Sb2S3 Heterojunction Photocatalyst Based on WO3 of

Apr 1, 2016 - electrochemical (PEC) water splitting. The antimony sulfide. (Sb2S3) was incorporated onto WO3 for the first time with the aim of improv...
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Novel WO3/Sb2S3 Heterojunction Photocatalyst Based on WO3 of Different Morphologies for Enhanced Efficiency in Photoelectrochemical Water Splitting Jing Zhang, Zhihua Liu, and Zhifeng Liu* School of Materials Science and Engineering, Tianjin Chengjian University, Tianjin 300384, China ABSTRACT: We report the fabrication of tungsten trioxide (WO3) with different morphologies applied in photoelectrochemical (PEC) water splitting. The antimony sulfide (Sb2S3) was incorporated onto WO3 for the first time with the aim of improving its photoelectrocatalytic activity under visible-light illumination. In the present work, WO3 of different morphologies were fabricated on FTO glass via adjusting the pH value via a facile hydrothermal method and the morphological effect on the photoelectrocatalytic activity of the obtained samples has been discussed. WO 3 /Sb 2 S 3 heterojunction photoelectrocatalysts were subsequently synthesized successfully to further improve the photoelectrocatalytic activity. Among them, WO3/Sb2S3 heterojunction photoelectrocatalyst based on WO3 micro crystals achieved an enhanced photocurrent of 1.79 mA/cm2 at 0.8 V versus RHE under simulated sunlight, compared to 0.45 mA/cm2 of pristine WO3 micro crystals. This excellent PEC performance benefits from the enhanced light absorbance, construction of suitable energy band gap, the improved photogenerated electron−hole pairs separation and transfer efficiency, which potentially provides new insights into PEC water splitting systems. KEYWORDS: WO3, morphology, Sb2S3, heterojunction, photoelctrochemical

1. INTRODUCTION Because fossil energy resources have been overconsumed so that consequence of environment pollution on the global scale, development of technologies for clean and sustainable energy has been attracted increasingly intense attention.1 Hydrogen energy is considered as a promising clean energy resource to ameliorate climate change and environmental issues associated with the combustion of fossil fuels.2 Because Fujishima and Honda found it feasible to decompose water via a photoelectrochemical system with a TiO2 electrode for the first time in 1972,3 photoelectrochemical (PEC) water splitting driven by solar power has attracted intensely attention. Currently, TiO2 (∼3.2 eV),4 ZnO (∼3.2 eV),5 α-Fe2O3 (∼2.2 eV),6 BiVO4 (∼2.4 eV),7 and WO3 (∼2.8 eV)8 are the most popular semiconductor photocatalysts for PEC water splitting. Among them, WO3 has good resilience to photocorrosion effect in aqueous solution as well as excellent electron transport properties.9 Several researches discussed different nanostructures of WO3 used for PEC water splitting,10−14 specifically, Li et al. met the challenge and prepared two-dimensional WO3 nanoflakes photoanodes via a postgrowth modification method, which shows an improved photocurrent density of 1.10 mA/ cm2 at 1.23 V vs RHE applied in PEC water splitting.10 However, pristine WO3 is not effective semiconductor photocatalysts because of the low sunlight absorption, high photogenerated electron−hole recombination rate and the difficulty in H2 evolution due to its lower position of conduction band (CB).15,16 Numerous tactics have been © 2016 American Chemical Society

adopted including morphology and dimension controlling, use of hierarchical nanostructures, addition of metal and nonmetal dopant and nanocomposite constructing so as to enhance the photoelectrocatalytic efficiency of semiconductor photocatalysts.17−22 Particularly, the coupling of WO3 with narrow energy band gap semiconductors including GaInP2, BiVO4, Cu2O or CdS is an excellent approach to enhance the light absorbance ability and photoelectrocatalytic activity of WO 3 . Peng et al. have synthesized 1D WO 3 −Bi 2 WO 6 heterojunctions for the first time via a facile hydrothermal method showed higher photoelectrocatalytic activities than pure WO3.23 Zhang et al. have structured a Z-scheme photocatalytic mechanism based on CdS/WO3 heterojunction which can produce H2 energy.24 To date, antimony sulfide (Sb2S3) attracts ,much attention as a brilliant candidate for high-performance photoelectrode because of a high optical absorption coefficient and an appropriate energy band gap of 1.7−1.9 eV, which is adapted to cover the whole solar spectrum.25−29 In this work, WO3 of different morphologies were synthesized via a hydrothermal method by changing the pH value in the precursor solution and the influence of pH value on controlling the morphologies of the samples was systematically investigated. The samples exhibited high chemical stability as Received: January 13, 2016 Accepted: April 1, 2016 Published: April 1, 2016 9684

