Bi2S3 Nanowire Composite

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C: Energy Conversion and Storage; Energy and Charge Transport 3

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Effective Formation of WO Nanoparticle/BiS Nanowire Composite for Improved Photoelectrochemical Performance Mira Park, Jong Hyeok Seo, Ji Hyeon Kim, Gisang Park, Joon Yong Park, Won Seok Seo, Hyunjoon Song, and Ki Min Nam J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05555 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 29, 2018

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The Journal of Physical Chemistry

Effective Formation of WO3 Nanoparticle/Bi2S3 Nanowire Composite for Improved Photoelectrochemical Performance Mira Park,†,§ Jong Hyeok Seo,‡,§ Ji Hyeon Kim,‡ Gisang Park,‡ Joon Yong Park,‡ Won Seok Seo,¶,* Hyunjoon Song,†,* and Ki Min Nam‡,* †

Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST),

291 Daehak-ro, Yuseong-gu, Daejeon 34341, Republic of Korea ‡

Department of Chemistry, Mokpo National University, 1666 Yeongsan-ro, Cheonggye-

myeon, Muan-gun, Jeonnam 58554, Republic of Korea ¶

§

Department of Chemistry, Sogang University, Seoul, 04107, Republic of Korea

These authors contributed equally to this work

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ABSTRACT A well-defined WO3/Bi2S3 composite comprised of single-crystalline Bi2S3 nanowire (Bi2S3NW) layers on top of the WO3 nanoparticles (WO3NP) was synthesized via an in-situ hydrothermal reaction. The single-crystalline Bi2S3 nanowires were uniformly grown on the surface of the WO3 nanoparticle layer. This in-situ hydrothermal process is also a general route for the synthesis of well-aligned Bi2S3 nanowires on various metal oxide substrates, such as TiO2, BiVO4, and ZnO. Compared to the sole Bi2S3 electrode, the resulting WO3NP/Bi2S3NW composite showed enhanced photoelectrochemical (PEC) activity. The origin of this enhanced activity is mainly attributed to the enhancement of charge separation on the Bi2S3 layer, due to the effective photogenerated electron transfer from the Bi2S3 conduction band to that of WO3. Furthermore, the single-crystalline longitudinal structure of the Bi2S3 nanowires can provide a direct electrical pathway through a single domain of nanowires.

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INTRODUCTION Given the existing requirements of clean energy and increasing concern for environmental issues, utilization of solar energy has been considered a realistic solution to counter the depletion of fossil fuels while avoiding environmental pollution.1 Since the discovery of photolysis of water by TiO2 in 1972,2 much attention has been paid to semiconductor materials that could be used as photoelectrodes.3-7 Investigation of the semiconductors in a photoelectrochemical (PEC) cell is a fast and simple method to characterize the electrochemical behaviors of semicoductors.3 Although many semiconductor materials, such as TiO2, WO3, BiVO4, CdS, CdSe, CuO, TaON, and Fe2O3, have been studied as photoelectrodes,8-11 most of them have poor PEC efficiency, due to limited visible light absorption and significant electron-hole recombination. Several strategies have recently been developed to improve PEC performance, such as morphology modification, electrocatalyst loading, and proper doping on semiconductor.4 Furthermore, hetero-structures, in which two semiconductor materials are connected in series using currently available semiconductors, such as WO3/BiVO4,12 WO3/Fe2O3,13 WO3/CuWO4,14 TiO2/CdS/Bi2S3,15 CaFe2O4/TaON,16 WO3/ZnWO4,17 Cu2O/TiO2,18 PbMoO4/Bi2O3,19 n-Si/n-TiO2,20 and CuFeO2/CuO,21 have been extensively studied as promising photoelectrodes. In these hetero-structures, the semiconductor can promote conversion efficiency through enhanced light absorption and charge separation. Although the advantages of pairing semiconductors are now widely known, understanding the origins of those advantages is still unclear. Among various semiconductors, bismuth sulfide (Bi2S3) has attracted great interest as a sensitizer for PEC and photovoltaic cells due to its narrow band gap (of about 1.3 eV) and large absorption coefficient.22-25 However, the reported photocurrent values under 1 sun irradiation are still much lower than the theoretical maximum (of about 30 mA/cm2). The 3



