Bi2S3 Nanorod Heterojunction as

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Three-Dimensional WO3 Nanoplates/Bi2S3 Nanorods Heterojunction as Highly Efficient Photoanode for Improved Photoelectrochemical Water Splitting Yidan Wang, Wei Tian, Liang Chen, Fengren Cao, Jun Guo, and Liang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11510 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017

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ACS Applied Materials & Interfaces

Three-Dimensional WO3 Nanoplates/Bi2S3 Nanorods Heterojunction as Highly Efficient Photoanode for Improved Photoelectrochemical Water Splitting

Yidan Wang, † Wei Tian,*,† Liang Chen,† Fengren Cao,† Jun Guo‡ and Liang Li*,†

†

College of Physics, Optoelectronics and Energy, Center for Energy Conversion

Materials & Physics, Jiangsu Key Laboratory of Thin Films, Soochow University, Suzhou 215006, P. R. China ‡

Analysis and Testing Center, Soochow University, Suzhou, P. R. China

Email: [email protected], [email protected]

KEYWORDS: heterojunction, WO3, Bi2S3, photoanode, photoelectrochemical water splitting

ACS Paragon Plus Environment


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ABSTRACT The rational design of semiconductor photoanodes with sufficient light absorption, efficient photo-generated carrier separation and fast charge transport is crucial for photoelectrochemical (PEC) water splitting. Incorporating small band gap semiconductor to large band gap material with matched energy band position is a promising route to improve the light harvesting and charge transport. Herein, we report the fabrication of three-dimensional heterojunction with uniform Bi2S3 nanorods on WO3 nameplates by hydrothermal process and chemical bath deposition. The seed layer strategy was used to assist the growth of Bi2S3 nanorods for perfect interface contact between WO3 and Bi2S3.The as-prepared WO3/Bi2S3 composite exhibited much enhanced photocurrent(5.95 mA/cm2 at 0.9 V vs. RHE), which is 35 and 1.4 times higher than that of pristine WO3 and WO3/Bi2S3 composite without seed layer. In addition, higher IPCE (68.8%) and photoconversion efficiency (1.70%) were achieved. The enhancement mechanism was investigated in detail, and the sufficient light absorption, efficient charge transport and high carrier density simultaneously contribute to the improved PEC activity. These findings will open up new opportunities to develop other highly efficient heterostructures as photoelectrodes for PEC application.


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INTRODUCTION Photoelectrochemical (PEC) water splitting provides a promising way to address the shortage of fossil fuels and environmental pollution issues, which can convert solar energy into hydrogen fuels.1-3 Since oxygen evolution reaction occurring at photoanodes is crucial for achieving efficient solar energy conversion in PEC water splitting, significant efforts have been devoted to developing highly efficient photoanode semiconductors, such as Si, TiO2, Fe2O3 and Ta3N5.4-7 Among various semiconductors, WO3 (Eg = 2.5-2.8 eV) is regarded as a promising candidate for PEC application, as it exhibits stronger sunlight absorption ability and superior electron transport properties (~12 cm2/Vs) than TiO2 (~0.3 cm2/Vs), longer hole diffusion length compared to Fe2O3, and higher resistance to photocorrosion in aqueous solution than Si and Ta3N5.8,9 Various WO3 nanostructures have been fabricated and utilized as photoanodes in PEC cells. For example, Ye et al. synthesized large-area WO3 photonic crystal photoanodes with inverse opal structure by a solution method using polystyrene (PS) templates. The improved light-harvesting ability benefits from the slow-light effect, giving rise to an enhanced incident photon-to-current conversion efficiency (IPCE) of 18%.10 Zheng et al. fabricated dual etched and reduced WO3 nanoflakes with much rougher surface and higher carrier density by a solution process, which exhibited an enhanced photocurrent density of 1.10 mA/cm2 at 1.0 V vs. Ag/AgCl.11 Park et al. prepared a WO3 overlayer with dual oxygen and tungsten vacancies to alleviate charge recombination at the electrode/electrolyte interface. The resulting photoanode presented approximately 2.4 times higher photocurrent than