DOI: 10.1021/acsami.6b00429 ACS Appl. Mater. Interfaces 2016, 8, 9684−9691

Research Article

ACS Applied Materials & Interfaces well as thermostability. However, they still could not meet the requirements for practical application due to the low photoelectric conversion efficiency. Therefore, it was significant to improve the photoelectrocatalytic efficiency by coupling with other semiconductors. In this condition, the WO3/Sb2S3 heterojunction photoelectrocatalysts were synthesized for the first time in order to improve the photoelectrocatalytic efficiency. Detailed growth mechanisms for WO3 nanostructures and WO3/Sb2S3 heterojunctions were investigated. The optical property of heterojunction structures was measured as well as the reason for why the absorbance spectrum enhanced has been discussed. The possible photoelectrocatalytic mechanisms and the main active species were also explored to further understand the specific photoelectrocatalytic process. And the results demonstrated that the heterojunction exhibited more efficient photoelectrocatalytic performance than that of pure WO3. This benefits from the enhanced light absorbance spectrum, construction of suitable energy band gap, the improved charge separation and transfer efficiency, which potentially provides new insights into PEC water splitting systems.

into reversible hydrogen electrode (RHE) potential after electrochemical experiments. The photocurrent−voltage curves of the samples were examined using a potentiostat (SVC-1.5, Shanghai, China). The stability of the obtained samples was examined by current−voltage (C−V) scanning. It was conducted at a potential from −0.60 to 1.20 V for 10, 50, and 100 periods under irradiation (100 mW cm−2) and the scanning speed of voltage is 10 mV s−1.

2. EXPERIMENTAL SECTION

Figure 1. Schematic illustration for the synthetic route of WO3 nanostructures with different morphologies and WO3/Sb2S3 heterojunction structures.

3. RESULTS AND DISCUSSION Figure 1 summarizes the formation processes of WO3 nanoplates (NPs), WO3 nanorods (NRs) and WO3 micro-

2.1. Synthesis of Different Morphologies WO3. All the reagents were of analytical grade and used without further purification. Sodium tungstate dihydrate (Na2WO4·2H2O), hydrochloric acid (HCl), and potassium oxalate (K2C2O4·H2O) were used as the raw materials. The WO3 nanostructures were fabricated on the average-size fluorine-doped tin oxide (FTO) glass via a hydrothermal method. First, 2.4 g of Na2WO4·2H2O and 0.2 g of K2C2O4·H2O were dissolved in 90 mL of deionized water and then kept stirring for 6 h. Subsequently, 2 M concentrated hydrochloric acid was slowly dropped into the precursor solution with stirring to control the pH values at 0.5, 1.0, and 1.5, respectively. Afterward, the solution was transferred into a Teflon-lined hydrothermal synthesis reactor with 100 mL capacity and maintained at 180 °C for 24 h. The samples were obtained after the hydrothermal treatment. At last, the as-obtained samples were washed with distilled water and dried at 60 °C in air. 2.2. Preparation of WO3/Sb2S3 Heterojunction Photoelectrodes. In a typical experimental, 0.04 M antimony chloride (SbCl3) was first introduced into 100 mL of distilled water and kept stirring for 15 min. Subsequently, 0.03 M sodium thiosulfate (Na2S2O3·5H2O) was added and kept stirring for half an hour. And then the mixed precursor solution was transferred into a Teflon-lined hydrothermal synthesis reactor with the obtained different morphologies of WO3 on FTO substrates immersed in it. WO3/Sb2S3 heterojunction structures were obtained after sealing and maintaining at 70 °C for 2 h. After the reaction, the final products were scoured by deionized water as well as ethanol for the removal of ions remaining in it and naturally cooled to room temperature in air. 2.3. Characterization. The morphology of the samples was observed by PHILIPS XL-30 scanning electron microscope (SEM) and JEOL JEM-2100 transmission electron microscopy (TEM and HRTEM). The surface area was studied on a Nova 3000e Surface Area Analyzers. Crystal structures of the as-obtained products were characterized by X-ray diffractometer (XRD) using a Rigaku-D/max2500 using Cu Kα radiation (λ = 0.154059 nm) at 40 kV and 200 mA. Optical absorption properties of the photoelectrodes were investigated through DU-8B UV−vis double-beam spectrophotometer. PEC water splitting capability was performed in H2SO4 electrolyte (1 mol/L) and which was conducted in a three-electrode configuration consisting of a working electrode, a saturated Ag/AgCl as reference electrode and a platinum foil as counter electrode via an electrochemical workstation (LK2005A, Tianjin, China). To be specific, the obtained WO3 nanostructures and WO3/Sb2S3 heterojunction structures on FTO were used as the working electrode and irradiated with a xenon lamp (CHF-XM500, 100 mW cm−2). Ag/AgCl potential was transformed

