Heterostructure Composite for Peroxymonosulfate Activation, Small, 14 (2018) 1801353. [19] S. Zhang, H. Gao, Y. Huang, X. Wang, T. Hayat, J. Li, X. Xu, X. Wang, Ultrathin g-C3N4 nanosheets coupled with amorphous Cu-doped FeOOH nanoclusters as 2D/0D heterogeneous catalysts for water remediation, Environmental Science: Nano, 5 (2018) 1179-1190. [20] S. Zhang, H. Yang, H. Huang, H. Gao, X. Wang, R. Cao, J. Li, X. Xu, X. Wang, Unexpected ultrafast and high adsorption capacity of oxygen vacancy-rich WOx/C nanowire networks for aqueous Pb2+ and methylene blue removal, Journal of Materials Chemistry A, 5 (2017) 15913-15922. [21] H. Liu, A. Wang, Q. Sun, T. Wang, H. Zeng, Cu Nanoparticles/Fluorine-Doped Tin Oxide (FTO) Nanocomposites for Photocatalytic H2 Evolution under Visible Light Irradiation, Catalysts, 7 (2017) 385. 21
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[22] Z. Zhan, F. Grote, Z. Wang, R. Xu, Y. Lei, Degenerating Plasmonic Modes to Enhance the Performance of Surface Plasmon Resonance for Application in Solar Energy Conversion, Advanced Energy Materials, 5 (2015) 1501654. [23] Z. Zhan, Y. Wang, Z. Lin, J. Zhang, F. Huang, Study of interface electric field affecting the photocatalysis of ZnO, Chemical Communications, 47 (2011) 4517-4519. [24] P.M. Rao, L. Cai, C. Liu, I.S. Cho, C.H. Lee, J.M. Weisse, P. Yang, X. Zheng, Simultaneously efficient light absorption and charge separation in WO3/BiVO4 core/shell nanowire photoanode for photoelectrochemical water oxidation, Nano letters, 14 (2014) 1099-1105. [25] S.J. Hong, S. Lee, J.S. Jang, J.S. Lee, Heterojunction BiVO 4/WO 3 electrodes for enhanced photoactivity of water oxidation, Energy & Environmental Science, 4 (2011) 1781-1787. [26] L. Zhou, C. Zhao, B. Giri, P. Allen, X. Xu, H. Joshi, Y. Fan, L.V. Titova, P.M. Rao, High light absorption and charge separation efficiency at low applied voltage from Sb-doped SnO2/BiVO4 core/shell nanorod-array photoanodes, Nano letters, 16 (2016) 3463-3474. [27] Y. Liang, T. Tsubota, L.P. Mooij, R. van de Krol, Highly improved quantum efficiencies for thin film BiVO4 photoanodes, The Journal of Physical Chemistry C, 115 (2011) 17594-17598. [28] S. Byun, B. Kim, S. Jeon, B. Shin, Effects of a SnO 2 hole blocking layer in a BiVO 4-based photoanode on photoelectrocatalytic water oxidation, Journal of Materials Chemistry A, 5 (2017) 6905-6913. [29] A.P. Singh, N. Kodan, B.R. Mehta, A. Held, L. Mayrhofer, M. Moseler, Band edge engineering in BiVO4/TiO2 heterostructure: enhanced photoelectrochemical performance through improved charge transfer, ACS Catalysis, 6 (2016) 5311-5318. [30] B.-Y. Cheng, J.-S. Yang, H.-W. Cho, J.-J. Wu, Fabrication of an efficient BiVO4–TiO2 heterojunction photoanode for photoelectrochemical water oxidation, ACS applied materials & interfaces, 8 (2016) 2003220039. [31] R. Tong, X. Wang, X. Zhou, Q. Liu, H. Wang, X. Peng, X. Liu, Z. Zhang, H. Wang, P.D. Lund, CobaltPhosphate modified TiO2/BiVO4 nanoarrays photoanode for efficient water splitting, International Journal of Hydrogen Energy, 42 (2017) 5496-5504. [32] W. Ye, F. Chen, F. Zhao, N. Han, Y. Li, CuWO4 Nanoflake Array-Based Single-Junction and Heterojunction Photoanodes for Photoelectrochemical Water Oxidation, ACS Applied Materials & Interfaces, 8 (2016) 9211-9217. [33] X. Chang, T. Wang, P. Zhang, J. Zhang, A. Li, J. Gong, Enhanced surface reaction kinetics and charge separation of p–n heterojunction Co3O4/BiVO4 photoanodes, Journal of the American Chemical Society, 137 (2015) 8356-8359. [34] R. Saito, Y. Miseki, K. Sayama, Highly efficient photoelectrochemical water splitting using a thin film photoanode of BiVO 4/SnO 2/WO 3 multi-composite in a carbonate electrolyte, Chemical Communications, 48 (2012) 3833-3835. [35] J.H. Baek, B.J. Kim, G.S. Han, S.W. Hwang, D.R. Kim, I.S. Cho, H.S. Jung, BiVO4/WO3/SnO2 doubleheterojunction photoanode with enhanced charge separation and visible-transparency for bias-free solar water-splitting with a perovskite solar cell, ACS applied materials & interfaces, 9 (2017) 1479-1487. [36] J. Su, L. Guo, N. Bao, C.A. Grimes, Nanostructured WO3/BiVO4 heterojunction films for efficient photoelectrochemical water splitting, Nano letters, 11 (2011) 1928-1933. [37] J. Resasco, H. Zhang, N. Kornienko, N. Becknell, H. Lee, J. Guo, A.L. Briseno, P. Yang, TiO2/BiVO4 Nanowire Heterostructure Photoanodes Based on Type II Band Alignment, ACS Central Science, 2 (2016) 80-88. [38] F. Zhan, J. Li, W. Li, Y. Liu, R. Xie, Y. Yang, Y. Li, Q. Chen, In situ formation of CuWO 4/WO 3 heterojunction plates array films with enhanced photoelectrochemical properties, International Journal of Hydrogen Energy, 40 (2015) 6512-6520. [39] Y. Pihosh, I. Turkevych, K. Mawatari, J. Uemura, Y. Kazoe, S. Kosar, K. Makita, T. Sugaya, T. Matsui, D. Fujita, Photocatalytic generation of hydrogen by core-shell WO3/BiVO4 nanorods with ultimate water splitting efficiency, Scientific reports, 5 (2015). [40] P. Chatchai, Y. Murakami, S.-y. Kishioka, A. Nosaka, Y. Nosaka, FTO∕ SnO2∕ BiVO4 composite photoelectrode for water oxidation under visible light irradiation, Electrochemical and Solid-State Letters, 11 (2008) H160-H163. 22