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pristine samples.12 However, pristine WO3 suffers from several shortcomings, including low solar spectrum absorption (about 12%), slow charge transfer at the WO3/electrolyte interface, sluggish water oxidation kinetics and high electron-hole recombination, which greatly limit the overall PEC performance of pristine WO3.13,14 In the past decade, extensive research efforts have been made to address these issues and enhance the PEC activity of WO3, including fabrication of low-dimensional nanostructures, element doping, deposition of cocatalysts, and so on.15-19 For instance, Yang et al. demonstrated that the 2 mol% Fe-doped WO3 photoanode achieved a photocurrent density of 0.88 mA/cm2 at 1.23 V vs. reversible hydrogen electrode (RHE), about 30% higher than that of the undoped WO3.20 To improve the oxidation selectivity and the Faradaic efficiency, the deposition of CoOx nanoparticles on WO3was reported.21 In particular, construction of hybrid structures by combing WO3 with small band gap semiconductors has been demonstrated as a promising approach to increase the light absorption efficiency, facilitate the separation and transfer of charge carriers and promote oxygen evolution dynamics. In recent, Liu et al. fabricated a WO3/Sb2S3 heterojunction photoanode, which achieved an enhanced photocurrent of 1.79 mA/cm2 at 0.8 V vs. RHE under simulated sunlight, compared to 0.45 mA/cm2 of pristine WO3.22 Among narrow bandgap semiconductors, Bi2S3 is an n-type semiconductor with a direct bandgap of 1.3 eV, as well as high absorption coefficient (104-105). In addition, it is reported that Bi2S3 has a more negative conduction band edge than that of WO3.23


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As a result, the photo-generated electrons in Bi2S3 can easily inject into WO3, reducing the charge recombination. Accordingly, WO3/Bi2S3 heterostructure is expected to be a promising photoanode to achieve highly efficient PEC application. As known, the contact quality at the interface of heterojunction plays a crucial role in charge separation and transport.24 Thus, it is of great importance to develop facile and effective approach to fabricate WO3/Bi2S3 heterostructure with perfect interfacial contact and less defects. Although there are several approaches, including successive ionic layer adsorption and reaction (SILAR) and in situ synthesis using Bi2WO6 as an interim product, to fabricate the WO3/Bi2S3 heterostructure based photoanodes, it is still challenging to rationally tune their interface quality and morphologies for enhanced PEC performance.25,26 Moreover, the PEC activity as a function of deposition amount of Bi2S3 on WO3 was not systematically investigated. In this paper, we rationally designed and fabricated three-dimensional (3D) WO3 nanoplates/Bi2S3 nanorods heterostructures on FTO substrates by combing hydrothermal method, SILAR process and chemical bath deposition (CBD) reaction. Particularly, a seed layer of Bi2S3 was utilized before the CBD process, leading to the uniform distribution of Bi2S3nanorods on WO3 and high-quality interfacial contact. As control samples, the heterostructures were prepared by a two-step process without seed layer. The composite photoanode(CBD time: 12 h) with seed layer exhibited the largely enhanced activity with a high photocurrent density of 5.95 mA/cm2 at 0.9 V vs. RHE, which is 35 and 1.4 times higher than that of pristine WO3 and heterostructure without seed layer, respectively. This excellent PEC performance benefits from the



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increased light absorption, matched energy band alignment, and improved separation and transfer efficiency of photo-generated electron-hole pairs.

EXPERIMENTIONAL SECTION Materials. Sodium tungstate dihydrate (Na2WO4·2H2O, AR, ≥99.5%) and sodium sulfide

nonahydrate

(Na2S·9H2O,

AR,

≥98.0%

Shanghai Aladdin Bio-Chem Technology Co., Ltd.

)

were

purchased

Diammonium

from oxalate

monohydrate ((NH4)2C2O4·H2O, GR, ≥99.8%), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, AR, ≥99.0%), thiourea (CH4N2S, AR, ≥99.0%) and ethylene glycol (C2H6O2, AR, ≥99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Hydrochloric acid (HCl, AR, 36-38%) was purchased from Chinasun Specialty Products Co., Ltd. Synthesis of WO3 nanoplates. The WO3 nanoplates films were grown on FTO glass by a hydrothermal method according to previous report.27 Firstly, 0.2474 g sodium tungstate dihydrate and 10 ml of 3 M hydrochloric acid were dissolved in 30 ml of deionized water. This solution was mixed with 30 ml aqueous solution containing 55 mM diammonium oxalate monohydrate under vigorous stirring for 10 min. Then 10 ml of the as-prepared precursor solution was transferred into a 25 ml of Teflon-lined stainless steel autoclave. A piece of clean FTO substrate was placed into the autoclave with the conductive side facing down. The hydrothermal reaction was conducted at 140 °C for 3 h. After cooling down to room temperature naturally, the FTO substrate with a layer of yellow film was washed with deionized water and ethanol for several


