Using Al2O3 defect levels to enhance the photoelectrocatalytic activity of SnS2 nanosheets

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Using Al2O3 defect levels to enhance the photoelectrocatalytic activity of SnS2 nanosheets Jianglong Mua, Hui Miaoa, Enzhou Liub, Lida Chena, Juan Fenga, Tongxin Hana, Ying Gaoa, ⁎ Jun Fanb, Xiaoyun Hua, a b

School of Physics, Northwest University, Xi’an, Shaanxi 710069, PR China School of Chemical Engineering, Northwest University, Xi’an, Shaanxi 710069, PR China

A R T I C L E I N F O

A BS T RAC T

Keywords: SnS2 Al2O3/SnS2 composite Photoelectrocatalysis Defect levels

Hexagonal SnS2 nanosheets and Al2O3/SnS2 composites were fabricated via a one-step hydrothermal synthesis method. The investigation indicates that hexagonal SnS2 with diameters of 100–200 nm are well dispersed on the surface of Al2O3. The band gap of SnS2 after coupling with 11 wt%-Al2O3 is reduced by 0.042 eV compared with the pure SnS2. The composite with 11 wt%-Al2O3 shows the highest photocurrent density of 37 μA/cm2 at 0.49 V (vs. Ag/AgCl) under visible light (λ > 420 nm), which is approximately 1.2 times that of the pure SnS2 nanosheets. Photoelectrocatalytic measurements demonstrate that an appropriate amount of Al2O3 can enhance the photoelectrocatalytic efficiency of SnS2. The 11 wt%-Al2O3/SnS2 composite (AOSS-11) can degrade 85.9% MB after 3 h under visible light illumination at an applied potential of 0.49 V (vs. Ag/AgCl). The highly effective photoelectrocatalytic activity of the Al2O3/SnS2 composite is attributed to the efficient separation of photoinduced electron-hole pairs based on the defect levels. This work may provide a new design idea for constructing the effective SnS2-based photocatalysts with other defective semiconductors.

1. Introduction

cogenide semiconductor material, exfoliated MoS2 nanosheets have drawn great attention in many new fields, such as solar cells [9], catalysis [10,11] and optoelectronic devices [12,13], because of their optical, electronic and catalytic properties. However, the use MoS2 semiconductor material in the conversion of solar energy field is unlikely due to its poor charge transport ability, weak stability, and the hydrophobic nature of the catalyst [14,15]. In the two-dimensional (2D) layered materials mentioned above, SnS2 nanomaterials, processing a narrow band gap of approximately 1.91–2.35 eV, have been known for the strong anisotropy of their optical properties and their potential for applications in photoelectrical detections, as well as photocatalysis [3,16]. As an important n-type semiconductor, SnS2 has a layered CdI2-type structure, where each layer of Sn atoms is sandwiched between two layers of hexagonally closed-packed S atoms, and the adjacent sulfur layers are connected by the weak van der Waals interactions [17]. SnS2 nanomaterials have several advantages for use as photocatalysts, possessing a high efficiency, good chemical stability in acid or a neutral aqueous solution and thermal stability in air (decomposition temperature is approximately 600 °C), as well as being non-toxic [18]. It has been confirmed that hexagonal SnS2 nanosheets with a high crystallinity display an excellent photocatalytic performance because of their special structure and morphology [19].

Currently, global environmental pollution and the energy crisis have become increasingly serious issues. Photocatalysis and photoelectrocatalysis are environmentally friendly “green” techniques to address global environmental pollution and the energy crisis and can effectively decompose pollutants and split water to production H2, and so on. However, their applications are still far from satisfactory because of the rapid recombination rate of electron-hole pairs and the poor visible light harvesting ability. Therefore, considerable efforts have been dedicated to develop semiconductor materials highly active in visible light. Among various semiconductor materials, two-dimensional (2D) layered materials, such as g-C3N4 [1], MoS2 [2], and SnS2 [3], have recently attracted substantial attention because of their unique structures and excellent properties. Graphene-like carbon nitride (g-C3N4) has received increasing amounts of attention because of its high photodegradation of organic pollutants [4] and photocatalytic performance towards splitting water [5–8] under visible light. However, the high recombination rate of the photoinduced electron-hole pairs and the low specific surface area of g-C3N4 still limit the photocatalysis. Molybdenum disulfide (MoS2) is an important transition metal dichal-

