Rational Design of Branched WO3 Nanorods Decorated with BiVO4

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Cite This: ACS Appl. Energy Mater. 2019, 2, 4535−4543

Rational Design of Branched WO3 Nanorods Decorated with BiVO4 Nanoparticles by All-Solution Processing for Efficient Photoelectrochemical Water Splitting Jae-Hyeok Kim,† Do Hong Kim,† Ji Won Yoon,† Zhengfei Dai,‡ and Jong-Heun Lee*,† †

Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China

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‡

S Supporting Information *

ABSTRACT: The formation of heterostructure between BiVO4 and WO3 is a promising strategy to design a high-performance photoanode. In this study, we prepared the highly crystalline branched WO3 nanorods decorated with BiVO4 nanoparticles by all-solution processes and achieved high photoelectrochemical (PEC) performances through the morphological design of WO3 bottom layer and BiVO4 decorations. WO3 nanorods with epitaxially grown nanobranches could be prepared via two-step hydrothermal method, and the BiVO4/WO3 heterostructure was formed by sequent electrodeposition of BiVO4 nanoparticles on branched WO3 nanorods. In comparison to bare WO3 nanorods counterpart, the macelike branched WO3 nanorods can present the enlarged surface area and improved light trapping properties from the morphological control of WO3 hierarchical nanostructures, endowing a 32.8% higher photocurrent around 0.85 mA/ cm2 at 1.23 V vs reversible hydrogen electrode (RHE). While decorated with BiVO4 nanoparticles, the as-fabricated BiVO4/ macelike WO3 nanorod heterostructure performs a much improved photocurrent of 3.87 mA/cm2 at 1.23 V vs RHE. Such a significant enhancement may result from the significantly enhanced light-harvesting and charge separation efficiency. This rational design of heterostructured photoanodes provides a facile, cost-effective, and scalable strategy to improve PEC performances. KEYWORDS: branched WO3 nanorod, BiVO4, type II heterojunction, all-solution process, photoelectrochemical water splitting

1. INTRODUCTION

The construction of BiVO4/WO3 heterostructure was reported as one of the most effective ways to improve the electron−hole separation and transport, because of its type II band alignment at the interfaces.33,38,39 For the heterostructure, WO3 will provide conductive pathway for charge transfer, whereas BiVO4 could absorb broad visible spectrum to facilitate light harvesting.32,40 Hence, the structural configuration of WO3 would play an essential role in the PEC performance of BiVO4/ WO3 composites. Among various WO3 nanoarchitectures, the single-crystalline one-dimensional WO3 nanorod is regarded as a promising candidate which can provide an enlarged surface area to maximize active sites and direct conduction pathways for generated charges with reduced charge recombination.41,42 By integrating abundant branches on nanorods, photocatalytic activity can be further enhanced owing to its maximized redoxactive sites and increased light-trapping characteristics, as revealed by previous efforts.43−46 In recent years, various

Hydrogen energy is deemed as the most environmental-friendly energy source to substitute fossil fuel.1 Among various hydrogen-harvesting techniques, photoelectrochemical (PEC) water splitting using a semiconductor electrode is considered ideal with its simple, direct, clean, and sustainable process.2−6 This concern has in turn spawned a flurry of research activity to search favorable photoelectrode materials for efficient photoenergy deployment.7−10 Bismuth vanadate (BiVO4) with a band gap of 2.4 eV is a widely investigated material because of its favorable band positions and narrow band gap energy for effective visible spectrum absorption.11−16 However, short carrier diffusion length and poor charge transfer have been key obstacles for BiVO4 PEC performances. To tackle this issue, many approaches have been used to improve photoactivity of BiVO4, mainly focusing on doping electron donors,11,17−19 introducing catalysts to promote oxygen evolution reaction,20−26 inserting hole extraction layer,27 designing morphology for reducing carrier transit distance,28,29 and forming heterostructure with other oxide materials.30−37 © 2019 American Chemical Society