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times and then dried at 70 °C for 1 h. Finally, the substrate was annealed at 450 °C for 1 h in air with a heating rate of 5 °C/min to obtain WO3 nanoplates films. Deposition of Bi2S3 seed layer on WO3 film. The Bi2S3 seed layer was prepared by a modified SILAR method. First, the FTO substrate with WO3 film was immersed in a solution of bismuth nitrate pentahydrate (0.05 M) in ethylene glycol for 1 min, in which Bi3+ ions were adsorbed on the surface of the substrate. This substrate was rinsed with deionized water for 10 s to remove the excess Bi3+. The substrate was then immersed in aqueous solution of sodium sulfide nonahydrate (0.1 M) for 1 min, in which S2- ions were reacted with previously adsorbed Bi3+ ions to form Bi2S3. This was followed by rinsing again in deionized water. The above steps complete one cycle of deposition. After 5 SILAR cycles, the obtained sample was dried at 40 °C in a vacuum oven and then annealed at 250 °C for 1 h in argon atmosphere. Synthesis of WO3/Bi2S3 heterojunction. The WO3/Bi2S3 photoanode was prepared by a CBD method. First, a precursor solution was prepared by mixing 0.1 M bismuth nitrate pentahydrate, 0.1 M thiourea and 40 ml ethylene glycol, and then stirred to clear condition. An FTO substrate grown with WO3 nanoplates (with or without Bi2S3 seed layer) was placed into the solution leaned against the wall of the beaker with the conductive side facing down. The reaction was kept at 60 °C for different time (4, 8, 12, 16, 20 and 24 h). The as-prepared samples were rinsed with deionized water and dried at 40 °C in a vacuum oven. Finally, the samples were annealed at 250 °C for 1 h with a heating rate of 2 °C/min in argon atmosphere to obtain WO3/Bi2S3 heterojunctions.



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Characterization. The surface morphology of prepared photoanodes was characterized by field emission scanning electron microscope (FE-SEM, Hitachi SU8010). The crystalline phase was measured by X-ray diffractometer (XRD, D/Max-III-B-40 kV, Cu Kα radiation; λ=0.15418 nm). The high resolution transmission electron microscopy (HRTEM, FEI Tecnai G20 F20 S-TWIN TMP) was operated at 200 kV to investigate the microstructure of samples. The elemental states were analyzed by X-ray photon spectroscopy (XPS, ESCALAB 250Xi). The Raman spectra of samples were measured by Raman spectroscopy (Horiba JY LabRAM HR800). The absorption spectra of samples were obtained using an UV-vis spectrophotometer (shimadzu UV-3600). Photoelectrochemical measurement. The photoelectrochemical measurements were performed with an electrochemical workstation (Autolab, PGSTAT 302N) using a three-electrode cell configuration, in which the as-prepared samples were used as the work electrodes with active area of 0.7 cm2. Meanwhile, a saturated Ag/AgCl (in 3 M KCl solution) electrode and a Pt mesh electrode served as the reference and counter electrode, respectively. An aqueous solution of 0.1 M Na2S and 0.1 M Na2SO3 (PH≈12) was used as the electrolyte. A solar simulator (Newport, 94043A) was employed to provide simulated solar light (AM 1.5G, 100 mW/cm2). Linear sweep voltammetry (LSV) curves were tested at a scan rate of 20 mV/s. Electrochemical impedance spectroscopy (EIS) was measured under light at the open-circuit potential in the frequency range of 0.01-105 Hz with a 5 mV amplitude. Mott-schottky plots were obtained in the electrolyte of 0.25 M Na2SO4 (PH≈6.8) in dark at an AC


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frequency of 1 kHz. The measured potential versus Ag/AgCl was converted to RHE potential by the Nernst equation28 ERHE = EAg/AgCl + 0.059pH + E°Ag/AgCl where ERHE is the potential versus the RHE, EAg/AgCl is the measured potential versus Ag/AgCl, and E°Ag/AgCl is 0.1976 V at 25°C. The IPCE was tested under monochromatic light produced by a lamp connected with light filters (CEAULIGHT, Beijing), and the light intensity was measured by alight power meter (Newport, Model 1918-R).