⁎

Corresponding author. E-mail address: [email protected] (X. Hu).

http://dx.doi.org/10.1016/j.ceramint.2017.01.006 Received 9 December 2016; Received in revised form 2 January 2017; Accepted 3 January 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Mu, J., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.01.006

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tamide (TAA, purity ≥ 99.0%), hydrochloric acid (HCl, 36–38%), aluminum oxide (Al2O3), absolute ethanol (C2H6O, purity ≥ 99.7%) were analytical grade; Triton X-100 (C34H62O11) was chemically pure. All of the chemical reagents were used without further purification, and all of the experiments were carried out using deionized (ID) water.

In particular, the oxidation process using heterogeneous photocatalysis is a promising technology to decompose pollutants and split water under visible light. Compared with a single semiconductor, the coupling between two different semiconductors has proved to be successful in developing high-performance photocatalysts [20–22]. Recently, coupling SnS2 with various matched-band-gap semiconductors to develop promising photocatalytic materials is a hot topic. Constructing a heterojunction is the main way to improve the photocatalytic performance of SnS2. SnS2 possesses a small band gap, so it is easy to form a heterojunction. For example, Liu et al. [23] prepared a SnS2/g-C3N4 heterojunction with different weight ratios of SnS2 by a facile in situ ion-exchange synthesis. The heterojunction shows the highest photocurrent density of 13.66 μA/cm2 at 0.8 V over 4.0-SnS2/ g-C3N4. Zhang et al. [18] used a plastic syringe for electrospinning to prepare a SnS2/TiO2 heterojunction that exhibited excellent visible light photocatalytic activities for the degradation of organic dyes (rhodamine B and methyl orange) and phenols (4-nitrophenol). Zhao et al. [24] synthesized a SnS2/SnO2 nanocomposite that exhibited a high photocatalytic activity toward the reduction of aqueous Cr(VI) under visible light irradiation. However, decorating SnS2 with a wide band-gap metal oxide (e.g., ZnO, Al2O3, and ZrO2) has rarely been studied. Hence, more appropriate materials should be further developed for designing composites with SnS2. Al2O3, as an “earth abundant” material, has been widely used as an assisted supporting material in catalysis fields [25–27]. Al2O3 is an excellent electron acceptor because of the existence of a large amount of defect levels, which benefit the separation of charge carriers [28,29]. This results in fewer recombinations of photoinduced electron–hole pairs before their participation in the redox process, which is a key issue for the photocatalytic efficiency enhancement of a photocatalyst [30]. Al2O3 is also a good reflective material because of its refractive index of 1.65 and its high reflectivity index of ~98%, which means Al2O3 can transmit visible light (600–800 nm) and absorb UV light (200–300 nm) [31]. It is conducive to improving the utilization rate of light for other composites. To our knowledge, there is no report that an efficient Al2O3-assisted material was applied in photocatalysis or photoelectrocatalysis fields. Photocatalysis is the mainstream of research now. The photocatalyst active sites can absorb photons with an energy equal to or higher than their band-gap to induce the electron-hole separation [32]. Furthermore, in photoelectrocatalysis, a positive bias is applied to the photoelectrode, which is more favorable for the photoelectron transfer into an external circuit, further preventing photoinduced electrons-hole pairs from recombining [33]. Therefore, photoelectrocatalysis has great potential for the degradation of pollutants or splitting water. In the present work, we report on an in situ method of the hydrothermal synthesis for constructing an Al2O3/SnS2 composite. Because of the high reflectivity and defect levels of Al2O3, the Al2O3/ SnS2 composite can greatly improve the utilization rate of light and promote the photoinduced electron-hole pair separation compared with pure SnS2. The photocurrent density and photoelectrocatalytic activity were measured by placing the Al2O3/SnS2 electrode under simulating solar light irradiation at an applied potential of 0.49 V (vs. Ag/AgCl). In addition, we discuss the transmission channels of electrons and the degradation ways of MB in the photoelectrochemical (PEC) processes by the mechanism diagram in detail. This work not only provides a new method to constructing SnS2-based composites with other defective semiconductor materials but also broadens the application of Al2O3 in photoelectrocatalysis fields.