crystals (MCs) by adjusting different pH and the synthetic route for WO3/Sb2S3 heterojunction structures. At first, the WO3 nanostructures of different morphologies were fabricated on the FTO glass during the hydrothermal reaction. It is generally known that pH value will have an effect on degree of supersaturation of solution, size of crystal particle and morphology of crystal during hydrothermal process.30,31 In this step, different morphologies of WO3 nanostructures were achieved by means of adjusting pH value in the growth solution. To be specific, WO3 nanoplates (NPs) are achieved when the pH value of solution is 0.5, the pH value varies to 1.0 WO3 nanorods (NRs) are achieved and if the pH value changes to 1.5 WO3 microcrystals (MCs) are achieved. The following chemical reaction could take place: Na 2WO4 ·2H 2O + 2HCl + nH 2O ⇋ H 2WO4 ·(n + 2)H 2O + 2NaCl

H 2WO4 ·(n + 2)H 2O ⇋ WO3 + (n + 3)H 2O

(1) (2)

Different morphologies of WO3 nanostructures were achieved as the chemical reaction above we can see, when the pH value of solution varies while the amount of H+ is changed, WO42− anions polymerize to form tungstic acid. The obtained samples followed by calcinations at 550 °C for 2 h after the first-step hydrothermal reaction in order to crystallize completely. Sequentially, Sb2S3 particles were prepared by putting the FTO substrates with the as-prepared different morphologies of WO3 in the mixed solution containing 0.04 M of antimony chloride (SbCl3) and 0.03 M of sodium thiosulfate (Na2S2O3· 5H2O) in a Teflon-lined hydrothermal synthesis reactor and then sealing and maintaining at 80 °C for 2 h. At last, WO3/ Sb2S3 heterostructures were achieved. The following chemical reactions may take place during this action SbCl3 → Sb3 + + 3Cl1 − 9685

(3) DOI: 10.1021/acsami.6b00429 ACS Appl. Mater. Interfaces 2016, 8, 9684−9691

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ACS Applied Materials & Interfaces Na 2S2 O3 ·5H 2O → 2Na1 + + S2 O32 − + 5H 2O

orientation by adjusting the pH values of the precursor solutions (Figure 2d−f). Each kind of WO3 nanostrutures represents the varied crystalline structure such as nanoplate and nanorod represent monoclinic phase while microcrystal represents hexagonal phase. Because of the high surface energy of (001) facet, it is primordially expected to be extremely positive whose crystalline structure shown in Figure 2f. The photocatalytic activity of a photocatalyst is strongly dependent on the electronic and surface atomic constructions, which affected by the presence of crystal facets with different crystal orientations.8 Figure 3 shows the XRD patterns of the WO3 NPs (a), WO3 NRs (b), and WO3 MCs (c), respectively. From curve a, most

(4)

2Sb3 + + 3S2 O32 − + 3H 2O → Sb2S3 + 3SO4 2 − + 6H1 + (5)

The morphologies of as-obtained products were characterized by SEM Figure 2 shows the SEM images a−c of the WO3

Figure 2. SEM images of WO3 nanostructures with different morphologies (a) WO3 NPs, (b) WO3 NRs, and (c) WO3 MCs. Crystalline structures of WO3 nanostructures with different morphologies: (d) monoclinic WO3 NPs, (e) monoclinic WO3 NRs, and (f) hexagonal WO3 MCs.