recombination occurring at the surface, as well as in the bulk state of Bi2S3, leads to unfulfilled PEC conversion efficiency. To suppress bulk recombination, Bi2S3 nanostructures, such as nanorods and nanoparticles, have been considered as photoelectrodes,22,26 which because of reduced hole diffusion length, can enhance the kinetic parameters of the PEC reactions. Unfortunately, these nanostructures are associated with several disadvantages, such as reduced space-charge region, and relatively low activity in the visible light region, due to quantum confinement arising from small grain sizes.6 Thus, further studies need to be conducted to optimize the size and shape of one-dimensional Bi2S3 for improved PEC performance. One-dimensional Bi2S3 such as nanorods and nanowires have been synthesized by various techniques, including hydrothermal, solvothermal, sonochemical, and chemical vapor deposition.27-30 In order to investigate the electrical and electrochemical properties of Bi2S3 nanowires, they must be grown on a substrate to enable efficient interactions under the controlled reaction conditions. Because most Bi2S3 nanowires are usually prepared as powders, their deposition on substrates is another challenging process.31 To reduce the contact resistance between the semiconductor (Bi2S3) and the substrate, in-situ growth processes are required. Accordingly, the facile synthetic method on the substrate is still demanding to reduce fabrication complexity of the devices. In this study, we report a facile formation of WO3NP/Bi2S3NW composite on a fluorinedoped tin oxide (FTO) substrate via an in-situ hydrothermal reaction for PEC characterization. The single-crystalline Bi2S3 nanowires were uniformly grown and well connected on the surface of the WO3 nanoparticles. This synthetic method is also a general route for the synthesis of well aligned Bi2S3 nanowires on various substrates, such as TiO2, BiVO4, and ZnO. Compared to the sole Bi2S3 electrode, the resulting WO3NP/Bi2S3NW composite 4


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showed enhanced PEC activity. The increase of photocurrent in the WO3NP/Bi2S3NW composite is mainly attributed to the enhanced charge separation of Bi2S3 resulting from the WO3NP layer. The photogenerated electron from the Bi2S3 conduction band rapidly transfers to that of WO3, which results in enhanced charge separation of the Bi2S3. Furthermore, the enhanced performances also originate from the directly grown Bi2S3 nanowire on WO3 nanoparticle, which shows single crystallinity of nanowires. The single-crystalline Bi2S3 nanowires can efficiently provide direct electrical pathways through a single domain of nanowire, and absorb a large fraction of solar light, because of the optimum length of the nanowires.

EXPERIMENT SECTION Materials. A fluorine-doped tin oxide (FTO, TEC 15, WY-GMS) coated glass was used as a substrate for thin film electrode. Ammonium metatungstate hydrate (99.99 %, SigmaAldrich), tungstic acid (99 %, Sigma-Aldrich), and bismuth (III) nitrate pentahydrate (99.999 %, Sigma-Aldrich) were used as metal precursor salts. Oxalic acid dihydrate (≥ 99 %, Sigma-Aldrich), thiourea (≥ 99 %, Sigma-Aldrich), hydrochloric acid (36.5 %, Junsei), ethylene glycol (≥ 99 %, Sigma-Aldrich), isopropyl alcohol (99.5 %, Junsei), sodium sulfite (≥ 98 %, Sigma-Aldrich), and sodium sulfide nonahydrate (96.0 %, Junsei) were also used as received. Deionized water was used as a solvent in all of the electrochemical experiments. Preparation of the WO3 Nanoparticle Electrode. WO3 nanoparticles were grown on a FTO substrate using a seed-mediated hydrothermal reaction according to our previous work.32 A seed layer was fabricated via a drop-casting technique on the FTO substrate, which was cleaned by deionized water and ethanol. An ammonium metatungstate hydrate solution (10 mM, 200 µL) in ethylene glycol was dropped onto the FTO substrate (1.5 cm × 2 cm), 5