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[41] S.-Y. Chen, J.-S. Yang, J.-J. Wu, Three-Dimensional Undoped Crystalline SnO2 Nanodendrite Arrays Enable Efficient Charge Separation in BiVO4/SnO2 Heterojunction Photoanodes for Photoelectrochemical Water Splitting, ACS Applied Energy Materials, 1 (2018) 2143-2149. [42] H. Liu, K. Zhao, T. Wang, J. Deng, H. Zeng, Facile preparation of cerium (Ce) and antimony (Sb) codoped SnO2 for hydrogen production in lactic acid solution, Materials Science in Semiconductor Processing, 40 (2015) 670-675. [43] H.S. Han, S. Shin, D.H. Kim, I.J. Park, J.S. Kim, P.-S. Huang, J.-K. Lee, I.S. Cho, X. Zheng, Boosting the solar water oxidation performance of a BiVO 4 photoanode by crystallographic orientation control, Energy & Environmental Science, 11 (2018) 1299-1306. [44] J.U. Kim, H.S. Han, J. Park, W. Park, J.H. Baek, J.M. Lee, H.S. Jung, I.S. Cho, Facile and controllable surface-functionalization of TiO2 nanotubes array for highly-efficient photoelectrochemical water-oxidation, Journal of Catalysis, 365 (2018) 138-144. [45] H. Dotan, K. Sivula, M. Grätzel, A. Rothschild, S.C. Warren, Probing the photoelectrochemical properties of hematite (α-Fe2O3) electrodes using hydrogen peroxide as a hole scavenger, Energy & Environmental Science, 4 (2011) 958-964. [46] M. Zhong, T. Hisatomi, T. Minegishi, H. Nishiyama, M. Katayama, T. Yamada, K. Domen, Bulky crystalline BiVO 4 thin films for efficient solar water splitting, Journal of Materials Chemistry A, 4 (2016) 9858-9864. [47] J. Feng, X. Zhao, S.S.K. Ma, D. Wang, Z. Chen, Y. Huang, Fast and Simple Construction of Efficient SolarWater-Splitting Electrodes with Micrometer-Sized Light-Absorbing Precursor Particles, Advanced Materials Technologies, 1 (2016) 1600119. [48] T. Lopes, L. Andrade, H.A. Ribeiro, A. Mendes, Characterization of photoelectrochemical cells for water splitting by electrochemical impedance spectroscopy, International Journal of Hydrogen Energy, 35 (2010) 11601-11608. [49] J.R. Macdonald, E. Barsoukov, Impedance spectroscopy: theory, experiment, and applications, History, 1 (2005).
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Highlight • Preparation of triple-layered TiO2/BiVO4/SnO2 (T/B/S) photoanode by solution spin-coating method • The T/B/S photoanode showed improved photoelectrochemical and optical properties
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• SnO2 increases lateral grain-size of BiVO4 and improves charge separation efficiency • TiO2 forms staggered conduction-band-edge with BiVO4 and protect underlying layers
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• High photocurrent densities (2.3 mA/cm2 at 1.23 VRHE) and stability are demonstrated
S0925-8388(19)30246-4
DOI:
https://doi.org/10.1016/j.jallcom.2019.01.251
Reference:
JALCOM 49289
To appear in:
Journal of Alloys and Compounds
Received Date: 11 September 2018 Revised Date:
16 January 2019
Accepted Date: 19 January 2019
Please cite this article as: S.W. Hwang, J.U. Kim, J.H. Baek, S.S. Kalanur, H.S. Jung, H. Seo, I.S. Cho, Solution-processed TiO2/BiVO4/SnO2 triple-layer photoanode with enhanced photoelectrochemical performance, Journal of Alloys and Compounds (2019), doi: https:// doi.org/10.1016/j.jallcom.2019.01.251. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Solution-processed TiO2/BiVO4/SnO2 Triple-layer Photoanode with Enhanced Photoelectrochemical Performance
Sung Won Hwang,a,b,1 Jin Un Kim,a,b,1 Ji Hyun Baek,c Shankara S. Kalanur,a Hyun Suk Jung,c Hyungtak Seo,a,b,* and In Sun Choa,b,*
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Department of Materials Science & Engineering, Ajou University, Suwon, 16499, South Korea Department of Energy Systems Research, Ajou University, Suwon, 16499, South Korea c School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon, 16419, South Korea
These authors contributed equally
*Corresponding Authors: Hyungtak Seo
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In Sun Cho
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Tel: +82-31-219-3532; Fax: +82-31-219-3532; Email: [email protected]
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Tel: +82-31-219-2468; Fax: +82-31-219-1613; Email: [email protected]
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ACCEPTED MANUSCRIPT Abstract The design of heterostructured multilayer oxide films for photoanodes enables the control of interfacial, charge transport/transfer and optical properties as well as stability, thus resulting in efficient photoelectrochemical (PEC) water splitting. Here, we report a triple-layered
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TiO2/BiVO4/SnO2 (T/B/S) photoanode that shows improved PEC water-oxidation performance and high visible transmittance at the wavelengths above 510 nm. The T/B/S photoanode was fabricated by a solution spin-coating method via a sequential deposition of the three layers. First, a bottom
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SnO2 layer with thickness ~200 nm was deposited on a F:SnO2 (FTO) substrate. Subsequently, a BiVO4 middle layer (~130 nm) and a TiO2 nanoparticle top layer (~100 nm) were deposited. The
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three distinct layers of TiO2, BiVO4, and SnO2 deposited on the FTO substrates were free of voids and cracks. Importantly, the bottom SnO2 layer caused an increase in the lateral grain size (up to ~600 nm) of the BiVO4 layer and formed a type-II heterojunction with the layer (similar to the BiVO4/WO3 case), thus efficiently improving charge separation and electron transport properties.
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Furthermore, the top TiO2 (anatase phase) layer formed a staggered conduction band structure with the BiVO4 layer and also protected the underlying layers against photocorrosion. The resultant T/B/S photoanode, which was devoid of any electrocatalyst, showed a higher photocurrent density
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of ~2.3 mA/cm2 and ~3.7 mA/cm2 at 1.23 V versus reversible hydrogen electrode for water oxidation and H2O2 oxidation, respectively, and a higher stability compared to those of BiVO4/SnO2
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and pristine BiVO4 photoanodes.
Keywords:
heterostructure,
BiVO4/SnO2,
TiO2
photoelectrochemical performance, photocurrent stability.