RESULTS AND DISCUSSION The typical SEM image of pristine WO3 in Figure 1a shows that the vertically aligned and uniform plate-like array was grown on FTO substrate. The thickness and edge length of the nanoplates is 50-200 nm and 0.7-1.5 µm, respectively. The cross sectional view of WO3 arrays (Figure 1b) reveals that the height of pristine WO3 film is about 790 nm. Figure 1c shows the SEM image of WO3/Bi2S3 heterojunction without Bi2S3 seed layer prepared by the CBD process with reaction time of 12 h. It is seen that bundles of Bi2S3 nanorods are randomly distributed on WO3 arrays. Meanwhile, some part of WO3 arrays is not covered by Bi2S3. Obviously, this is not an ideal heterojunction configuration with good interfacial contact. In order to increase the amount of Bi2S3 and obtain uniform heterojunction, a Bi2S3 seed layer was deposited on pristine WO3 using SILAR method. As shown in Figure S1, after the SILAR process, the surface of WO3 becomes rougher than that of bare WO3 (Figure



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1a), indicating there are a large number of Bi2S3 nanoparticles loaded on the WO3 nanoplates, which serve as a seed layer for the subsequent CBD reaction. In the presence of a seed layer, there are more nucleation centers on WO3 nanoplates, which may induce uniform deposition and growth of Bi2S3 nanorods. The SEM image of WO3/Bi2S3 heterostructures (CBD time: 12 h) with seed layer is shown in Figure 1d. Once the seed layer is introduced, the WO3 plates are uniformly covered by dense Bi2S3 nanorods with diameter of about 30 nm. The average height of the composite is about 950 nm, which is 160 nm higher than that of pristine WO3 film. The increased height is equal to the length of Bi2S3 nanorods, which can be confirmed by the following TEM images. More importantly, the plate-like morphology of WO3 is still preserved. The uniform and entire coverage of Bi2S3 on WO3 implies better contact between WO3 and Bi2S3, which is beneficial for the transport of charge carriers. The morphology of WO3/Bi2S3 heterostructures with different CBD time (4, 8, 12, 16, 20 and 24 h) was shown in Figure S2. For WO3/Bi2S3 composites without seed layer, the length and diameter of Bi2S3 nanorods increase with reaction time. Although abundant of Bi2S3 nanorods are grown on WO3 film, there are still some bare WO3 even if the reaction time prolongs to 20 h. In contrast, the morphology of the heterostructures with seed layer is totally different. For the sample with CBD reaction time of 4 h, there are no obvious Bi2S3 nanorods on WO3 nanoplates except for a layer of nuclei. When the deposition time is prolonged to 8 h, the Bi2S3 nanorods appear and become more apparent as the time further increases to 12 h. Increasing the reaction time to 16 and 24 h, abundant of Bi2S3 nanorod arrays are almost vertically


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aligned on the WO3 plates. Overall, whatever the heterostructures are prepared with or without seed layer, the prolonged deposition time will increase the length, diameter and quantity of Bi2S3 nanorods. For heterostructures with seed layer, Bi2S3 nanorods distribute more uniformly and have better coverage on WO3 arrays, which may contribute to the improvement in light absorption and electron transport between WO3 and Bi2S3. The XRD patterns of pristine WO3 film and WO3/Bi2S3 composite are shown in Figure 1f. For the pristine WO3, besides the diffraction peaks of FTO substrate, other peaks are well indexed to a monoclinic structured WO3 (JCPDS: 83-0950). With respect to the heterostructure with seed layer, the peaks marked with blue color can be ascribed to the orthorhombic Bi2S3 (JCPDS: 17-0320). Moreover, the diffraction peaks of WO3 are obviously observed in WO3/Bi2S3 composites, indicating that the phase of WO3 is retained after the CBD process. No other obvious peaks are observed, suggesting the high purity of the heterostructure. To further investigate the composition and chemical state of WO3 and WO3/Bi2S3 films, XPS measurement was performed. As shown in Figure S3a, the spin orbit doublet peaks at 35.6 and 37.7 eV are ascribed to W 4f7/2 and W 4f5/2 respectively.11 The splitting of 2.1 eV between two core levels is in accordance with previous report, indicating the W6+ state in WO3. The dissymmetric peaks of O 1s are shown in Figure S3b. The broad peak in high binding energy side indicates the existence of several different oxygen species. The peak at 530.2 eV corresponds to O2- state with a bond manner of W=O in the WO3 lattice. The peaks at 531.2 and 532.5 eV are assigned to