Received: April 17, 2019 Accepted: May 29, 2019 Published: May 29, 2019 4535

DOI: 10.1021/acsaem.9b00776 ACS Appl. Energy Mater. 2019, 2, 4535−4543

Article

ACS Applied Energy Materials

Figure 1. Schematic diagram illustrating the formation of WO3 NRs, branched WO3 NRs, and BiVO4/branched WO3 NRs on FTO/glass substrate, respectively. maintained at 5 × 10−6 Torr. The E-beam voltage and current were 7.0 kV, and 30 mA, respectively. The samples were converted to crystalline phase by annealing at 550 °C for 2 h in air. WO3 nanorods were grown on WO3-seeded FTO/glass substrate by hydrothermal reaction. In a typical synthesis, 0.0693 g of ammonium paratungstate ((NH4)10H2(W2O7)6, 99.99%, Sigma-Aldrich) was dissolved in 28.5 mL of deionized (DI) water. Then 3 mL of hydrochloric acid (HCl, 35−37%, Samchun chemical) and 1 mL of hydrogen peroxide (H2O2, 30%, JUNSEI) were added to above aqueous solution in sequence. The solution was transferred to a Teflon-lined autoclave (volume: 100 mL) and the seed-layer-coated FTO substrate was placed in the middle of Teflon-liner with the conducting side facing down. The hydrothermal synthesis was conducted at 180 °C for 6−48 h to grow WO3 nanorods. In addition, branched WO3 nanorods were also synthesized by two-step hydrothermal process. The secondary hydrothermal reaction was performed at 180 °C for 6 h using the same precursor solution for the growth of WO3 branches. After the reaction, the sample was rinsed with DI water and annealed at 550 °C for 2 h to prepare crystalline monoclinic phase of WO3 nanorods and to remove impurities. 2.2. Preparation of BiVO4/Branched WO3 Nanorod Heterostructure. The BiVO4 particles were coated on the branched WO3 nanorods by electrodeposition. Electrodeposition was performed in a strandard three-electrode system. The branched WO3 nanorods on FTO/glass substrate, a platinum (Pt) mesh, and Ag/AgCl were used as the working electrode, counter electrode, and reference electrode, respectively. Precursor solution was prepared by dissolving 1.141 g of vanadium oxide sulfate hydrate (VOSO4·xH2O, 99.9%, Alfa Aesar), 10 mL of nitric acid (HNO3, 60%, Samchun Chemical), 0.97014 g of bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, 98.0%, JUNSEI), and 32.812 g of sodium acetate (CH3COONa·3H2O, 98.5%, JUNSEI) in an aqueous solution of 190 mL of DI water. Amorphous Bi−V−O layer was deposited potentiostatically at 0.8 V vs Ag/AgCl for 200 s at room temperature. The sample was annealed at 500 °C for 6 h to convert the coating layer into a crystalline monoclinic phase of BiVO4. 2.3. Materials Characterization. The morphologies of obtained WO3 nanorods, branched WO3 nanorods, and BiVO4/branched WO3 nanorods were characterized by field emission scanning electron microscope (FESEM, SU-70, Hitachi Co. Ltd.). The crystallinity and phase of the sample were investigated by X-ray diffraction (XRD, Cu Kα, D/MAX-2500 V/PC, Rigaku) and high-resoution transmission electron microscopy (HR-TEM, TALOS F200X, FEI Co. Ltd.). The optical absorption spectra of the photoanode were measured using UV−vis spectroscopy (UV−vis, Jasco V-650). 2.4. Photoelectrochemical (PEC) Measurements. PEC measurements were carried out in 1 M sodium sulfite (Na2SO3) aqueous solution containing 0.5 M potassium phosphate buffer (pH 7.3) using a potentiostat (Ivium Technologies). Sodium sulfite, which is kinetically and thermodynamically more oxidative than water, is known as the effective holes scavenger. This electrolyte enables the investigation of inherent PEC performances of BiVO4-based photoanodes with poor water oxidation kinetics.11,52 In this perspective, there has been research