2.2. Methodology 2.2.1. Preparation of SnS2 In a typical hydrothermal synthesis of hexagonal SnS2 nanosheets, 0.45 g of SnCl2·2H2O and 0.1502 g thioacetamide (TAA) were dispersed into a mixture consisting of 46 ml of ID water and 4 ml of HCl. Then, 0.4 ml of Triton X-100 was added to the as-received mixture. After vigorous stirring for 30 min, the above mixture was transferred into a 100 ml Teflon-lined autoclave, sealed, heated at 180 °C for 10 h, and then the system was cooled to room temperature naturally. The final yellow product was collected by centrifuging the mixture, washing with ID water and absolute ethanol 3 times, and then drying at 60 °C for 6 h in vacuum for further characterization. 2.2.2. Synthesis of Al2O3/SnS2 Al2O3/SnS2 composite was also fabricated via a one-step hydrothermal synthesis method. The amount of Al2O3 powder was controlled to be 0, 4%, 11%, 18% and 22% to SnCl2·2H2O in mass ratio. The preparation process for the composite was the same as the preparation process of pure SnS2; the only difference was adding the Al2O3 to the mixture. clearly, the process for forming the Al2O3/SnS2 composite is simple and convenient. 2.2.3. Fabrication of Al2O3/SnS2 photoelectrode First, fluorine-doped tin oxide (FTO) glass substrates (Pilkington, 7 Ω cm−1, 20 mm×30 mm, 2.2 mm) were successively cleaned with acetone, ethanol and deionized water under ultra-sonication for 30 min. Then, they were stored in clean ID water and dried in a vacuum oven before using. Second, 10 mg of as-prepared sample was dissolved in 1 ml of absolute ethanol with ultrasonic dispersion for 2 h. After that, the dispersed sample solution was dropped onto the FTO conductive surface until it dried naturally. Lastly, the photoelectrode, which was put into an alumina crucible with a cover, was placed in a muffle furnace and heated to 200 °C at a heating rate of 5 °C/min and continued to heat for 2 h. After 5 h of reaction, the alumina crucible was cooled to room temperature and then the photoelectrode was collected. Clearly, the annealing was beneficial to reduce the contact resistance and improve the quality of photoelectrode. 2.3. Characterization The phase composition of the samples was identified by a Shimadzu X-ray diffraction (XRD)-6000 powder diffractometer at 40 kV and 30 mA with Cu Kɑ radiation (λ=0.15406 nm) in the 2ɵ range from 10° to 80° with a scanning rate of 0.02°/s. The surface area of the catalysts was measured by N2 adsorption isotherms conducted using a Quantachrome NOVA 2000e via the Brunauer-Emmett-Teller (BET) method. The morphology, elements and microstructure of the samples were observed by a scanning electron microscope (SEM, JSM-6390A) equipped with energy-dispersive X-ray (EDS) and transmission electron microscopy (TEM, Tecnai G2F20 S-twin field emission transmission). X-ray photoelectron spectroscopy (XPS) was carried out using a Kratos AXIS NOVA spectrometer. The optical properties of the samples were analyzed by UV–vis absorption spectra recorded on a Shimadzu UV-3600 UV/Vis/NIR spectrophotometer.