and then dried at 120 °C in air. For the fabrication of a WO3 seed layer, the film was annealed at 500 °C for 3 h in air. The seeded FTO substrate was horizontally placed into a Teflon-lined stainless steel autoclave with a volume of 50 mL. A reaction solution was prepared by adding tungstic acid (0.20 mmol) and oxalic acid (2.8 mmol) into deionized water (30 mL). The reaction solution was dissolved in water and then transferred into an autoclave. The sealed autoclave was heated in an electric oven at 170 °C for 6 h and then cooled to room temperature. Preparation of the WO3NP/Bi2S3NW Composite Electrode. Bi2S3 nanowires were directly grown on the surface of the WO3 nanoparticle layer using a seed-mediated hydrothermal reaction. A bismuth (III) nitrate pentahydrate solution (80 mM, 50 µL) in ethylene glycol was dropped onto the WO3 nanoparticle electrode (1.5 cm × 2 cm) and then dried at 120 °C in air. For the fabrication of a bismuth oxide layer on the WO3 nanoparticle, the electrode was annealed at 500 °C for 3 h in air. The electrode was horizontally placed into a Teflon-lined stainless steel autoclave with a volume of 50 mL. A reaction solution was prepared by adding thiourea (7.9 mmol) and hydrochloric acid (2.0 M, 0.10 mL) into deionized water (30 mL). The reaction solution was transferred into an autoclave and then heated in an electric oven at 140 °C for 4 h. The resulting WO3NP/Bi2S3NW composite structure electrode was dried at 50 °C in air. The Bi2S3 nanorods on the FTO substrate were also synthesized through the same processes without the formation of the WO3 nanoparticle layer. Preparation of Powdered Bi2S3 Nanorods and WO3NP/Bi2S3NR(Drop) Electrode. Powdered Bi2S3 nanorods were synthesized using a hydrothermal reaction. The Bi2O3 powder (0.93 mg), thiourea (7.9 mmol), and hydrochloric acid (2.0 M, 0.10 mL) were added into deionized water (30 mL). The solution was transferred into an autoclave, and then heated in 6


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an electric oven at 140 °C for 4 h. The resulting Bi2S3 nanorods (Bi2S3NR) were washed with ethanol, and were then dispersed in isopropyl alcohol. The WO3NP/Bi2S3NR(Drop) was fabricated by drop-casting of the Bi2S3 nanorods solution onto the surface of WO3 nanoparticles. Preparation of WO3NP/Bi2S3(SILAR) Electrode. Bi2S3 thin layer has been deposited onto WO3 nanoparticles film by a successive ionic layer adsorption and reaction (SILAR) method.33 First, the FTO substrate with WO3 nanoparticle was immersed in 20 mM ethylene glycol solution of Bi(NO3)3·5H2O for 2 min, for the adsorption of Bi3+ onto the WO3 surface, and then washed with ethylene glycol and completely dried at 90 °C to remove the solvent. It was then dipped in a 20 mM methanol solution of Na2S for 2 min, and thoroughly washed with methanol. The S2− reacted with the absorbed Bi3+, and formed Bi2S3. This two-step dipping procedure was repeated 12 times. Preparation of the ZnO/Bi2S3NW, TiO2/Bi2S3NW, and BiVO4/Bi2S3NW Composite Electrodes. Bi2S3 nanowires were directly grown on the surface of the various metal oxide substrates such as ZnO, TiO2, and BiVO4 layers using a seed-mediated hydrothermal reaction. The synthetic method used in these cases was identical to the synthesis of the WO3NP/Bi2S3NW, except for the use of TiO2, BiVO4, and ZnO as substrates. Preparation of the WO3NP/CdS, and WO3NP/CoS2 Composite Electrodes. CdS and CoS2 layers were directly grown on the surface of the WO3 nanoparticle layer using a seedmediated hydrothermal reaction. The synthetic methods used in these cases were identical to the synthesis of the WO3NP/Bi2S3NW, except for the use of CdO and CoO as precursors, respectively. Electrochemical Characterization of the Bi2S3 Electrodes. Photoelectrochemical (PEC) characterization was performed in a specially designed cell in a three-electrode configuration 7



with the thin film as a working electrode, a Pt wire counter electrode, and an Ag/AgCl reference electrode. The working electrode with the actual geometric area of 0.28 cm2 was exposed to electrolyte solution. A 150 W Xenon lamp (ABET technologies) was used as the light source in the PEC characterization step, and light illumination area was 0.28 cm2. Chopped light linear sweep voltammetry (LSV) was utilized to obtain the photocurrent responses using a DY2321 potentiostat (Digi-Ivy) and a CHI Instruments Model 630 potentiostat (Austin, TX). The PEC measurements were conducted in a 0.1 M Na2SO3 and 0.1 M Na2S solution. The PEC measurements were also taken in aqueous solutions of Na2SO4 (0.1 M) with a phosphate buffer (pH 7) for water oxidation. In all tests, the intensity of the lamp on the sample was measured and found to be 100 mW/cm2 using a Si solar cell (AIST). To provide only visible light illumination, a 425 nm long-pass filter was used to cut the UV portion of the spectrum. A monochromator (ORIEL) was used to obtain the action spectra of photo-response as a function of the wavelength. Materials Characterization. The electrodes were characterized by scanning electron microscopy (SEM, Magellan 400 operated at 10 kV). Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HR-TEM), and scanning transmission electron microscopy (STEM) were conducted using a Talos F200X instrument at 200 kV. A focused ion beam (FIB, Helios Nanolab 450 F1) was used to prepare the samples for STEM measurement. The X-ray diffraction (XRD) pattern was measured using Cu Kα radiation at 40 kV and 300 mA (Rigaku, D/MAX-2500). The UV-vis-NIR absorption spectra were acquired with a UV-3600 UV-VIS-NIR spectrophotometer using solid sample holder for wavelengths ranging from 300 to 1500 nm with the FTO substrate as the reference.