Figure 3. XRD patterns of (a) WO3 NPs, (b) WO3 NRs, and (c) WO3 MCs.

nanoplates (NPs), WO3 nanorods (NRs) and WO3 microcrystals (MCs) prepared from a sodium tungstate using a simple hydrothermal method with the addition of different amount of hydrochloric acid. It is indicated in Figure 2a that the as-synthesized sample has monoclinic plate-like morphology with an average size of ∼900 nm, which presents that the product is regular because that large amount of nanoplates were aggregated. The mean single NP approximated the thickness at 200 nm. As can be seen from Figure 2b, the average diameter of the rod-like morphology sample is 250 nm approximately. Figure 2c shows a top view of WO3 microcrystals with approximate octahedral morphology. It is observed that pH value of precursor solution had an effect on the morphology of the final products.32 From the top view images of the samples, it is found that the nanoplates grow as aggregation and then distribute in different direction gradually (Figure 2a). The well-aligned WO 3 nanorods and microcrystals grow vertically on the substrate (Figure 2b, c). It is reasonable to form such morphologies under the hydrothermal conditions of high temperature and high pressure. Additionally, the specific surface area of WO3 with different morphologies as nanoplates, nanorods and microcrystals was measured with value 18.1, 18.7, and 17.6 m2 g−1, respectively. It is significant to fine-tune WO3 nanostructures by exposing specific facets by means of different facets with different capabilities inherent.8 On the basis of the above, we prioritize the fine-tuning of WO3 nanostructures by exposing specific facets with dominant facets, such as the (001) facet. The crystal preferential orientation of the WO3 nanostructures depends mainly on the pH values in the solution of hydrothermal process because pH value can not only influence the solubility of a substance but also affect the growth of the crystal efficiency and change the structure of the growth in the solution. In one word, the crystal structure/shape/size and start crystallization temperature are eventually decided by pH value.33 So we achieved various WO3 nanostructures of different crystal

of the diffraction peaks are indexed by the monoclinic WO3 with the lattice spacing of a = 7.297 Å, b = 7.539 Å, c = 7.688 Å, β = 90.91°and a space group of P21/n14 (Joint Committee on Powder Diffraction Standards, JCPDS Card No. 43−1035), indicating the samples are pure. The diffraction peaks in curve (a) with 2θ value around 23.1, 23.6, 24.4, 49.9, and 56.0° could be observed, which can be indexed to the (002), (020), (200), (140), and (420) plane, respectively. The XRD pattern presents a strong {200} preferred orientation, as proved by the strengthened intensity of the (200), (020) and (002) facets (Figure 3a). In great contrast to the XRD pattern of the WO3 NPs, besides a strong (020) diffraction, (200) diffraction, and (420) diffraction, diffraction (001), (120), (220), (221), (140), and (240) with 2θ value around 23.2, 26.7, 34.2, 42.0, 50.0, and 54.9° can be detected in the XRD pattern of the WO3 NRs (Figure 3b) indexed by the monoclinic with the lattice spacing of a = 7.285 Å, b = 7.517 Å, c = 3.835 Å, β = 90.15° and a space group of P21/a14 (JCPDS Card No. 05−0363), indicating that the WO3 NRs has a strongly preferential orientation parallel to the WO3 NPs with a monoclinic structure. As can also be seen from Figure 3c, all diffraction peaks from the hexagonal phase of WO3, with lattice spacing of a = b = 7.317 Å and c = 3.894 Å and a space group of P6/mmm (JCPDS Card No. 33−1387). 2θ values 14.4°, 22.7°, 24.3°, 28.7°, 36.6°, 50.0° and 55.5° corresponding to (100), (001), (110), (220), (201), (220), and (221). The exposure of {100} and {110} crystal facets of WO3 microcrystals depended mainly on the priority growth direction along [100], [001] and [110] orientation. Therefore, the product synthesized by the hydrothermal method is monoclinic and hexagonal WO3 with different lattice constants. The optical properties of WO3 NPs, WO3 NRs and WO3 MCs have been characterized which is illustrated in Figure 4a. Obviously, the absorption edge of the WO3 NPs is ∼330 nm, WO3 NRs is ∼340 nm, and WO3 MCs is near 350 nm. In addition, the band edges of WO3 NPs, WO3 NRs, and WO3 9686

DOI: 10.1021/acsami.6b00429 ACS Appl. Mater. Interfaces 2016, 8, 9684−9691

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ACS Applied Materials & Interfaces

Figure 4. (a) UV−vis absorbance spectra and (inset of a) the energy gap of WO3 with different morphologies; (b) photocurrent density− voltage curves and (inset of b) the photographs of FTO and WO3 nanostructures of the samples.