2
overlayer,
sol-gel
spin-coating,
ACCEPTED MANUSCRIPT Introduction Photoelectrochemical (PEC) water splitting that involves the direct conversion of sunlight into hydrogen fuel and its storage, is an attractive renewable energy technology because it is relatively simple (analogous to photosynthesis in nature) and produces clean hydrogen gas from
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earth’s most abundant energy source without pollutant byproducts.[1-4] To date, tremendous efforts have been devoted to the design and development of an efficient photoanode that possesses longterm stability and generates sufficiently high photocurrent density for commercially viable PEC
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devices.[5-8]
Bismuth vanadate (BiVO4) is one of the most attractive and promising photoanode because
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of its high visible-light absorption, suitable band-edge position for oxygen evolution reaction (OER), and modest stability in neutral electrolytes.[9-11] Nevertheless, BiVO4 still suffers from low electron mobility (~0.01 cm2/V•s), short hole collection-depth, and sluggish OER kinetics, thereby reducing the PEC performance (photocurrent generation and long-term stability) to values far below
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than those expected theoretically.[10, 12, 13]
Design of heterostructures enables control of optical, charge transfer/transport, interfacial properties, and stability of photoanodes.[11, 14-23] Recently, diverse heterostructures based on
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BiVO4, such as BiVO4/WO3,[24, 25] BiVO4/SnO2,[26-28] BiVO4/Sb:SnO2,[26] TiO2/BiVO4,[29-31] BiVO4/CuWO4,[32] Co3O4/BiVO4,[33] BiVO4/SnO2/WO3,[34] and BiVO4/WO3/SnO2,[35] have
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been demonstrated to form heterojunctions (i.e., band alignments), thereby improving PEC performance by enhancing charge separation, transport, and transfer efficiencies.[24, 36-38] For instance, BiVO4/WO3 has been extensively studied and diverse nanostructures have been synthesized due to its appropriate band alignment and band-gap energies.[11, 24, 36, 39] The BiVO4/SnO2 heterojunction was firstly reported by Chatchai et al.[40] The heterojunction showed efficient charge separation in BiVO4/SnO2 photoanode by transferring electrons from BiVO4 to SnO2 and inhibiting the transfer of holes from BiVO4 to surface of a F-doped tin oxide (FTO) 3
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substrate. More recently, Zhou et al.[26] and Chen et al.[41] have reported one-dimensional nanowire array-based BiVO4/SnO2 (or Sb:SnO2) heterojunction photoanodes that show improved light-trapping and charge-separation efficiencies for PEC water oxidation. Byun et al. have investigated the effect of the thickness of an SnO2 buffer layer on the BiVO4/SnO2 photoanode
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characteristics.[28] They showed the thickness of the SnO2 plays an important role in the determination of crystalline phase, grain size, film roughness, as well as the band-gap of BiVO4, which in turn determines the PEC performance. The combination of three layers: BiVO4, WO3, and
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SnO2 has been studied by Saito et al.[34] and Baek et al.[35]; the triple-layered photoanode showed a much better PEC performance than that of single and double-layered photoanodes. In addition,
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TiO2 (both the anatase and rutile phases), owing to its high photocatalytic activity and corrosion stability, has also been combined with BiVO4 to obtain heterostructures.[29, 31] In this study, we report a triple-layered TiO2/BiVO4/SnO2 (T/B/S) photoanode with high PEC performance, light absorption, and stability. The photoanodes were fabricated by a facile
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solution spin-coating route via sequential deposition of the three layers. First, a SnO2 nanoparticle film (~200 nm thick) was coated on a FTO substrate. Subsequently, a BiVO4 middle layer (~130 nm) and a TiO2 nanoparticle top layer (~100 nm thick) were deposited. The three distinct layers of
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TiO2, BiVO4, and SnO2 deposited on FTO substrates were free of voids and cracks. Importantly, we found that the bottom SnO2 layer causes an increase in the lateral grain size (up to ~600 nm) of the
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BiVO4 layer and forms a type-II heterojunction with the layer (i.e., staggered conduction and valence bands alignment in between SnO2 and BiVO4 layers), thereby resulting in improved charge separation and electron transport properties. Furthermore, the top TiO2 layer forms a staggered conduction band edge with BiVO4 and protects the underlying layers against photocorrosion. The resultant T/B/S photoanode, devoid of any electrocatalyst deposition, shows a photocurrent density of ~2.3 mA/cm2 and ~3.7 mA/cm2 at 1.23 V versus reversible hydrogen electrode (RHE) for water
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oxidation and H2O2 oxidation, respectively, and higher stability compared with that of BiVO4/SnO2 and pristine BiVO4 photoanodes.
Experimental
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Preparation of TiO2/BiVO4/SnO2 (T/B/S) triple-layer photoanode The T/B/S photoanodes were fabricated by a solution spin-coating method. To prepare the SnO2 coating solution (0.2 M), we dissolved 0.701 g of tin (IV) chloride pentahydrate (SnCl4·5H2O,
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98%, Samchun Chemicals) in 10 mL of 2-methoxyethanol (2-ME, ≥99.3%, Alfa Aesar) with ultrasonication for 30 min [42]. The BiVO4 coating solution (0.1 M) was prepared by dissolving
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0.485 g of bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O, ≥98%, Sigma-Aldrich) and 0.265 g of vanadyl acetylacetonate (98%, Sigma-Aldrich) in a solvent mixture of 2ME/acetic acid (99%, Duksan)/2,4-pentanedione (99%, Alfa Aesar) = 8:2:2 mL with ultrasonication (30 min) and stirring (12 h).[43] Similarly, we prepared the TiO2 coating solution (0.1 M) by dissolving 0.340 g of
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titanium (IV) butoxide (Ti(OCH2CH2CH2CH3)4, 97%, Sigma-Aldrich) in a solvent mixture of isopropyl alcohol (99.5%, Daejung)/deionized (DI) water/nitric acid (64-66%, Duksan) = 10:0.9:0.12 mL with ultrasonication (60 min) and stirring (24 h).[44] The coating solutions were
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sequentially spin-coated onto FTO substrates (Pilkington, TEC-8) at 3000 rpm for 40 s. After each layer was coated, we performed intermediate annealing on a hot-plate at 350 °C for 10 min; the
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final annealing was performed at 500 °C for 1 h. The average film thickness per coating cycle was predetermined (~15-20 nm/cycle), and the optimal thickness for each film was determined from the photocurrent-potential (J-V) curves (Fig. S1). Material characterization The morphology and thickness of the films were examined using a field-emission scanning electron microscope (FE-SEM, S-4800, Hitachi). Transmission electron microscopy (TEM) images 5
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of the samples were obtained using a focused ion beam (FIB) system (JIB-4601F, Jeol) and crosssectional TEM and scanning TEM (STEM) images as well as energy dispersive X-ray spectroscopy (EDS) data of the photoanodes were obtained using a high-resolution TEM (JEM-2100F, Jeol). The crystal structure/phase of the photoanodes were examined using X-ray diffraction (XRD, Ultima III,
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Rigaku). The transmittance (T%) and reflectance (R%) spectra of the photoanodes were measured using a UV-Vis spectrometer (Cary 5000, Agilent Technologies). The absorption (A) was calculated from T and R (i.e., A = 100 − %T – %R).