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the hydroxyl group (-OH) and adsorbed oxygen on the WO3 surface, respectively.29,30 The XPS survey spectrum of WO3/Bi2S3 composite demonstrates the elements of W, O, Bi, S and C without obvious impurities (Figure 2a). The peaks at 163.8 eV and 159.4 eV in Figure 2b fit well with Bi 4f5/2 and Bi 4f7/2, indicating the +3 valence state of Bi.25,31 The peaks of S 2p1/2 (162.3 eV) and S 2p3/2 (161.1 eV) are assigned to S2- in Bi2S3.23 The Raman spectra of pristine WO3 shows four typical peaks corresponding to the strongest vibration modes in monoclinic WO3 (Figure 2c). The peaks at 268.8 and 321.8 cm-1 are attributed to the O-W-O bending vibration, and those at 709.0 and 805.8 cm-1 are due to the stretching vibration. The lattice vibration possibly leads to the peaks below 200 cm-1. The shoulder peak at about 640 cm-1 can be ascribed to the stretching vibration mode in the residual hydrated tungsten.32,33 After the coating of Bi2S3 nanorods, the peaks of WO3 become weaken, and only the peak at 805.9 cm-1 is visible (Figure 2d). The peaks at 183.1, 234.7 and 257.6 cm-1 correspond to the Ag, Ag and B1g mode of Bi2S3, respectively. The additional peak at 121.5 cm-1 may be due to the displacement defects in the Bi2S3 lattice.34 All the above results clearly confirm that the WO3/Bi2S3 composite is successfully fabricated. Figure 3a and Figure S4 show the TEM image of as-prepared WO3/Bi2S3 composite. It is apparent that WO3 nanoplate is surrounded by a large number of Bi2S3 nanorods. The length of Bi2S3 nanorods is from 100 to 200 nm and the diameter is from 10 to 50 nm, which are in accordance with the SEM image. The HRTEM image (Figure 3b) of the dotted box region in Figure 3a reveals that the lattice fringe with interface spacing


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of 0.35 and 0.28 nm is ascribed to the (310) and (221) plane of Bi2S3, respectively, suggesting that the Bi2S3 nanorod is of orthorhombic structure in agreement with the XRD results. The corresponding fast Fourier transform (FFT) pattern (inset in Figure 3b) shows the high crystallinity. The lattice spacing of 0.26 nm is assigned to the (202) plane of monoclinic WO3. In order to confirm the elements distribution, elemental mapping at the interface of nanoplate and nanorod was conducted (Figure S5). It is shown that W and O elements distribute mainly on the plate, while both Bi and S elements distribute on plate and nanorod part. It demonstrates that the heterostructure is made of WO3 nanoplate and Bi2S3 nanorod. The appearance of Bi and S signals on the nanoplate is due to the loading of Bi2S3 seed layer on WO3 plate. The light absorption spectra were utilized to reveal the light harvesting ability of different samples. Figure 4a shows that bare WO3 has poor absorption in the visible light region with absorption edge of about 446 nm, which is due to its large indirect band gap of 2.78 eV (Figure 4b). After the CBD process of Bi2S3 without seed layer, the absorbance of the film enhances in all measured spectral range, and it has an absorption edge of about 920 nm, corresponding to the small direct band gap (1.35 eV) of Bi2S3 (Figure 4b). Moreover, the light harvesting of WO3/Bi2S3 film synthesized with seed layer has been further great enhanced. The photographs of the different samples are shown in Figure S6. Among the samples, the bare WO3 film is light yellow (Figure S6a) and becomes brown after coupling with Bi2S3 seed layer (Figure S6b). After CBD process for 12 h, the WO3/Bi2S3 without seed layer shows dark brown (Figure S6d), whereas the sample prepared with seed layer is totally black