MCs are inserted in Figure 4a. The band gap (Eg) is identified by the following equation.34 (αhν)n = A(hν − Eg )

Figure 5. (a)SEM image, (b) XRD pattern, (c) HRTEM image, and (inset of c) TEM image of the obtained WO3 NPs/Sb2S3 structure.

(6)

Eq 6 is used for calculating the optical band gap energy (Eg) of samples, α represents the absorbance coeffcient, hν represents the incident light intensity, A is a constant. When n is equivalent to 2 shows a direct band gap and 1/2 shows an indirect band gap. According to eq 6, the band gaps of WO3 NPs, WO3 NRs, and WO3 MCs were caculated to be 2.96, 2.82, and 2.75 eV, respectively. The photocurrent density−voltage curves of as-obtained WO3 structures with different morphologies are researched as exhibited in Figure 4b. It is indicated that the densities of photocurrent of WO3 NPs, WO3 NRs and WO3 MCs were 0.36, 0.39, and 0.45 mA cm−2 at 0.8 V versus RHE, respectively. From the comparison, the photoelectrocatalyst which obtained based on hexagonal WO3 microcrystals when the precursor solution pH is 1.5 have a correspondingly high photocurrent. Furthermore, to investigate the relevance of surface area in the photocurrent difference, total surface area has been characterized. The surface areas of WO3 with different morphologies as nanoplates, nanorods, and microcrystals are 18.1, 18.7, and 17.6 m2 g −1, respectively. As the results shown the surface area of different morphologies has little effect on the photocurrent difference. In addition, the high response of this hexagonal microcrystal material may be attributed to the following reasons. On one hand, the product has a good crystalline character. The high crystallization will result in low bulk defects, thus lowering the conductivity of the catalyst body. On the other hand, the hexagonal structure has trim edges, which will form grain boundaries in the catalyst body. According to Zheng et al., grain boundaries as well as grain junctions are usually considered as the surface active sites, so that they could responde positively to the light.8 To further improve the photoelectrocatalytic properties of the WO3, we subsequently synthesized WO3/Sb2S3 heterojunction photoelectrocatalysts successfully. As displayed in Figure 5a, the random Sb2S3 nanoparticles are deposited on the surface of WO3 NPs, whose diameter is ∼200 nm. As shown in Figure 5b, except for the diffraction peaks assigned to WO3 NPs (JCPDS N0. 43−1035), the additional peaks are identical to the orthorhombic phase of Sb2S3 (JCPDS NO. 02−0374), revealing the presence of Sb2S3 loaded on the WO3 nanoplates substrates. To be specific, the crystalline peaks at 2θ values if 17.7, 25.1, and 43.0° correspond to the (210), (310), and (421) planes. Figure 5c shows the HRTEM images of WO3 NPs deposited with Sb2S3 and the inset graph shows the sample at a

low magnification, respectively. From the HRTEM image of sample, the monoclinic WO3 (200) plane with lattice distance of 0.365 nm and the orthorhombic Sb2S3 (310) plane with lattice distance of 0.335 nm can be scanned clearly, respectively. The above experimental results showed that the WO3 NPs/ Sb2S3 was fabricated successfully. It can be observed in Figure 6a that the obtained samples inherit the morphology of preprepared WO3 NRs. When the

Figure 6. (a) SEM image, (b) XRD pattern, (c) HRTEM image, and (inset of c) TEM image of the obtained WO3 NRs/Sb2S3 structure.

Sb2S3 nanoparticles modification was conducted onto the surface of WO3 NRs, the facets of nanorods became flattened as reported.35 This latter observation reveals that the Sb2S3 nanoparticles were indeed loaded on the surface of the WO3 NRs/Sb2S3. To further confirm the chemical composition as well as the crystal structure, we performed the XRD pattern and it is shown in Figure 6b. When compared with the WO3 NRs diffraction pattern (JCPDS, NO. 05−0363), the additional peaks are identical to the orthorhombic structure of Sb2S3 (JCPDS NO. 02−0374), which is the same as WO3 NPs/Sb2S3. The characterization of HRTEM (Figure 6c) supports the construction of the specific facets described above as well. The 9687