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PEC measurements
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The PEC and electrochemical properties of all photoanodes were measured with a potentiostat (SP-200, Biologics) under a simulated solar light (AM1.5G, 100 mW/cm2) illumination using a solar simulator (Model 94011A, Newport). A standard three-electrode cell with an Ag/AgCl reference electrode (3M KCl, V° = 0.210 V), the photoanode (working electrode), and a Platinum wire counter electrode were employed. Before all measurements, we calibrated the intensity of solar
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simulator using a reference silicon diode (Mode 91150V, Newport). The active area of the working electrode was 0.18 cm2, as defined by a mask. We used 0.5 M phosphate buffer solution (PBS, pH
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7) or a 0.5 M phosphate buffer solution with H2O2 as a electrolyte. For J-V measurements, a potential scan rate of 50 mV/s was used and the potentials vs. Ag/AgCl were recorded with
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correction using the Nernst equation: VRHE = VAg/AgCl + (0.059 × pH) + 0.210 where VAg/AgCl is the applied bias potential and 0.210 corresponds to the reference potential of Ag/AgCl electrode with respect to RHE. Electrochemical impedance spectroscopy (EIS) spectra were recorded in three electrode configuration. The sinusoidal voltage-amplitude of 10 mV was used, and the frequency range recorded was from 7 MHz to 1 Hz. The incident photon to current conversion efficiency (IPCEs) was measured at 1.23 V vs. RHE using an IPCE system (Dongwoo Optron Co., Ltd). A 250 W tungsten-halogen lamp equipped with a monochromator (MonoRa150i, Dongwoo Optron Co., Ltd) was used to generate a monochromatic light. The charge transport/transfer efficiencies were 6
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determined by using a H2O2 under simulated solar-light irradiation.[45]
Results and Discussion Fabrication of TiO2/BiVO4/SnO2 (T/B/S) triple-layer photoanode As described in the experimental section, the T/B/S photoanode was prepared using the
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solution spin-coating method (Fig. 1a). A dense SnO2 layer with a thickness ~200 nm was deposited on a FTO substrate using a SnO2 coating solution (0.2 M, 9 cycles, annealed at 500 °C/1 h). Subsequently, a BiVO4 layer (0.1 M, 9 cycles, annealed at 500 °C/1 h) with thickness ~130 nm
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and a TiO2 nanoparticle layer with thickness ~100 nm (0.1 M, 6 cycles) were obtained by spincoating the respective coating solutions. Anatase TiO2 was formed after annealing at 500 °C for 1 h.
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Figure 1b shows a SEM image (cross-sectional) and a photograph of the fabricated T/B/S triplelayer photoanode. As observed, the layers of SnO2 (200 nm), BiVO4 (130 nm), and TiO2 (100 nm) are in intimate contact and free of voids and cracks. Notably, the BiVO4 layer exhibits larger grains (~300 nm in lateral size) compared to those of the SnO2 and TiO2 layers (~30 nm) (See Fig. S2).
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Additionally, the T/B/S photoanode is transparent to visible-light and exhibited bright-yellow color (inset of Fig. 1b). Figure 1c shows the XRD pattern of the T/B/S photoanode. All the peaks matched with those of rutile SnO2 (JCPDS: #41-1445), monoclinic BiVO4 (JCPDS: #75-1867), and
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anatase TiO2 (JCPDS: #21-1272), indicating the formation of three layers, i.e., SnO2, BiVO4, and TiO2 layers. Figure 1d shows the schematic band alignment of SnO2, BiVO4, and TiO2, as
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estimated from the Mott-Schottky measurement and band-gap values[35] (see Fig. S3). The SnO2 and BiVO4 layers form a type-II heterojunction, similar to the WO3 and BiVO4 case, promoting electron/hole separation and transfer between the layers.[40] In the case of the top TiO2 layer, it passivates the surface defects of the BiVO4 layer and protects it against photocorrosion.[29] Figure 2 shows the effect of the bottom SnO2 layer on the lateral grain size of the BiVO4 layer. Compared to the BiVO4 layer deposited on bare FTO substrate (lateral grain size ~110 nm), the BiVO4 deposited on the SnO2 nanoparticle film exhibits three times larger grains (~290 nm). 7
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This result indicates that the underlying SnO2 layer causes a decrease in the surface roughness as well as an increase in the surface energy of the substrate (See Fig. S4), leading to high wettability of BiVO4 droplets the contact, and eventually facilitating the grain growth of BiVO4.[28] The detailed microstructure and elemental distributions of the fabricated T/B/S photoanode
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were investigated using TEM and EDS, respectively (Fig. 3). As shown in the TEM image (Fig. 3a), the three distinct layers, which are in intimate contact, can be identified. The total thickness of the layers is ~450 nm, which is in agreement with the value obtained from the cross-sectional SEM
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image (Fig. 1b). In addition, the bottom SnO2 and top TiO2 layers are composed of nanoparticles smaller than those of the middle BiVO4 layer. To confirm the crystal structure and crystallinity,
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high-resolution TEM analysis was performed on each layer at spots A (SnO2), B (BiVO4), and C (TiO2) (Fig. 3b). The images show highly crystalline, interconnected SnO2 nanoparticles of size ~5– 30 nm. The high-resolution TEM image confirms the crystal structure of the SnO2 nanoparticles to be tetragonal rutile. The middle BiVO4 layer is found to be dense; the HR-TEM image confirms the
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formation of a highly crystalline BiVO4 layer with monoclinic scheelite structure. In addition, the HR-TEM image corresponding to spot C confirms the crystal structure of the top TiO2 layer to be anatase. Moreover, the STEM image (Fig. 3c) clearly shows the three distinct layers, and the EDS
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elemental maps confirm uniform elemental distribution within the layers. Notably, the good contact/ adhesion among the three layers without voids helps to inhibit interfacial recombination.