synthetic routes have been employed to prepare branched hierarchical WO3 nanorods, including thermal evaporation of W powders,47 hot electrospinning with polystyrene-colloid-template,48 and hot-wire chemical vapor deposition.49 Although these methods could provide relatively acceptable control of hierarchical morphology, the demand of high thermal energy and/or vacuum environment hampers the cost-effective synthesis of large photoelectrochemical cells, which are critically important for real applications. In stark contrast, two-step hydrothermal reaction can be a promising and viable alternative that can synthesize highly crystalline branched nanostructures with facile, cost-effective, and scalable manner.50,51 However, to the best of our knowledge, there was no research about the design of branched WO3 nanorods for photoanodes using twostep hydrothermal reaction. Note that light-absorbing BiVO4 overlayer can be also coated by solution-based electrodeposition. This enables us to prepare branched WO3 nanorods uniformly decorated with BiVO4 nanoparticles via all-solution processing. In the present study, high-PEC performances were achieved by controlling the length of WO3 nanorods and subsequent epitaxial growth of nanobranches using two-step hydrothermal reaction, which were further enhanced through the establishment of type II heterostructure between nanocrystalline BiVO4 and branched WO3 nanorods via simple solution-based electrodeposition. In comparison to the bare WO3 nanorod counterpart, the macelike branched WO3 nanorods present the enlarged surface area and improved light-trapping properties from the morphological control of WO3 hierarchical nanostructures, endowing a 32.8% higher photocurrent around 0.85 mA/ cm 2 at 1.23 V vs RHE. While decorated with BiVO 4 nanoparticles, the as-fabricated BiVO4/macelike WO3 nanorod heterostructure performs a much improved photocurrent of 3.87 mA/cm2 at 1.23 V vs RHE. The origin of high photocurrent density was investigated and discussed by incident photon-toelectron conversion efficiency (IPCE) and electrochemical impedance spectroscopy (EIS). This result provides a rational design of hetero-nanoarchitectures to improve photoanode efficiency with all-solution based process for a wide range of applications.

2. EXPERIMENTAL SECTION 2.1. Synthesis of WO3 Nanorods. A thin (∼50 nm) WO3 seed layer was deposited on the fluorine-doped tin oxide (FTO)/glass substrate using electron-beam evaporator (Rocky Mountatin Vacuum Tech, Englewood, CO). The base pressure in the vacuum chamber was 4536

DOI: 10.1021/acsaem.9b00776 ACS Appl. Energy Mater. 2019, 2, 4535−4543

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Figure 2. Cross-sectional SEM images of WO3 NRs grown on FTO/glass substrates with hydrothermal times of (a) 6, (c) 12, and (e) 24 h. Plain-view SEM images of WO3 NRs grown on FTO/glass substrates with hydrothermal time of (b) 6, (d) 12, and (f) 24 h. Cross-sectional (g) and plain-view (h) of SEM images of branched WO3 NRs grown on FTO/glass substrates. to use Na2SO3-contained electrolyte in order to analyze WO3/BiVO4 heterostructures in the literature.33,52,53 A Xe lamp was used as a light source, and the light intensity was calculated to 1 sun (AM 1.5G filter, 100 mW/cm2) using a reference silicon (Si) photodiode. The incident photon-to-current conversion efficiency (IPCE) was measured by a monochromator (MonoRa150) under illumination. The potential versus reversible hydrogen electrode (ERHE) was measured from the following Nernst equation54 0 E RHE = EAg/AgCl + EAg/AgCl + 0.059*pH

(1)

where E0Ag/AgCl = 0.1976 V and EAg/AgCl is the measured potential against Ag/AgCl reference. Electrochemical impedance spectroscopy (EIS) was measured in applied voltage of 0.56 V versus Ag/AgCl near the onset potential. The frequency range was from 100 kHz to 1 Hz using alternating current with an amplitude of 10 mV, and the data were fitted using the Z plot 2.x software. Rs (series resistance), Cct (constant phase element; CPE), and Rct (charge-transfer resistance at the interface between the electrode and the electrolyte) values of the circuit were obtained from the fitting and analyzing of EIS curves.