DOI: 10.1021/acsami.6b00429 ACS Appl. Mater. Interfaces 2016, 8, 9684−9691

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ACS Applied Materials & Interfaces monoclinic WO3 (200) plane with lattice distance of 0.364 nm and the orthorhombic Sb2S3 (310) plane with lattice distance of 0.335 nm can be observed clearly, respectively. What’s more, the inset graph presents that a large amount of smaller particles seems to cover the surface of WO3 NPs with an average diameter about 200 nm. Figure 7 shows the (a) SEM images, (b) XRD pattern, (c) HRTEM image, and (inset of c) TEM imageof the obtained

Figure 8. (a) UV−vis absorbance spectra of (1) WO3 NPs/Sb2S3 heterojunction structures, (2) WO3 NRs/Sb2S3 heterojunction structures, (3) WO3 MCs/Sb2S3 heterojunction structures, (4) WO3 NPs, (5) WO3 NRs, and (6) WO3 MCs; (b) energy gap of (1) WO3 NPs/Sb2S3 heterojunction structures, (2) WO3 NRs/Sb2S3 heterojunction structures, and (3) WO3 MCs/Sb2S3 heterojunction structures, respectively. The inset graphs show the appearance of WO3/Sb2S3 compared to pristine WO3.

step from pristine WO3 to WO3/Sb2S3 heterostructure. It is thus clear that the loaded Sb2S3 on WO3 plays a significant role in the photoelectronic property of light absorbance. According to eq 6, energy gap (Figure 8b) of (1) WO3 NPs/ Sb2S3 heterojunction structures, (2) WO3 NRs/Sb2S3 heterojunction structures, and (3) WO3 MCs/Sb2S3 heterojunction structures were caculated to be 2.75, 2.50, and 2.28 eV from the absorbance curves, respectively, indicating that the absorbance range of WO3 was improved by coupling with narrow band gap Sb2S3. The WO3/Sb2S3 heterostructure system shows the better performance for photoelectrocatalytic in accordance with their band gaps. These results show that the fabrication of WO3/ Sb2S3 heterostructure can effectively enhance the photoelectrocatalytic activity of WO3 crystal. The much more improved photoelectrocatalytic activity of WO3/Sb2S3 heterostructure than those of pure WO3 is ascribed to the strengthening of the interface charge carrier transfer between Sb2S3 and WO3. After introducing Sb2S3 onto the interface of WO3, electron transfer (WO3/Sb2S3) driven by the excitation of Sb2S3 was speculated as a main path, which effectively promotes the separation of photogenerated charge carriers. Because of the significance of PEC water spliting as solar power conversion devices, WO3/Sb2S3 heterojunctions were employed in a three-electrode PEC system as photoanode with pristine WO3 nanostructures as comparison references. As indicated in Figure 9a, the photocurrent density−voltage (I−V)

Figure 7. (a) SEM image, (b) XRD pattern, (c) HRTEM image, and (inset of c) TEM image of the obtained WO3 MCs/Sb2S3.

WO3 MCs/Sb2S3. It can be seen that the obtained samples inherit the morphology of the WO3 MCs, which fits with the hydrothermal reaction mechanism. Moreover the top facets of the microcrystal became flattened after Sb2S3 nanoparticles modification were loaded on the surface of it as shown in Figure 7a. The uniform diameter of Sb2S3 nanoparticles is about 200 nm. Besides the WO3 MCs diffraction pattern (JCPDS, NO. 33−1387), all additional peaks could be indexed to Sb2S3 (JCPDS file No. 02−0374) as mentioned above. The intensity of (310) diffraction peak is relatively stronger than others which reveals a preferential direction of Sb2S3 particles in the (310) direction. In addition, from the HRTEM observation shown in Figure 7c, we can see that the lattice distance of orthorhombic Sb2S3 (310) plane is 0.335 nm. Besides, the lattice distance of the hexagonal WO3 (001) plane is 0.634 nm, which is consistent with the XRD analysis above. Consequently, it can be summarized that crystalline Sb2S3 on WO3 microcrystals has been synthesized. To examine the optical properties of WO3/Sb2S3 heterojunctions, we studied the UV−vis absorption spectra compared to pristine WO3 (Figure 8a). The curves display the roomtemperature absorbance spectra of (1) WO3 NPs/Sb2S3, (2) WO3 NRs/Sb2S3, (3) WO3 MCs/Sb2S3, (4) WO3 NPs, (5) WO3 NRs, and (6) WO3 MCs, respectively. It can be seen that the absorbance peaks appeared at nearly 390 nm (WO3 NPs/ Sb2S3), 410 nm (WO3 NRs/Sb2S3), and 440 nm (WO3 MCs/ Sb2S3), respectively. Compared with pristine WO3 nanostructures (∼330 nm on average), the peak of WO3/Sb2S3 shows a red shift, which reveals an improvement in light absorbance. In contrast, WO3 MCs/Sb2S3 heterojunction shows a best absorbance in the visible region. Accordingly, the appearance of WO3/Sb2S3 nanostructures shows orange compared to pristine WO3 showing light green (inset of Figure 8a). As we can see, the photoelectrocatalytic activity is enhanced step-by-