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Furthermore, the larger BiVO4 grains help to decrease bulk recombination of charge carriers.[46,
Optical properties
Figure 4a is a photograph showing the fabricated T/B/S, B/S, and BiVO4 (BVO) photoanodes. The T/B/S photoanode is bright yellow, while B/S and BVO photoanodes are greenish-yellow. The color variation is attributed to the optical interference in the layers, which 8
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have different thicknesses and refractive indices (Fig. S5), and this affects the optical properties of the fabricated photoanodes. We have analyzed the light absorption, reflectance, and transmittance spectra of the photoanodes. Figures 4b and 4c show transmittance and reflectance spectra, respectively, of the three photoanodes. Notably, the T/B/S photoanode shows higher transmittance
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and lower reflectance than those of B/S and BVO photoanodes at wavelengths above ~510 nm, which is beneficial for the construction of tandem PEC cells.[35] Additionally, the T/B/S photoanode exhibits a slightly higher light absorption than that of the other two photoanodes in the
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wavelength range 350–480 nm (Fig. 4d). The integrated light absorption value of the T/B/S photoanode is ~76%, which is higher than that of B/S (68%) and BVO (65%) photoanodes. As
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shown in Fig. S6, estimated band gap values of T/B/S photoanode was ~2.4 eV, which is comparable to the band gap value of pure BiVO4.
Photoelectrochemical performances
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We have examined the PEC performance of the T/B/S photoanode for water oxidation and compared them with those of B/S, BVO, and BiVO4/TiO2/SnO2 (B/T/S) photoanodes. Schematic structures of the four photoanodes are shown in Fig. 5a. Fig. 5b presents the photocurrent-potential
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(J-V) curves (the characteristic measurement for a PEC device) of the four photoanodes. The J-V curves of individual materials (i.e., BiVO4, TiO2, and SnO2 films) are shown in Fig. S7. Compared
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to the BVO (~0.07 mA/cm2 at 1.23 V vs. RHE), the B/S photoanode exhibits a photocurrent density (~2.1 mA/cm2 at 1.23 V vs. RHE) that is 30 times higher. As shown in Fig. 4c, there is little difference in the light absorption (i.e., charge generation) between samples. Therefore, the higher value is attributed to the formation of a heterojunction (type-II) between the BiVO4 and SnO2 layers of the B/S photoanode.[40] The electrons/holes generated from the BVO layer can be separated effectively by the internal electric field; subsequently, the electrons are transferred to the conduction band of SnO2 and holes are blocked by the SnO2 layer, thereby improving the charge separation and 9
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transport efficiencies.[28, 40] Additionally, the larger grain size of the BiVO4 layer in the B/S photoanode (see Fig. 2) causes a reduction in bulk recombination, which in turn leads to superior charge transport properties.[46] However, the T/B/S photoanode exhibits a slightly higher photocurrent density (~2.3 mA/cm2 at 1.23 V vs. RHE) than that of the B/S photoanode. As shown
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in Fig. S8, the IPCEs of T/B/S, B/S, and BVO photoanodes exhibit a similar trend as the J-V results. The integrated photocurrent-density values (shown in parenthesis) also correspond with those from the J-V results. On the other hand, for the B/T/S photoanode, with a TiO2 layer between
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the BiVO4 and SnO2 layers, the photocurrent density decreases drastically (~0.32 mA/cm2), indicating that the TiO2 layer should be deposited on the BiVO4 layer to obtain higher photocurrent
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density. The two-electrode J-V curve of the BVO photoanode was measured to estimate the performance in real application. As shown in Fig. S9, there was little differences in between 2- and 3-electrode measurements.
To understand the effects of the top TiO2 and bottom SnO2 layers, the charge transfer
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(ηtransfer) and transport efficiencies (ηtransport) of the four photoanodes have been estimated using a hole scavenger method[44, 45] (Figs. 5c and 5d). As shown in Fig. 5c, T/B/S and B/S photoanodes exhibit similar ηtransfer (~62%), which is much higher than those of B/T/S (~33%) and BVO (~5%)
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photoanodes. Similarly, T/B/S and B/S photoanodes exhibit ~2–4 times higher ηtransport (~72%) than those of B/T/S (~27%) and BVO photoanodes (~18%). These results confirm that the deposition of
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the bottom SnO2 layer in T/B/S and B/S photoanodes greatly improves both ηtransfer and ηtransport. The advantageous effect is ascribed to the formation of a type-II heterojunction between SnO2 and BiVO4 layers (similar to the case of BiVO4/WO3). In addition, the large grains in the BiVO4 layer improve charge separation/transport efficiencies and reduce bulk recombination. On the other hand, deposition of the TiO2 layer on top of BiVO4/SnO2 effectively improves the electron transport, resulting from the cascade alignment of the conduction-band edges (Fig. 1d & Fig. S10).[29] To further understand the superior PEC performance of T/B/S and B/S photoanodes, we have 10
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performed electrochemical impedance spectroscopy on the samples under 1 sun illumination. Figure 5e shows the Nyquist plots of the four photoanodes. The inset is a schematic equivalent circuit model, which was used to fit the EIS spectra; R1 is the series resistance, R2 is the bulk resistance, R3 is the charge transfer resistance, C1 is the chemical capacitance (= cµL, L: film
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thickness) of the photoanode, and C2 is Helmholtz layer capacitance[48, 49]. Table S1 summarized the extracted R1, R2, and R3 values; the data show that T/B/S and B/S photoanodes have much lower R2 and R3 values than those of B/T/S and BVO photoanodes. In addition, R2 and R3 of the
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T/B/S photoanode are slightly lower than those of the B/S photoanode. These results indicate that T/B/S and B/S photoanodes have superior charge transport and transfer properties, in accordance
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with the results of the ηtransfer and ηtransport measurements.
The PEC performance of the T/B/S photoanode has been investigated for H2O2 and Na2SO3 oxidation reactions, as well. Figure 6 shows the J-V curves of T/B/S, B/S, and BVO photoanodes. Both H2O2 and Na2SO3 act as hole scavengers during oxidation and the kinetics of
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peroxide and sulfite oxidation are thermodynamically much faster than that of water oxidation. Because of the fast kinetics of the reactions, even the BVO layer exhibits slightly high photocurrent density (~1.3 mA/cm2 at 1.23 V vs. RHE) for both H2O2 and Na2SO3 oxidations. Importantly, T/B/S
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and B/S photoanodes exhibit significantly high photocurrent density. In particular, the T/B/S photoanode shows a photocurrent density of ~3.7 mA/cm2 at 1.23 V vs. RHE for H2O2 oxidation.