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RESULTS AND DISCUSSION Preparation of WO3NP/Bi2S3NW Composite Electrode. The WO3NP/Bi2S3NW composite was synthesized by a sequential layer formation (Scheme 1). First, WO3 nanoparticles were synthesized on a FTO substrate through a seed-mediated hydrothermal reaction.34 The scanning electron microscopy (SEM) images show high uniformity of the WO3 nanoparticle film (WO3NP) with an average diameter of 145 ± 40 nm and a thickness of approximately 310 nm, with full coverage of the FTO substrate (Figure 1a). Second, bismuth oxides were formed by simple drop-casting of Bi(NO3)3·5H2O solution onto the surface of WO3NP, and subsequent annealing at 500 °C for 3 h in air (Figure S1, Supporting Information). Then, Bi2S3 nanowires were grown directly on the surface of WO3NP electrode by a hydrothermal reaction in the presence of thiourea (WO3NP/Bi2S3NW). Figure 1b shows the SEM images of the Bi2S3 nanowires that were produced on the WO3 nanoparticles, which nanowires had an average width of 59 ± 24 nm and a length of 1.1 ± 0.2 µm (inset). The widths of the nanowires were uniform throughout their length. The nanowires fully covered the WO3NP substrate, and the majority were slightly tilted on the substrate.

Scheme 1. Schematic image showing the preparation of WO3NP/Bi2S3NW composite on the FTO substrate.

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Figure 1. SEM top-view and cross-section images (inset) of (a) WO3 Nanoparticles and (b) WO3NP/Bi2S3NW composite on the FTO substrate.

In order to obtain reliable structural information, X-ray diffraction (XRD) was carried out. Figure 2 and Figure S1 show the XRD patterns of the pristine WO3NP, WO3NP/Bi2O3, and WO3NP/Bi2S3NW films. The diffraction peaks of WO3NP are indexed to the triclinic structure (JCPDS No. 20-1323), indicating highly crystallized WO3. The diffraction peaks of WO3NP/Bi2O3 are in good agreement with those of orthorhombic Bi2WO6 (JCPDS No. 732020) and tetragonal Bi14W2O27 (JCPDS No. 39-0061) phases (Figure S1b). Figure 2b shows the XRD patterns of the WO3NP/Bi2S3NW composite. New diffraction peaks are observed in addition to the WO3 peaks, which correspond to the orthorhombic Bi2S3 (JCPDS No. 17-0320) phase. Secondary phases such as Bi2O3 and WS2 were not observed in the XRD patterns 10


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(Figure 2b).

Figure 2. XRD patterns of (a) WO3 Nanoparticles, and (b) WO3NP/Bi2S3NW composite on the FTO substrate.

Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analysis were used to identify the structural composition of WO3NP/Bi2S3NW composite (Figures 3 and 4). The HRTEM and corresponding fast Fourier transform (FFT) patterns indicate the 11



single-crystal structure of Bi2S3 nanowire (Figure 3), reflecting the highly crystalline growth nature of the (220) lattice plane.35 The FFT diffractogram is in agreement with the (110), (220), and (130) planes of orthorhombic Bi2S3 (Figure 3c), and that of the bottom layer is consistent with the (002), (022), and (020) planes of triclinic WO3 (Figure 3d). Consequently, the single-crystalline Bi2S3 nanowires were uniformly grown on the surface of the WO3 nanoparticle layer. Spatial elemental mapping was also performed on a WO3NP/Bi2S3NW structure to study the distribution of each element on the hetero-structure (Figure 4). The elemental mapping results confirm that the Bi2S3 nanowires were directly grown on the WO3 nanoparticle layer. Energy-dispersive X-ray (EDX) spectrometry of the Bi2S3 nanowires yielded an average atomic ratio of 38 : 59 (Bi : S), indicative of 2 : 3 atomic compositions.

Figure 3. TEM images of (a) Bi2S3 nanowire, and (b) WO3NP/Bi2S3NW junction regions. FFT patterns of (c) Bi2S3 and (d) WO3 regions.