3. RESULTS AND DISCUSSION Figure 1 shows the schematic diagram of the synthetic process for the BiVO4/branched WO3 nanorods (NRs) heterostructure via two-step hydrothermal reaction followed by electrodeposition. Optical photographs of as-prepared WO3 NRs, branched WO3 NRs and BiVO4/branched WO3 NRs photoanodes are shown in Figure S1. After annealing, each sample exhibits homogeneous color distribution throughout the entire film surface, which implies that highly uniform photoanodes can be successfully prepared by hydrothermal and further electrodeposition procedure. X-ray diffraction (XRD) patterns show that monoclinic WO3 nanorods (JCPDS no. 32-1395) are prepared by the single-step hydrothermal reaction (Figure S2a). To control the length and diameter of nanorods, the reaction time was varied from 6 to 48 h. All the samples showed uniform surface coverage (Figure 2 and Figure S3). The lengths of WO3 nanorods grown for 6, 12, and 24 h were estimated to be ∼0.6, 1.5, and 3.0 μm (Figure 2a− f). However, further increase of reaction time to 36 and 48 h did not lead to the growth of nanorods (Figure S3). Accordingly, the optimal reaction time was fixed to 24 h. The crystal structure of WO3 nanorods was confirmed again by high-resolution TEM analysis, as shown in Figure 3a,b. The observed fringe spacings of 0.376 and 0.185 nm corresponds to (02̅0) and (301̅) planes of monoclinic WO3, respectively. The single crystallinity of nanorods is advantageous to minimize the electron−hole recombination by reducing surface defects and grain boundaries,

Figure 3. TEM image of (a) WO3 NRs and (c) branched WO3 NRs. High-resolution TEM image with indexing. Inset: SAED pattern of (b) WO3 NRs and (d) branched WO3 NRs with indexing.

which can provide highly conductive pathways for charge transportation. PEC performances of WO3 nanorods were measured in 1 M sodium sulfite electrolyte containing 0.5 M potassium phosphate buffer. We used front-side illumination for measuring the PEC properties of the WO3 nanorods. The photocurrent density is observed to increase along with the nanorod length from 0.6 to 3.0 μm (Figure 4a). The highest photocurrent density is 0.64 mA/cm2 at 1.23 V vs RHE when the nanorods are 3.0 μm long. The higher photocurrent density in longer WO3 nanorods can be explained by high surface area for photoexcitation and consequent promotion of interface reaction between oxide electrode and electrolytes.55 The photocurrent density of WO3 nanorods grown for 24, 36, and 48 h were similar (Figure S4), confirming again that PEC performance is primarily dependent upon the nanorod length. The results suggest that the hydrothermal method is a promising synthetic route to control and design photoanodes performances. By second hydrothermal reaction, a number of branches were grown on WO3 nanorods forming the macelike heterostructure (Figure 2g,h), which can be explained by the nucleation at the 4537

DOI: 10.1021/acsaem.9b00776 ACS Appl. Energy Mater. 2019, 2, 4535−4543

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Figure 4. (a) J−V curves of WO3 NRs prepared with different thicknesses under calibrated 1 sun (100 mV/cm2) illumination. Inset: Current density of WO3 NRs with thicknesses of 0.6, 1.5, and 3.0 μm at 1.23 V versus RHE. (b) Comparison of photocurrent density between WO3 NRs and branched WO3 NRs at 1.23 V versus RHE. (c) Amperometric current density-time profiles for WO3 NRs and branched WO3 NRs. (d) IPCE measurements of WO3 NRs and branched WO3 NRs.

Figure 5. (a) Cross-sectional SEM image of BiVO4/branched WO3 NRs. (b) Plain-view SEM image of BiVO4/branched WO3 NRs. (c) TEM image of BiVO4/branched WO3 NRs. (d) High-resolution TEM image with indexing. Inset: FFT image of BiVO4/branched WO3 NRs with indexing. (e) EDS element maps of W, O, Bi, and V. (f) XRD pattern of BiVO4/branched WO3 NRs.

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DOI: 10.1021/acsaem.9b00776 ACS Appl. Energy Mater. 2019, 2, 4535−4543

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Figure 6. (a) Comparison of photocurrent density between branched WO3 NRs and BiVO4/branched WO3 NRs at 1.23 V versus RHE. (b) Amperometric current density−time profiles for branched WO3 NRs and BiVO4/branched WO3 NRs. (c) IPCE measurements of branched WO3 NRs and BiVO4/branched WO3 NRs. (d) EIS analysis of WO3 NRs, branched WO3 NRs, and BiVO4/branched WO3 NRs.