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As a result, the T/B/S photoanode shows enhanced photocurrent density for water, H2O2, and sulfite oxidation reactions compared to the B/S and BVO photoanodes, which is ascribed to the improved charge transport and transfer efficiencies resulting from the intimate contact, type-II heterojunction, and larger grains of BiVO4. We have also evaluated the photocurrent stability of T/B/S and B/S photoanodes under simulated solar-light irradiation (Fig. 7). As shown in Fig. 7a, the T/B/S photoanode exhibited a more stable photocurrent density than that of the B/S photoanode for 5 h. This result indicates that 11
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the top TiO2 layer plays a crucial role in protecting the BiVO4 against photocorrosion/degradation. The SEM images obtained after the stability test (Figs. 7b & 7c) show that the portion (or thickness) of the remaining BiVO4 layer in the T/B/S photoanode is much larger compared to that in the B/S photoanode. This result clearly indicates that the top TiO2 layer helps in stagnating the degradation
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of the BiVO4 layer. However, the BiVO4 layer in the T/B/S photoanode has undergone some degradation during the stability test, indicating that the coverage of the TiO2 layer is not perfect (Fig. S11). Therefore, some other TiO2 deposition method (e.g., atomic layer deposition, ALD)
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and/or electrocatalyst coating, needs to be explored to achieve higher stability for the T/B/S photoanode. Lastly, the T/B/S photoanode also showed a high faradaic efficiency over 90% for the
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water-oxidation (Fig. S12).
Conclusion
In summary, we prepared a triple-layered TiO2/BiVO4/SnO2 (T/B/S) planar-type photoanode
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that showed enhanced visible-light transmittance and PEC performance. The T/B/S was fabricated by a solution spin-coating route. A bottom SnO2 layer (~200 nm thick) was first coated on FTO substrates. Subsequently, a middle BiVO4 layer (~130 nm) and a top TiO2 nanoparticle layer (~100
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nm) were deposited. From the TEM and EDS results, we confirmed the formation of three distinct, highly crystalline layers in intimate contact and free of voids and cracks. Additionally, we found
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that the bottom SnO2 layer not only caused an increase in the lateral grain size of the BiVO4 layer but also formed a type-II heterojunction with the layer, which led to a great improvement in charge separation and electron transport properties. Furthermore, the top TiO2 layer protected the BiVO4 layer against photocorrosion. The optimized T/B/S photoanode exhibited high photocurrent densities (~2.3 mA/cm2 and 3.7 mA/cm2 at 1.23 V vs. RHE for water oxidation and H2O2 oxidation, respectively). The T/B/S photoanode also exhibited relatively stable photocurrent density for 5 h under simulated solar light irradiation compared to those of BiVO4/SnO2 and BiVO4. We believe 12
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that the design and development of heterostructures with multi-layered films for oxide-based photoanodes is a facile and effective route to achieve a simultaneous enhancement in charge
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transport/transfer efficiencies, thus allowing the fabrication of efficient PEC water-splitting device.
Acknowledgments
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This research was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF2015R1C1A1A01053785).
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Figure 1. Fabrication and characterization of TiO2/BiVO4/SnO2 (T/B/S) triple-layer photoanode. (a) Schematic of fabrication. (b) Cross-sectional SEM image and photograph of the T/B/S photoanode. (c) XRD pattern of the T/B/S photoanode. (d) Band alignment of the three layers.
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Figure 2. Effect of the bottom SnO2 layer on the lateral grain size of the BiVO4 layer. Top-view SEM images and statistical grain size distribution of (a) the BiVO4 layer directly deposited on FTO and (b) the BiVO4 layer deposited on the SnO2 layer.
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Figure 3. TEM characterization of the TiO2/BiVO4/SnO2 triple-layer photoanode. (a) The cross-sectional TEM image and (b) Magnified TEM images corresponding to the spots A, B, and C in (a). Insets are high-resolution TEM images (scale bar = 2 nm). (c) Dark-field STEM image and EDS elemental maps.
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Figure 4. Optical properties of the photoanodes. (a) A photograph showing T/B/S, B/S, and BVO photoanodes with variation in color and visible transparency. (b) Transmittance, (c) Reflectance and (d) Light absorption spectra (= 100 –%R–%T). The corresponding integrated light absorption values are given in parentheses in (d).
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Figure 5. Water oxidation: PEC characterization of T/B/S, B/S, BVO, and B/T/S photoanodes. (a) A schematic layer structure of the photoanodes. (b) Typical photocurrent-potential (J-V) curves. (c) Charge transfer efficiency. (d) Charge transport efficiency. (e) Electrochemical impedance spectra (Nyquist plots); inset is the schematic equivalent circuit model.
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Figure 6. J-V curves of T/B/S, B/S, and BVO photoanodes. (a) H2O2 oxidation and (b) Na2SO3 oxidation reactions performed under AM1.5G (100 mW/cm2) simulated solar light illumination.
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Figure 7. (a) Result of the photocurrent stability test performed on T/B/S and B/S photoanodes for 4 h under simulated solar light illumination (measured at 1 V vs. Pt). The inset is a photograph of the wrapped with epoxy resin. Top-view SEM images of (b) B/S photoanode and (c) T/B/S photoanode obtained after J-t curve measurement for 2 and 4 h, respectively.