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Figure 4. (a) TEM image of WO3NP/Bi2S3NW composite, and the element distribution maps of (b) Bi, (c) S, and (d) W on the WO3NP/Bi2S3NW composite.

During the hydrothermal reaction, the S2- ions were generated by the decomposition of thiourea.25 Then, the Bi3+ ions in the bismuth (III) oxides (Bi2WO6 and Bi14W2O27 noted simply as Bi2O3) layer reacted with S2- ions to form the Bi2S3 nanowires. To obtain a reliable exchange mechanism, SEM images were taken during the reaction process (Figure S2). The surface of Bi2O3 was initially smooth, but after the hydrothermal reaction for 10 min began to roughen, indicating the initiation of sulfidation (Figure S2a). At 30 min, small Bi2S3 nanoparticles and nanorods were formed on the surface of the Bi2O3 (Figure S2b). Subsequently, the Bi2S3 particles grew bigger, and were then reorganized to form Bi2S3 nanorods after the 1 h reaction. In a further 4 h hydrothermal reaction, the Bi2S3 nanowire growth reached saturation, even when the reaction time was extended. Because there was no 13



extra addition of the Bi3+ source during the process, the Bi2O3 layer acted as a substrate as well as a Bi3+ source for the growth of Bi2S3 nanowires. Because the Bi2S3 has far smaller solubility than Bi2O3 (Ksp: 1 × 10-97 vs. 4 × 10-31),35,36 the hydrothermal sulfidation of Bi2O3 may spontaneously follow what Chen et al. describes as the “etching and regrowth mechanism”.35 In the presence of a large amount of S2- , the dissolved Bi3+ from Bi2O3 reacts with S2-, and the balance between Bi2O3 and Bi3+ is shifted towards the Bi3+ formation. In other words, the formation of Bi2S3 promotes the dissolution of Bi3+ from the Bi2O3, and then the Bi2S3 grows rapidly on the surface of the Bi2O3 layer. The growth direction of the nanowires is closely related to the substrate as well as the synthetic conditions.37 The Bi2S3 nanowires tend to grow rapidly along the direction perpendicular to the substrate in a solution containing a large amount of thiourea.35 In this synthetic process, the S2- ions were abundantly generated by the decomposition of thiourea and thus the Bi2S3 nanowires were grown vertically on the WO3 substrate. The present synthetic method can be employed to other metal oxide substrates, such as BiVO4, ZnO, and TiO2. The synthetic methods used for these substrates were identical to the synthesis of the WO3NP/Bi2S3NW, except for the use of BiVO4, ZnO, and TiO2 as substrates (Figure S3). This synthetic method is a simple and fast route for the synthesis of well aligned Bi2S3 nanowires on various substrates in general. Furthermore, it can also apply for other metal sulfide structures such as CdS, and CoS2 (Figure S4), which are known to be useful semiconductor electrodes, on the WO3 substrate. Therefore, our approach is not limited only for WO3/Bi2S3, but can be expanded to various hetero-structures with precisely well-defined interfaces.

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Photoelectrochemical Characterization of WO3NP/Bi2S3NW Composite Electrode. The hetero-structures such as WO3/BiVO4, WO3/CuWO4, and CuWO4/BiVO4, which show good alignment of conduction and valence band positions, have been studied as promising PEC electrodes due to efficient carrier separation in the composites.12,14,38,39 Although the improved PEC properties of the hetero-structures are now widely known, their origins are far from fully understood. Further studies need to be conducted to optimize the semiconductor morphology, crystallinity, and interface with other layers for improved PEC performances. To compare the photoelectrochemical (PEC) perfomance of the WO3NP/Bi2S3NW composite, WO3NP/Bi2S3(SILAR) and WO3NP/Bi2S3NR(Drop) hetero-structures were prepared by the successive ionic layer adsorption and reaction (SILAR)33 and drop-casting (Drop) methods, respectively. The SILAR method, WO3NP/Bi2S3(SILAR), does not contain a calcination process, but only involve a wet-chemical deposition. Nevertheless, it has a reliable interfacial contact of the two components in the composite film. On the other hand, the WO3NP/Bi2S3NR(Drop) was fabricated by drop-casting of the Bi2S3 nanorods onto the surface of WO3 nanoparticles, and therefore the weak interfacial contact would lead to a high resistance in the WO3NP/Bi2S3NR(Drop) hetero-structure interface. Figure S5 shows the SEM images and XRD patterns of the WO3NP/Bi2S3(SILAR) and WO3NP/Bi2S3NR(Drop) hetero-structures. No secondary phase was observed in the XRD patterns. All electrode samples were adjusted with the same thickness of Bi2S3 layers using the UV-visible absorption spectra, and showed nearly identical optical behaviors, particularly in the absorption intensity (Figure S6). A sacrificial electron donor, sodium sulfite (Na2SO3), was used to investigate the PEC performance without surface recombination. Sulfite anions have been recognized as an efficient scavenger of hydroxyl radicals and are known to react in nearly diffusion-controlled 15