photoexcitation reaction. In addition, the promoting effect of branched structures on light scattering61 is also supported by the literature that branched 1D nanorods exhibited enhanced light trapping and scattering capability.57,62,63 Finally, the creation of new branches should not affect the charge transport. In general, grain boundaries between the different direction of crystals are known to hinder the carrier transport across the adjacent crystals.45,62 However, in the present study, no distinctive interfaces between nanorods and nanobranches were found because of epitaxial branch growth, and thus high charge transport efficiency can be maintained. For further enhancement of PEC performances, WO3, and BiVO4 heterojunction with type II band alignment was employed (Figure S6). The appropriate design of band structure with proper band gap energies of WO3 and BiVO4 enables the fluent transport of holes to the film surface and photoelectrons to the charge collector. In this context, amorphous Bi−V−O layer deposited on WO3 nanorods by electrodeposition was converted into a discrete configuration of crystalline BiVO4 nanoparticles by thermal annealing at 500 °C for 6 h. This is confirmed by the X-ray diffraction (Figure 5f). At a glance, it was difficult to find the existence of the BiVO4 nanoparticles simply from SEM images (Figure 5a,b). Accordingly, the sample was further analyzed using TEM (Figure 5c,d). The BiVO4 nanoparticles showed the size distribution of 60−100 nm which are uniformly decorated on the WO3 nanorod surfaces (Figure 5e). Near the WO3/BiVO4 interfaces, the fringe spacing of 0.365 and 0.376 nm matches the (200) and (02̅0) planes of monoclinic WO3, whereas fringe spacing of 0.312 nm corresponds to the (103) lattice plane of BiVO4 (Figure 5c,d). This confirms that the coating of highly crystalline BiVO4 nanoparticles is loaded on branched WO3 nanorods. Further,

energetically favorable sites on nanorods and subsequent growth.56−59 These nanobranches are also identified as monoclinic WO3 (Figure S2b) and show the same crystal directions of nanowidth WO3 nanorods (Figure 3c,d). It means that WO3 nanobranches grow epitaxially on the WO3 nanorods. This epitaxial growth manner can maximize specific surface area without deteriorating the charge transfer at the nanorod/ nanobranch interfaces.60 The branched WO3 nanorods exhibit a photocurrent density of 0.85 mA/cm2 at 1.23 V versus RHE (Figure 4b). The photocurrent density enhanced about 32.8% compared to bare WO3 nanorods. Amperometric photocurrents of WO3 nanorods and branched WO3 nanorods were measured as a function of time at a bias voltage of 1.23 V versus RHE (Figure 4c). For both photoanodes, photocurrent appears instantaneously after initial illumination and relaxed to a steady state, which indicates the immediate and stable reaction to illuminated light. Figure 4d is the IPCE spectra of the WO3 nanorods and branched WO3 nanorods. Both photoanodes demonstrated photoresponse on about 300−460 nm wavelengths. Particularly, the IPCE of branched WO3 nanorods showed higher value over the entire photoreactive wavelengths than that of bare WO3 nanorods, suggesting the enhanced PEC performances. Furthermore, both WO3 nanorods and branched WO3 nanorods exhibited relatively stable photocurrent for 25 000 s (Figure S5), which implies high stability of the branched hierarchical structure. The improved performances can be understood by the increased surface area which promotes light absorption, enhanced light scattering, and effective charge transport across the interfaces between nanorods and nanobranches with minimal charge recombination. The growth of nanobranches increases the surface-area-to-volume ratio to facilitate the 4539