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References
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[1] M. Grätzel, Photoelectrochemical cells, nature, 414 (2001) 338. [2] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Solar water splitting cells, Chemical reviews, 110 (2010) 6446-6473. [3] A.J. Bard, M.A. Fox, Artificial photosynthesis: solar splitting of water to hydrogen and oxygen, Accounts of Chemical Research, 28 (1995) 141-145. [4] Y. Tachibana, L. Vayssieres, J.R. Durrant, Artificial photosynthesis for solar water-splitting, Nature Photonics, 6 (2012) 511. [5] C. Jiang, S.J. Moniz, A. Wang, T. Zhang, J. Tang, Photoelectrochemical devices for solar water splitting– materials and challenges, Chemical Society Reviews, 46 (2017) 4645-4660. [6] K. Sivula, R. Van De Krol, Semiconducting materials for photoelectrochemical energy conversion, Nature Reviews Materials, 1 (2016) 15010. [7] M.D. Bhatt, J.S. Lee, Recent theoretical progress in the development of photoanode materials for solar water splitting photoelectrochemical cells, Journal of Materials Chemistry A, 3 (2015) 10632-10659. [8] H.M. Chen, C.K. Chen, R.-S. Liu, L. Zhang, J. Zhang, D.P. Wilkinson, Nano-architecture and material designs for water splitting photoelectrodes, Chemical Society Reviews, 41 (2012) 5654-5671. [9] T.W. Kim, K.-S. Choi, Nanoporous BiVO4 photoanodes with dual-layer oxygen evolution catalysts for solar water splitting, Science, (2014) 1245026. [10] Y. Park, K.J. McDonald, K.-S. Choi, Progress in bismuth vanadate photoanodes for use in solar water oxidation, Chemical Society Reviews, 42 (2013) 2321-2337. [11] J.H. Kim, J.S. Lee, BiVO4-based heterostructured photocatalysts for solar water splitting: a review, Energy and Environment Focus, 3 (2014) 339-353. [12] A.J.E. Rettie, H.C. Lee, L.G. Marshall, J.-F. Lin, C. Capan, J. Lindemuth, J.S. McCloy, J. Zhou, A.J. Bard, C.B. Mullins, Combined Charge Carrier Transport and Photoelectrochemical Characterization of BiVO4 Single Crystals: Intrinsic Behavior of a Complex Metal Oxide, Journal of the American Chemical Society, 135 (2013) 11389-11396. [13] F.F. Abdi, T.J. Savenije, M.M. May, B. Dam, R. van de Krol, The Origin of Slow Carrier Transport in BiVO4 Thin Film Photoanodes: A Time-Resolved Microwave Conductivity Study, The Journal of Physical Chemistry Letters, 4 (2013) 2752-2757. [14] C.X. Kronawitter, L. Vayssieres, S. Shen, L. Guo, D.A. Wheeler, J.Z. Zhang, B.R. Antoun, S.S. Mao, A perspective on solar-driven water splitting with all-oxide hetero-nanostructures, Energy & Environmental Science, 4 (2011) 3889-3899. [15] J. Yin, F. Di, J. Guo, K. Zhang, W. Xu, Y. Wang, S. Shi, N. Chai, C. Chu, J. Wei, W. Li, X. Shao, X. Pu, D. Zhang, X. Ren, J. Wang, J. Zhao, X. Zhang, X. Wei, F. Wang, H. Zhou, Tuning Ni-Foam into NiOOH/FeOOH Heterostructures toward Superior Water Oxidation Catalyst via Three-Step Strategy, ACS Omega, 3 (2018) 11009-11017. [16] J. Huang, X. Deng, H. Wan, F. Chen, Y. Lin, X. Xu, R. Ma, T. Sasaki, Liquid Phase Exfoliation of MoS2 Assisted by Formamide Solvothermal Treatment and Enhanced Electrocatalytic Activity Based on (H3Mo12O40P/MoS2)n Multilayer Structure, ACS Sustainable Chemistry & Engineering, 6 (2018) 5227-5237. [17] E. Ahn, B.-S. Kim, Multidimensional Thin Film Hybrid Electrodes with MoS2 Multilayer for Electrocatalytic Hydrogen Evolution Reaction, ACS Applied Materials & Interfaces, 9 (2017) 8688-8695. [18] H. Gao, H. Yang, J. Xu, S. Zhang, J. Li, Strongly Coupled g-C3N4 Nanosheets-Co3O4 Quantum Dots as 2D/0D Heterostructure Composite for Peroxymonosulfate Activation, Small, 14 (2018) 1801353. [19] S. Zhang, H. Gao, Y. Huang, X. Wang, T. Hayat, J. Li, X. Xu, X. Wang, Ultrathin g-C3N4 nanosheets coupled with amorphous Cu-doped FeOOH nanoclusters as 2D/0D heterogeneous catalysts for water remediation, Environmental Science: Nano, 5 (2018) 1179-1190. [20] S. Zhang, H. Yang, H. Huang, H. Gao, X. Wang, R. Cao, J. Li, X. Xu, X. Wang, Unexpected ultrafast and high adsorption capacity of oxygen vacancy-rich WOx/C nanowire networks for aqueous Pb2+ and methylene blue removal, Journal of Materials Chemistry A, 5 (2017) 15913-15922. [21] H. Liu, A. Wang, Q. Sun, T. Wang, H. Zeng, Cu Nanoparticles/Fluorine-Doped Tin Oxide (FTO) Nanocomposites for Photocatalytic H2 Evolution under Visible Light Irradiation, Catalysts, 7 (2017) 385. 21
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[22] Z. Zhan, F. Grote, Z. Wang, R. Xu, Y. Lei, Degenerating Plasmonic Modes to Enhance the Performance of Surface Plasmon Resonance for Application in Solar Energy Conversion, Advanced Energy Materials, 5 (2015) 1501654. [23] Z. Zhan, Y. Wang, Z. Lin, J. Zhang, F. Huang, Study of interface electric field affecting the photocatalysis of ZnO, Chemical Communications, 47 (2011) 4517-4519. [24] P.M. Rao, L. Cai, C. Liu, I.S. Cho, C.H. Lee, J.M. Weisse, P. Yang, X. Zheng, Simultaneously efficient light absorption and charge separation in WO3/BiVO4 core/shell nanowire photoanode for photoelectrochemical water oxidation, Nano letters, 14 (2014) 1099-1105. [25] S.J. Hong, S. Lee, J.S. Jang, J.S. Lee, Heterojunction BiVO 4/WO 3 electrodes for enhanced photoactivity of water oxidation, Energy & Environmental Science, 4 (2011) 1781-1787. [26] L. Zhou, C. Zhao, B. Giri, P. Allen, X. Xu, H. Joshi, Y. Fan, L.V. Titova, P.M. Rao, High light absorption and charge separation efficiency at low applied voltage from Sb-doped SnO2/BiVO4 core/shell nanorod-array photoanodes, Nano letters, 16 (2016) 3463-3474. [27] Y. Liang, T. Tsubota, L.P. Mooij, R. van de Krol, Highly improved quantum efficiencies for thin film BiVO4 photoanodes, The Journal of Physical Chemistry C, 115 (2011) 17594-17598. [28] S. Byun, B. Kim, S. Jeon, B. Shin, Effects of a SnO 2 hole blocking layer in a BiVO 4-based photoanode on photoelectrocatalytic water oxidation, Journal of Materials Chemistry A, 5 (2017) 6905-6913. [29] A.P. Singh, N. Kodan, B.R. Mehta, A. Held, L. Mayrhofer, M. Moseler, Band edge engineering in BiVO4/TiO2 heterostructure: enhanced photoelectrochemical performance through improved charge transfer, ACS Catalysis, 6 (2016) 5311-5318. [30] B.