rates.40 Thus, the sulfite oxidation is an excellent model reaction for measuring the degree of recombination at the semiconductor (bulk recombination), and it is considered to be 100% in surface transfer efficiency during the PEC measurement due to its fast kinetics at the semiconductor-sulfite interface.41

Figure 5. (a) LSVs of WO3NP/Bi2S3NW composite under UV-visible illumination (back-side) in a 0.1 M Na2SO3 and 0.1 M Na2S solution. Scan rate: 20 mV/s. Light intensity: 100 mW/cm2. (b) Photocurrent density of electrodes in WO3/Bi2S3 hetero-structures under UVvisible (black) and visible illumination (red) at an applied potential of -0.10 V vs. Ag/AgCl.

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The PEC performance of all electrodes was investigated using linear sweep voltammetry (LSV) for sulfite oxidation (0.1 M Na2S + 0.1 M Na2SO3). The LSVs were conducted from 0.7 to 0.0 V vs. Ag/AgCl at a scan rate of 20 mV/s with chopped light under UV-visible irradiation (Figure 5 and Figure S7). All electrode samples successfully generated anodic photocurrents, which confirmed the n-type characteristics of the electrodes. The photocurrent of WO3NP/Bi2S3NW composite is 5.2 mA/cm2 at -0.1 V (vs. Ag/AgCl), while the WO3NP/Bi2S3(SILAR) electrode generates 2.5 mA/cm2, and the WO3NP/Bi2S3NR(Drop) electrode only generates 1.4 mA/cm2 at the same potential under the back-side UV-visible illumination (black bars in Figure 5b). Under the visible light irradiation (> 425 nm), the photocurrent of WO3NP/Bi2S3NW composite is also several times higher than those of the WO3NP/Bi2S3NR(Drop) and WO3NP/Bi2S3(SILAR) electrodes (red bar in Figure 5b). Note that the bare WO3NP and Bi2S3NR electrodes showed negligible photocurrent compared to the WO3/Bi2S3 hetero-structures (Figure 5b and Figure S7). The WO3NP/Bi2S3NW composite exhibited a slight enhancement of the photocurrent by the back-side illumination compared to that of the front-side illumination (Figure S8). In this nanowire sample, the electron mobility rather than the hole mobility dominates the charge extraction efficiency.42 When the nanowire length exceeds the carrier diffusion length (~70 nm for Bi2S3),43 holes in the middle of nanowires may be trapped or recombine with majority electrons. Thus, electron transport to the FTO interface becomes the limiting factor in the photocurrent density. When the WO3NP/Bi2S3NW composite is illuminated backward, there is a shorter diffusion length for the electrons to reach the contact than the case of the frontside illumination. Therefore, the PEC efficiency under the back-side illumination is improved compared to that of the front-side illumination. Figure 6a shows the action spectrum of WO3NP/Bi2S3NW composite, which indicates the 17



typical photocurrent depending on the wavelength with a 10 nm interval. The band gaps were determined from the wavelengths for the onset of the photocurrent. The action spectrum shows a band gap of approximately 1.45 eV, and its profile is in good agreement with the UVvisible absorption spectrum (Figure 6b). The WO3NP/Bi2S3NW composite shows the same onset wavelength as for Bi2S3 (1.3-1.7 eV),26 which indicates that the Bi2S3NW is the main absorber for the PEC reaction. In order to determine the detailed PEC properties depending on the length of nanowires, the PEC performance under UV-visible irradiation was evaluated for electrodes consisting of Bi2S3 nanowires prepared with different growth times. The photocurrents increased as the nanowires were grown from Bi2S3 seeds, and then saturated at 4 h, even though the growth reaction period of the Bi2S3 nanowires continued further (Figure S9). To understand the origin of enhanced PEC activity in the composite, the flat band potentials for WO3 and Bi2S3 were measured. The Mott-Schottky plots were investigated in 0.1 M Na2SO4 solution at 500 Hz (Figures 6c and 6d).24 The Mott-Schottky plot according to the slope in the quasi-linear region of the Bi2S3 indicated that the flat band potential for Bi2S3 is around 0.15 V (vs. NHE) with an n-type behavior. The flat band potential for WO3 was estimated from the Mott-Schottky plot to be around 0.45 V (vs. NHE). The conduction band edge in many n-type semiconductors is often considered to be more negative than its flat band potential by about 0.1 V.24 Thus, the typical conduction band edge (ECB) and valence band edge (EVB) of Bi2S3 are about 0.05 and 1.50 eV (vs. NHE), and the ECB and EVB of WO3 are about 0.35 and 3.20 eV (vs. NHE), respectively, which also match well with the values in the literature.24,25 Figure 7a shows photogenerated electron-hole pathways between WO3 and Bi2S3 based on the Mott-Schottky plots (Figure 6), and TEM images in Figure 4 (WO3 surface is completely covered by Bi2S3). 1) electron transfer from the conduction band of 18