DOI: 10.1021/acsaem.9b00776 ACS Appl. Energy Mater. 2019, 2, 4535−4543

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ACS Applied Energy Materials the elemental mappings illustrate the uniform distribution of BiVO4 nanoparticles on WO3 nanobranches (Figure 5e). Figure 6a compares the PEC properties of BiVO4/branched WO3 nanorods and pure branched WO3 nanorods. For BiVO4/ branched WO3 nanorods, we investigated PEC performances under front-side and back-side photo illumination, respectively. The photocurrent density of BiVO4/branched WO3 nanorods under front-side illumination increased significantly to 3.87 mA/ cm2 at 1.23 V vs RHE, which is about 4.6 times higher than that of pure branched WO3 nanorods. This result demonstrates that the deposition of BiVO4 on branched WO3 nanorods can significantly enhance the light absorption, charge separation, and transport efficiency of the photoanode. With respect to BiVO4 material, it can absorb the broad wavelength range of the visible spectrum with its narrow band gap energy. Therefore, uniform loading of BiVO4 nanocrystalline particles on branched WO3 nanorods enhances light harvesting. Meanwhile, the photocurrent density of heterostructure under backside illumination was 2.78 mA/cm2, which is lower than that of frontside illumination. This result is not consistent with some previous research that reported the higher backside illuminated photocurrent for the BiVO4/WO3 heterostructures.21,33 This difference can be understood in the viewpoint of carrier transport through different dimension and configuration of BiVO4 and WO3. BiVO4 is known for poor carrier transport and short electron diffusion length (∼100 nm).64−67 However, in our case the electron diffusion will be less likely to be ratedetermining because BiVO4 nanoparticles are sufficiently small (size, 60−100 nm). In addition, small-sized BiVO4 nanoparticles do not block the light and electrolyte pathways between branched nanorods, which are also advantageous for high accessibility of light and electrolyte into the overall surfaces under front-side illumination. In order to examine the effect of WO3 layer, we vary the thicknesses of WO3 thin films (45, 75, 150, 300, 450, and 600 nm) by E-beam evaporation for the BiVO4/WO3 heterostructures. The photocurrent densities under frontside illumination were similar or slightly higher for thick (300, 450, and 600 nm) WO3-based photoanodes, whereas back-illuminated photocurrent is higher for relatively thin (45, 75, and 150 nm) WO3-based photoanodes (Figure S7). This can be explained by the difficult hole transport from the bottom WO3/FTO interface to the top WO3/BiVO4 or WO3/ electrolyte interface. This result demonstrates that the front or back illuminated PEC performances are closely dependent upon the thickness of WO3 film. It is reasonable to consider the short hole diffusion length of WO3 (∼150 nm).68 That is, the charge transport properties of WO3 backbone is a critical factor of PEC performances as well as that of BiVO4. Figure 6b shows PEC properties at amperometric photocurrent versus time (t) curves at a voltage of 1.23 V versus RHE. The heterostructured photoanode showed high stability and photoreactivity. The IPCE spectra of the BiVO4/branched WO3 nanorods at 1.23 V versus RHE are shown in Figure 6c. The heterostructure demonstrated higher values on overall wavelengths. In particular, significantly high values on about 400− 500 nm wavelengths are attributed to BiVO4 decoration on WO3 nanorods, confirming that the formation of WO3/BiVO4 heterostructure can effectively increase photocurrent density.31,69 The EIS results of the WO3 nanorods, branched WO3 nanorods, and BiVO4/branched WO3 nanorods were measured at 1.23 V vs RHE (Figure 6d). The equivalent circuit of Figure 6d inset was used to analyze measured data. As shown in Table 1, the BiVO4/branched WO3 nanorods showed the lowest inter-

Table 1. Fitted Charge-Transfer Resistance photoanode

Rs (Ω cm2)

Rct1 (Ω cm2)