Figure 9. (a) Photocurrent density−voltage curves and (b) photocurrent density−time curves at an applied potential of 0.80 V vs RHE under illumination with 60 s light on/off cycles, collected with a scan rate of 10 mV s−1 of as-prepared (1) WO3 NPs/Sb2S3 heterojunction structures, (2) WO3 NRs/Sb2S3 heterojunction structures, (3) WO3 MCs/Sb2S3 heterojunction structures, (4) WO3 NPs, (5) WO3 NRs, and (6) WO3 MCs, respectively. 9688

DOI: 10.1021/acsami.6b00429 ACS Appl. Mater. Interfaces 2016, 8, 9684−9691

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ACS Applied Materials & Interfaces curves of the WO3/Sb2S3 composites photoanodes (1) WO3 NPs/Sb2S3, (2) WO3 NRs/Sb2S3, (3) WO3 MCs/Sb2S3, and pristine WO3 nanostructures photoanodes (4) WO3 NPs, (5) WO3 NRs, and (6) WO3 MCs versus RHE under visible light illumination were measured. It can be observed that the density of photocurrent as follows: (1) WO3 NPs/Sb2S3 was 0.93 mA cm−2 (2) WO3 NRs/Sb2S3 was 1.20 mA cm−2, and (3) WO3 MCs/Sb2S3 was 1.79 mA cm−2 at 0.8 V versus RHE, respectively. The highest density of photocurrent is 1.79 mA cm−2, which is obtained by using WO3 MCs/Sb2S3 as electrode. In contrast, the photocurrent density of WO3 MCs/Sb2S3 is about 3.98 times greater than the pristine WO3 MCs (0.45 mA cm−2), which indicates WO3/Sb2S3 heterojunctions have high photoelectrocatalytic activity because Sb2S3 modification has enhanced the photoelectrocatalytic effienciency successfully by means of narrow band gap Sb2S3 improving visible-light harvesting and trapping. The WO3 MCs/Sb2S3 heterojunction structures generated the highest photocurrent density of 1.79 mA cm−2, which is 2.92 and 1.49 times higher than that of WO3 NPs/Sb2S3 of 0.93 mA cm−2 and WO3 NRs/Sb2S3 of 1.20 mA cm−2, respectively. It indicates that the photocurrent density of WO3 MCs/Sb2S3 heterojunction possesses stronger response to the visible light, which might result from the nanocomposite construction and the band gap structure of WO3 MCs/Sb2S3 heterojunction.36 For further investigating the photoresponse of the WO3/ Sb2S3 heterojunctions, specific curves of chopped transient photocurrent density versus time have been investigated at 0.8 V versus RHE along with the period of light on/off every 60 second under light irradiation, scanned with a rate of 10 mV s−1, respectively (Figure 9b). All photoanodes show a rapid rise in photocurrent with the light on, and quickly return back to zero while the light turned off, indicating that the carrier transporting of the synthesized photoanodes is quite fast.37 Among them, the WO3 MCs/Sb2S3 heterojunction photoelectrode possessed the maximum photocurrent density during the light-on cycle, consistent with Figure 9a. The relationship between light absorbance and the density of photocurrent based on different WO3/Sb2S3 heterojunctions photoelectrodes were further investigated and presented by incident phototo-current conversion efficiency (IPCE) measurements as shown in Figure 10a. Apparently, all the WO3/ Sb2S3 heterojunction electrodes showed a specific photoactivity enhancement, which is much higher than the pure WO3 in visible light. Moreover, IPCE profile of the WO3 MCs/Sb2S3