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Bi2S3 to that of WO3, 2) electron-hole recombination at the Bi2S3 valence band, 3) transfer of a hole from the valence band of WO3 to that of Bi2S3, and 4) oxidation at the valence band of Bi2S3.

Figure 6. (a) Action spectrum of WO3NP/Bi2S3NW composite at an applied potential of 0.40 V vs Ag/AgCl in a 0.1 M Na2SO3 and 0.1 M Na2S solution. (b) UV-visible absorption spectrum and Tauc plot (inset) of WO3NP/Bi2S3NW composite. Mott-Schottky plots of (c) WO3, and (d) Bi2S3 obtained from the AC impedance capacitance measurements in 0.1 M Na2SO4 solution.

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Figure 7. (a) Photogenerated electron-hole pathways between two semiconductors (WO3 and Bi2S3). (b) Current-time response curves of WO3NP, WO3NP/Bi2S3NW composite, and WO3NP/Bi2S3(SILAR) electrodes under visible light illumination at an applied potential of 0.40 V vs. Ag/AgCl in a 0.1 M Na2SO3 and 0.1 M Na2S solution.

The ECB and EVB of WO3 are more positive than those of Bi2S3, which are favorable for the charge separation of Bi2S3. The origin of enhanced PEC activity of the hetero-structures (WO3NP/Bi2S3NR(Drop),

WO3NP/Bi2S3(SILAR),

and

WO3NP/Bi2S3NW

electrodes)

compared to the bare Bi2S3 electrode is mainly attributed to the enhanced charge separation 20


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of Bi2S3 on the WO3 electrode (pathways 1 and 3 in Figure 7a) under solar light irradiation. To assess the detailed interaction of the Bi2S3, chronoamperometry was carried out at -0.4 V vs. Ag/AgCl under visible light irradiation only (Figure 7b). The photocurrents from the WO3NP/Bi2S3NW composite and WO3NP/Bi2S3(SILAR) electrodes show subsequent interaction under visible irradiation. However, the WO3 nanoparticles show negligible photocurrent under visible irradiation, which indicates that Bi2S3 was the main absorber, and the relevant photocurrent was associated with pathway 1 rather than 3 in Figure 7a. Furthermore, the WO3NP/Bi2S3NW composite has an effective interface by the in-situ growth method, which may lead to a lower resistance in the composite interface (reduced pathway 2 in Figure 7a), compared to that of the WO3NP/Bi2S3NR(Drop) electrode. The optical bandgap of samples have been calculated by the Tauc equation.44 As shown in the Figure S6, the direct band gaps of hetero-structures (WO3NP/Bi2S3NR(Drop), WO3NP/Bi2S3NW, and WO3NP/Bi2S3(SILAR)) are estimated to be 1.35, 1.45, and 1.65 eV, respectively. The WO3NP/Bi2S3(SILAR) electrode is composed of multi-layers of nanoparticles, and these nanostructures are associated with a relatively low activity in the visible light region due to quantum confinement caused by small grain sizes.44 Thus, the UVvisible absorbance of WO3NP/Bi2S3(SILAR) showed limited absorptions in the visible and near IR regions compared to that of WO3NP/Bi2S3NW. Consequently, the increase in photocurrent from the WO3NP/Bi2S3NW composite compared to the other hetero-structures is attributed to the directly grown Bi2S3 nanowire on WO3 nanoparticle with less interfacial defects, and single crystallinity with optimum length of nanowires. To assess the stability of the hetero-structures over time, chronoamperometry was carried out at -0.5 V (vs Ag/AgCl) under UV-visible irradiation (Figure S10). The current transients upon turning on light was usually caused by the dynamics balance of photogenerated charges 21