-Y. Cheng, J.-S. Yang, H.-W. Cho, J.-J. Wu, Fabrication of an efficient BiVO4–TiO2 heterojunction photoanode for photoelectrochemical water oxidation, ACS applied materials & interfaces, 8 (2016) 2003220039. [31] R. Tong, X. Wang, X. Zhou, Q. Liu, H. Wang, X. Peng, X. Liu, Z. Zhang, H. Wang, P.D. Lund, CobaltPhosphate modified TiO2/BiVO4 nanoarrays photoanode for efficient water splitting, International Journal of Hydrogen Energy, 42 (2017) 5496-5504. [32] W. Ye, F. Chen, F. Zhao, N. Han, Y. Li, CuWO4 Nanoflake Array-Based Single-Junction and Heterojunction Photoanodes for Photoelectrochemical Water Oxidation, ACS Applied Materials & Interfaces, 8 (2016) 9211-9217. [33] X. Chang, T. Wang, P. Zhang, J. Zhang, A. Li, J. Gong, Enhanced surface reaction kinetics and charge separation of p–n heterojunction Co3O4/BiVO4 photoanodes, Journal of the American Chemical Society, 137 (2015) 8356-8359. [34] R. Saito, Y. Miseki, K. Sayama, Highly efficient photoelectrochemical water splitting using a thin film photoanode of BiVO 4/SnO 2/WO 3 multi-composite in a carbonate electrolyte, Chemical Communications, 48 (2012) 3833-3835. [35] J.H. Baek, B.J. Kim, G.S. Han, S.W. Hwang, D.R. Kim, I.S. Cho, H.S. Jung, BiVO4/WO3/SnO2 doubleheterojunction photoanode with enhanced charge separation and visible-transparency for bias-free solar water-splitting with a perovskite solar cell, ACS applied materials & interfaces, 9 (2017) 1479-1487. [36] J. Su, L. Guo, N. Bao, C.A. Grimes, Nanostructured WO3/BiVO4 heterojunction films for efficient photoelectrochemical water splitting, Nano letters, 11 (2011) 1928-1933. [37] J. Resasco, H. Zhang, N. Kornienko, N. Becknell, H. Lee, J. Guo, A.L. Briseno, P. Yang, TiO2/BiVO4 Nanowire Heterostructure Photoanodes Based on Type II Band Alignment, ACS Central Science, 2 (2016) 80-88. [38] F. Zhan, J. Li, W. Li, Y. Liu, R. Xie, Y. Yang, Y. Li, Q. Chen, In situ formation of CuWO 4/WO 3 heterojunction plates array films with enhanced photoelectrochemical properties, International Journal of Hydrogen Energy, 40 (2015) 6512-6520. [39] Y. Pihosh, I. Turkevych, K. Mawatari, J. Uemura, Y. Kazoe, S. Kosar, K. Makita, T. Sugaya, T. Matsui, D. Fujita, Photocatalytic generation of hydrogen by core-shell WO3/BiVO4 nanorods with ultimate water splitting efficiency, Scientific reports, 5 (2015). [40] P. Chatchai, Y. Murakami, S.-y. Kishioka, A. Nosaka, Y. Nosaka, FTO∕ SnO2∕ BiVO4 composite photoelectrode for water oxidation under visible light irradiation, Electrochemical and Solid-State Letters, 11 (2008) H160-H163. 22
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[41] S.-Y. Chen, J.-S. Yang, J.-J. Wu, Three-Dimensional Undoped Crystalline SnO2 Nanodendrite Arrays Enable Efficient Charge Separation in BiVO4/SnO2 Heterojunction Photoanodes for Photoelectrochemical Water Splitting, ACS Applied Energy Materials, 1 (2018) 2143-2149. [42] H. Liu, K. Zhao, T. Wang, J. Deng, H. Zeng, Facile preparation of cerium (Ce) and antimony (Sb) codoped SnO2 for hydrogen production in lactic acid solution, Materials Science in Semiconductor Processing, 40 (2015) 670-675. [43] H.S. Han, S. Shin, D.H. Kim, I.J. Park, J.S. Kim, P.-S. Huang, J.-K. Lee, I.S. Cho, X. Zheng, Boosting the solar water oxidation performance of a BiVO 4 photoanode by crystallographic orientation control, Energy & Environmental Science, 11 (2018) 1299-1306. [44] J.U. Kim, H.S. Han, J. Park, W. Park, J.H. Baek, J.M. Lee, H.S. Jung, I.S. Cho, Facile and controllable surface-functionalization of TiO2 nanotubes array for highly-efficient photoelectrochemical water-oxidation, Journal of Catalysis, 365 (2018) 138-144. [45] H. Dotan, K. Sivula, M. Grätzel, A. Rothschild, S.C. Warren, Probing the photoelectrochemical properties of hematite (α-Fe2O3) electrodes using hydrogen peroxide as a hole scavenger, Energy & Environmental Science, 4 (2011) 958-964. [46] M. Zhong, T. Hisatomi, T. Minegishi, H. Nishiyama, M. Katayama, T. Yamada, K. Domen, Bulky crystalline BiVO 4 thin films for efficient solar water splitting, Journal of Materials Chemistry A, 4 (2016) 9858-9864. [47] J. Feng, X. Zhao, S.S.K. Ma, D. Wang, Z. Chen, Y. Huang, Fast and Simple Construction of Efficient SolarWater-Splitting Electrodes with Micrometer-Sized Light-Absorbing Precursor Particles, Advanced Materials Technologies, 1 (2016) 1600119. [48] T. Lopes, L. Andrade, H.A. Ribeiro, A. Mendes, Characterization of photoelectrochemical cells for water splitting by electrochemical impedance spectroscopy, International Journal of Hydrogen Energy, 35 (2010) 11601-11608. [49] J.R. Macdonald, E. Barsoukov, Impedance spectroscopy: theory, experiment, and applications, History, 1 (2005).
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Highlight • Preparation of triple-layered TiO2/BiVO4/SnO2 (T/B/S) photoanode by solution spin-coating method • The T/B/S photoanode showed improved photoelectrochemical and optical properties
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• SnO2 increases lateral grain-size of BiVO4 and improves charge separation efficiency • TiO2 forms staggered conduction-band-edge with BiVO4 and protect underlying layers
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• High photocurrent densities (2.3 mA/cm2 at 1.23 VRHE) and stability are demonstrated