heterojunction structure is broadened and strengthened compared with that of WO3 NPs/Sb2S3 and WO3 NRs/Sb2S3 over the visible-light region. To further probe the property of the samples, we measured the electrochemical impedance spectroscopy (EIS). Figure 10b shows the EIS Nyquist plots of the WO3/Sb2S3 heterojunction photoanodes (1) WO3 NPs/ Sb2S3, (2) WO3 NRs/Sb2S3, (3) WO3 MCs/Sb2S3, and pristine WO3 nanostructures photoanodes (4) WO3 NPs, (5) WO3 NRs, and (6) WO3 MCs. It is obviously seen that the WO3 MCs/Sb2S3 heterojunction structure displays the smallest arc radius, which indicates that the fastest charge transporting and the longest service life of charge carriers in it. Namely, the WO3 MCs/Sb2S3 heterojunction structure can absorb more photons in the visible-light range so as to photogenerate more electron vacancies because of the lower optical band gap of the microcrystal structure. Moreover, the combination of WO3 MCs and Sb2S3 lead to effective carrier collection because of the efficient charge transfer and low electron-hole recombination rate. In addition, the current−voltage measurement results show a decrease in 10, 50, and 100 cycles of 1.9, 13.2, and 25.7%, respectively, indicating better photocorrosion resistance ability relatively. For the purpose of explaining the probable cause of the improved photosensitivity for WO3 MCs/Sb2S3 heterojunction under visible light illumination, the energy band diagram vs RHE was shown in the schematic diagram in Figure 11. As the

Figure 11. Schematic diagram of the WO3 MCs/Sb2S3 heterojunction structures electrode and determined valence band and conduction band edges vs RHE, showing the charge-transfer processes.

position of conduction band (CB) and valence band (VB) of Sb 2 S 3 particles is more negative than that of WO 3 respectively,38 the electron−hole pairs photoexcited in Sb2S3 under visible light illumination as well as the photogenerated electrons in Sb2S3 are successfully transferred from the CB of Sb2S3 to that of WO3 MCs because of the potential difference, and suppressing the recombination of photogenerated electron−hole pairs in turn as shown in the EIS measurements (Figure 10b). Furthermore, the recombination of electron− hole could be indeed reduced owing to the traveling of the photogenerated holes in the opposite direction from VB of WO3 to that of Sb2S3 and participating in the oxidation reaction.35 Meanwhile, a hydrothermal method used for the synthesis of WO3 MCs/Sb2S3 heterojunction structure can remain the original morphology of the products. In other words, the crystal defects will be decreased so that the lightharvesting capability of the semiconductor electrode is improved as well as the photoelectric performance.

Figure 10. (a) IPCE plots in the range of 300−800 nm at 0.8 V vs RHE and (b) electrochemical impedance spectroscopy of as-prepared (1) WO3 NPs/Sb2S3 heterojunction structures, (2) WO3 NRs/Sb2S3 heterojunction structures, (3) WO3 MCs/Sb2S3 heterojunction structures, (4) WO3 NPs, (5) WO3 NRs, and (6) WO3 MCs, respectively.

4. CONCLUSIONS In summary, different morphologies of WO3 nanostructures and WO3/Sb2S3 heterojunction structures were successfully 9689

DOI: 10.1021/acsami.6b00429 ACS Appl. Mater. Interfaces 2016, 8, 9684−9691

Research Article

ACS Applied Materials & Interfaces fabricated. This is the first example of WO3/Sb2S3 heterojunction with Sb2S3 particles as a photoelectrode for efficient PEC water splitting. The resulting WO3 MCs/Sb2S3 heterojunction showed significantly improved PEC water splitting performance, including a high photocurrent density of 1.79 mA/cm2 and well stability, which is better than pristine WO3 photoelectrodes. The (i) enhanced light harvesting; (ii) appropriate energy gap structure; (iii) efficient separation of photogenerated carriers and charge transfer efficiency; (iv) and the specific microcrystal nanostructural characteristic of the hexagonal WO3 conduce to the prominent PEC activity used for water splitting collectively. Our results demonstrated attractive strategies to design and construct WO3/Sb2S3 heterojunction photoelectrodes used in practical sun-energydriven PEC water splitting.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 22 23085236. Fax: +86 22 23085110. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from National Nature Science Foundation of China (51102174) and Stat e Key Labo rat ory o f Heavy Oil Processing (SKLHOP201505).

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