and their consumption at the semiconductor/electrolyte interface.45,46 This is a typical sign of surface recombination processes on the WO3NP/Bi2S3NW composite even in the presence of sacrificial reagents.47 Although the hetero-structures showed an initial drop in photocurrent, the photocurrent stabilized at a steady-state value in the presence of sacrificial reagent (Figure S10). For the stability in air, choronoamperometry was carried out after the exposure to air for 30 days (Figure S11). Although the WO3NP/Bi2S3NW showed a small drop in photocurrent, the PEC performance showed almost identical value. The WO3NP/Bi2S3NW is robust in air. All electrode samples were also tested for the PEC water oxidation in a pH 7 phosphate buffer solution without sacrificial reagents. The LSV was conducted from - 0.3 to 0.8 V vs. Ag/AgCl at a scan rate of 20 mV/s with chopped light under UV-visible irradiation (Figure S12). The photocurrent of WO3NP/Bi2S3NW composite was 0.9 mA/cm2 at 0.6 V (vs. Ag/AgCl), while the WO3NP/Bi2S3(SILAR) electrode generated 0.6 mA/cm2, and the WO3NP/Bi2S3NR(Drop) electrode only generated 0.4 mA/cm2 at the same potential. Similar to Figure 6b, the WO3NP/Bi2S3NW composite showed enhanced PEC water oxidation compared to the WO3NP/Bi2S3(SILAR) or WO3NP/Bi2S3NR(Drop) hetero-structures. Note that the PEC water oxidation value of WO3NP/Bi2S3NW composite is higher than the other reported results using Bi2S3 materials.24,47 However, the WO3NP/Bi2S3NW composite showed a continuous decrease of photocurrent without reaching a steady-state photocurrent indicating that the WO3NP/Bi2S3NW composite was unstable during water oxidation. The absolute photocurrent values were much smaller than those with the sacrificial reagent due to the slow rate of water oxidation on the semiconductor surface with less stability,24 but these shortcomings would be able to overcome by the addition of passivation layers and/or electrocatalysts on the Bi2S3 surface through further studies. 22


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CONCLUSION The WO3NP/Bi2S3NW composite was synthesized via an in-situ hydrothermal reaction. The single-crystalline Bi2S3 nanowires were uniformly grown on the surface of the WO3 nanoparticle layer. The spontaneous growth of Bi2S3 from the Bi2O3 substrate by the etching and regrowth mechanism provides a good interaction with the WO3 layer. This synthetic method can be a general route for the synthesis of well aligned Bi2S3 nanowires on various substrates such as TiO2, BiVO4, and ZnO. Furthermore, it is also effective for the formation of other metal sulfide structures, such as CdS, and CoS2 which are known to be useful semiconductor electrodes. The resulting WO3NP/Bi2S3NW composite showed enhanced PEC activity compared to those of the other Bi2S3 electrodes. These enhanced levels of activity mainly originate from three factors: (1) the enhanced charge separation of Bi2S3 on the WO3 layer due to the ECB and EVB of WO3 being more positive than those of Bi2S3, which are favorable for the charge separation of Bi2S3, (2) the effective composite interface by the insitu growth method, and (3) the single crystallinity of Bi2S3 with optimum length of nanowire, which provides a direct electrical pathway through a single domain of nanowire. These findings provide important guidance for the design of highly efficient electrodes, particularly bearing on hetero-structures, for enhanced PEC performances.

ASSOCIATED CONTENT Supporting Information. The TEM and SEM images, XRD patterns, UV-visible spectra of the electrodes are shown. This information is available free of charge via the Internet at http://pubs.acs.org 23



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by the Basic Science Research Program through the National Research

Foundation

of

Korea

(NRF)

funded

by the Ministry of

Science, ICT & Future Planning (NRF-2017R1E1A1A01074224, NRF-2017R1A2B3004096, NRF-2017R1D1A1B03031892). H.S. acknowledges the financial support of National Research Foundation under "Next Generation Carbon Upcycling Project” (Project No. 2017M1A2A2046713) of the Ministry of Science and ICT, Republic of Korea.

ABBREVIATIONS PEC, photoelectrochemical; FTO, fluorine-doped tin oxide; SEM, scanning electron microscopy; TEM, transmission electron microscopy; HR-TEM, high-resolution TEM; FIB, focused ion beam; FFT pattern, fast Fourier transform pattern; XRD, X-ray diffraction; LSV, linear sweep voltammetry.

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Bi2S3 Nanowire Composite

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