WO3 NRs branched WO3 NRs BVO/branched WO3 NRs

7.39 8.60 1.79

1760.35 1233.89 439.40

face resistance and thus indicated higher charge transfer efficiency. This result demonstrates the facilitation of interfacial charge transfer through design of heterostructures. For further explanation of outstanding PEC performances, light absorption and charge separation efficiencies were analyzed. Light harvesting efficiency (LHE) can be calculated from measured transmittance and reflectance of photoanodes (Figure 7a and Figure S8). Dominant light absorption is found in λ = 300−457 nm for WO3 nanorods, λ = 300−462 nm for branched WO3 nanorods, and λ = 300−497 nm for BiVO4/ branched WO3 nanorods. The extended light absorption range of BiVO4/branched nanorods heterostructure can be explained by the smaller band gap energy of BiVO4 than WO3.70,71 The water splitting photocurrents (JH2O) is a function of theoretical maximum photocurrent (Jmax), light-harvesting efficiency (ηabs), charge separation efficiency (ηsep), and charge transfer efficiency (ηtrans) from the equation JH2O = Jmax × ηabs × ηsep × ηtrans.21,72 In this study, sodium sulfite electrolyte has hole scavenging effect which induces almost 100% of ηtrans.73 ηabs values of WO3 nanorods, branched WO3 nanorods, and BiVO4/ branched WO3 nanorods were calculated from AM 1.5G solar spectrum, which were 73.6%, 78.9%, and 83.4%, respectively (Figure S9). From the result, we can obtain ηabs × ηsep product, and the final ηsep values of 22.8% for WO3 nanorods, 25.9% for branched WO3 nanorods, and 74.0% for BiVO4/branched WO3 nanorods with the dark current subtracted (Figure 7b,c). This analysis illustrates that the formation of BiVO4/branched WO3 nanorod heterostructure significantly enhanced charge separation efficiency, which implies restricted charge recombination lead to outstanding PEC performances. The PEC stability of BiVO4/branched WO3 nanorods was investigated for 25 000 s (Figure 8a), and the long-term stability was also examined for 24 days (Figure 8b) at 1.23 V vs RHE under simulated solar illumination. There was no degradation of photocurrent during measuring, which indicates that the BiVO4/branched WO3 nanorod photoanode demonstrates great long-term stable PEC performances. The stepwise improvement of PEC performances and excellent long-term stability suggest that photoanode design via two-step hydrothermal reaction and electrodeposition is a promising approach. Moreover, the all-solution process can provide facile, uniform, cost-effective, and large-scale synthesis, carrying great advantages for practical applications.

4. CONCLUSIONS BiVO4/branched WO3 nanorods were synthesized via allsolution process. The PEC performances of the WO3 photoanode was greatly improved by the formation of branched hierarchical structures with enhanced active surface area and light scattering ability using two-step hydrothermal reaction. The BiVO4/branched WO3 nanorods heterostructure were prepared by additional electrodeposition, which demonstrated significantly high photocurrent of 3.87 mA/cm2 at 1.23 V versus RHE. The improved performances of these photoanodes are attributed to the enhancement of light absorption and charge separation through morphology control of branched WO3 4540

DOI: 10.1021/acsaem.9b00776 ACS Appl. Energy Mater. 2019, 2, 4535−4543

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Figure 7. (a) LHE (%) of WO3 NRs, branched WO3 NRs, and BiVO4/branched WO3 NRs. (b) Product of light absorption and charge separation efficiency (ηabs × ηsep) and (c) charge separation efficiency (ηsep) versus potential with the dark currents subtracted.

Figure 8. (a) The chronopotentiometry curve of BiVO4/branched WO3 NRs in a 1 M Na2SO3 electrolyte at 1.23 V versus RHE with illuminated sunlight. (b) Long-term stability measurement of BiVO4/branched WO3 NRs for 24 days.

nanorods and rational design of type II heterojunction between WO3 and BiVO4. This work suggests facile, cost-effective, and highly controllable synthetic strategy to increase PEC performances and opens up the possibilities for various energy harvesting applications.

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Jong-Heun Lee: 0000-0002-3075-3623 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the National Research Foundation of Korea (NRF), which was funded by the Korean government (Ministry of Education, Science, and Technology (MEST), Grant 2016R1A2A1A05005331).

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00776. Photographs of synthesized photoanodes; XRD patterns of WO3 and branched WO3 nanorods; SEM images of WO3 nanorods prepared with different hydrothermal time; J−V curves of WO3 nanorods prepared with different hydrothermal time; the chronopotentiometry curve of bare WO3 NRs and branched WO3 NRs with illuminated sunlight; schematic energy band diagram of BiVO4/WO3 heterostructure; J−V curves of heterostructures with BiVO4 and various thickness-controlled WO3 nanofilms under front and back illumination; transmittance and reflectance of as prepared photoanodes; electron flux of AM 1.5 G solar spectrum and electron flux of as prepared photoanodes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +82-2-928-3584. Tel: +82-2-3290-3282. ORCID

Zhengfei Dai: 0000-0002-3709-8895 4541

DOI: 10.1021/acsaem.9b00776 ACS Appl. Energy Mater. 2019, 2, 4535−4543

Article

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DOI: 10.1021/acsaem.9b00776 ACS Appl. Energy Mater. 2019, 2, 4535−4543