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(Figure S6c), indicating its strong light harvesting ability. The influence of deposition time on light harvesting ability has also been studied. Figure S7 shows that whether the composite is prepared with seed layer or not, the prolonged deposition time will enhance the light absorption ability, because more reaction time results in more Bi2S3 deposited on WO3. The above results demonstrate that the increased amount and uniform distribution of Bi2S3 improve the light absorption ability of the heterostructures by introducing a seed layer. Figure 5a shows the LSV plots of bare WO3 and WO3/Bi2S3 under illumination. The bare WO3 film has a very low photocurrent density of 0.17mA/cm2 at 0.9 V vs. RHE. After coating with Bi2S3, the photocurrent obviously improves. As the deposition time increases from 4 to 24 h, the photocurrent density increases from 0.64 to 5.45 mA/cm2 at 0.9 V vs. RHE. This is attributed to the enlarged light absorption range and enhanced absorption ability of Bi2S3 that with a small band gap. Moreover, the presence of seed layer can further improve the PEC activity of the photoanode. As shown in Figure 5b, the WO3 film with Bi2S3 seed (before CBD) has photocurrent density of 3.19 mA/cm2 at 0.9 V vs. RHE. After CBD process of 4 h, the photocurrent density of WO3/Bi2S3 with seed layer is 4.30 mA/cm2 and further increases to 6.56 mA/cm2 when the deposition time is prolonged to 20 h. Whereas the photocurrent of 24 h sample has no obvious enhancement compared to the 20 h sample, suggesting that the photoactivity of 20 h sample has reached to a plateau, and the prolonged CBD time cannot further increase the photocurrent density. Figure 5c shows the photocurrent density of samples with different deposition time, which is obtained on


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the basis of average data of four photoelectrodes for each sample. The corresponding values are summarized in Table S1. It is obviously observed that the samples with seed layer (red line) have higher photocurrent density for all the deposition time, compared to the samples without seed layer. The enhanced PEC property is in accord with the UV-vis results. It is worth noting that the WO3/Bi2S3 composites with CBD time of 16 h or longer time have large dark current (Figure S8). According to the SEM images (Figure S2), it can be found that when the CBD is prolonged to 16 h, the Bi2S3 nanorod becomes almost vertically aligned on WO3 plates. Moreover, the diameter and length of the Bi2S3 nanorod further enhance with the increasing of CBD time, forming a thick film. As a result, the composites with 16 h or longer CBD time have much larger specific surface area, providing more active area for redox reaction, thus contributing to the large dark current. On the other hand, when the Bi2S3 nanorods are too long, the photo-generated carriers may not contribute to water splitting but recombine during transporting, which is not conducive to the enhancement of photocurrent. In order to confirm the optimized sample, we calculate the net photocurrent density (Jph) of each sample by subtracting dark current from total current. As shown in Figure 5d and table S2, the sample with CBD time of 12 h has the largest Jph of 5.64 mA/cm2 at 1.13 V vs. RHE. In comparison with the samples with shorter CBD time (4, 8 h), the 12 h-composite has higher light harvesting ability, which leads to high photocurrent. Contrary to the samples deposited for longer time (16, 20, 24 h), the WO3/Bi2S3 (12 h) has moderate length of Bi2S3 nanorod. As a result, the photo-generated carriers can easily transport to the surface and contribute to water



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splitting. Figure 5e shows the I-t curves of samples deposited for 12 h at 0.9 V vs. RHE under the chopped light illumination. All photoanodes have fast response and the WO3/Bi2S3 with seed layer has the highest photocurrent, which is in agreement with the LSV results. The stability of the as-prepared sample was tested at 0.9 V vs. RHE for 600 s under illumination (Figure 5f). The photocurrent density of the photoanode decreases slowly from 5.32 to 4.47 mA/cm2, indicating the photoanode suffers from photocorrosion. In order to explore the photoactivity of photoanodes under monochromatic light, the IPCE was measured at 0.9 V vs. RHE and the values are calculated by the equation:35 IPCE = (1240Jph) / λPlight where Jph is the net photocurrent density (mA/cm2), λ is the wavelength (nm) of the incident light and Plight is the measured incident light power density (mW/cm2) at the wavelength λ. As is shown in Figure 6a, the IPCE curve of WO3/Bi2S3 is similar in shape to that of bare WO3 in short wavelength region (λ

Bi2S3 Nanorod Heterojunction as

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