2. Experimental section 2.4. Photoelectrochemical measurements (PEC) 2.1. Raw materials The photoelectrochemical measurements were conducted on a standard CHI 660E electrochemical workstation (CHI 660E Chenhua

Tin(II) chloride dehydrate (SnCl2·2H2O, purity ≥ 98.0%), thioace2

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Instrument Company, Shanghai, China) in a conventional threeelectrode configuration with a Pt wire as the counter electrode, a Ag/ AgCl electrode as the reference electrode and the prepared photoelectrode as the working electrode. A potential of 0.49 V vs. Ag/AgCl was applied, to which measured potentials were converted to V vs. RHE (ERHE=EAg/AgCl+0.059 VХpH+0.197 V). The visible light irradiation was provided by a solar stimulation source (Zolix, 150 W, AAA) with a UV cut off filter (λ > 420 nm). A Na2SO4 (0.5 M, pH=7.0) aqueous solution was used as the electrolyte. 2.5. Photoelectrocatalysis performance evaluating The photoelectrocatalysis performance of the Al2O3/SnS2 composite was evaluated by the photoelectric degradation of MB with an initial concentration of 10 mg/l under visible light irradiation using a 300 W xenon lamp (Beijing Perfect Light Co. Ltd., Beijing) with a cut off filter (λ > 420 nm) as the light source. The photoelectrode was clamped under the photoelectrochemical reaction device with a 0.875 cm2 effective illumination area. The supporting electrolyte was a 0.5 M Na2SO4 solution. Before illumination, air was bubbled into the MB solution while in the dark for 10 min to ensure the establishment of adsorption/desorption equilibrium with the working electrode. During the reaction under visible light irradiation at the applied potential 0.49 V (vs. Ag/AgCl), 3.0 ml of the MB solution was collected at 20 min intervals for analysis. The whole experimental process was under the condition of room temperature with air continued to be bubbling in the MB. The concentration of MB was determined by measuring the absorption intensity at its maximum absorbance wavelength (664 nm) using a Shimadzu UV–vis spectrophotometer. To ensure the photoelectrocatalysis performance of the samples, cycling experiments were carried out. After cleaning with ID water and drying with nitrogen gas, the photoelectrode was used to repeat the previous degradation experiment with fresh MB under the same conditions.

Fig. 1. XRD patterns of as-fabricated pure SnS2, AOSS-4, AOSS-11, AOSS-18, AOSS-22 and commercial Al2O3.

Al2O3 content less than 11%. The SnS2 peak at 14.95° can be indexed as the (001) diffraction plane, which is attributed to the S-Sn-S interlayer structural packing arrangement vertical to the c-axis. The XRD results indicate that the morphology of SnS2 was affected by Al2O3. Moreover, no other impurity diffraction peaks are discovered in any of these samples, which confirm the high purity of the composites. To further demonstrate the specific surface area (SBET) and the porous structure of the pure SnS2 and the Al2O3/SnS2 composite, N2 adsorption/desorption isotherms were used to characterize the SBET and porosity of the catalysts. Fig. 2 exhibits that the similar shapes of their hysteresis loops indicate the similar pore shapes. All of the obtained specific surface area (SBET) data and the total pore volumes are summarized in Table 1. The surface area and pore volume are 9.704 m2/g and 0.037 cm3/g for SnS2, while those for AOSS-11 are 11.337 m2/g and 0.051 cm3/g, respectively. As expected, the SBET of the AOSS-11 composite is higher than that of pure SnS2. The larger SBET and the porous structure of AOSS-11 could be ascribed to Al2O3 as the supporting material, which is beneficial to provide more active sites. The general morphology and the elemental composition of the samples were characterized by scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS). Fig. 3a clearly shows the morphology of the as-prepared pure SnS2, which consists of irregular large sheets and particle materials mixed and stacked together in disorder. As displayed in Fig. 3b-c, the morphology of SnS2 shows hexagonal nanosheets with diameters of 100–200 nm that are randomly dispersed and decorating the Al2O3 content that is less than 11%. The obtained SnS2 presents irregular shapes again (Fig. 3d) and occurs as clumped materials (Fig. 3e) when the Al2O3 content is

2.6. Trapping experiment of reactive species To explore the reactive species of the photoelectrochemical (PEC) degradation, the superoxide anion radical (•O2-), photoinduced holes (h+), and hydroxyl radicals (•OH) were trapped by various scavengers including p-benzoquinone (BQ, a scavenger of •O2-), triethanolamine (TEOA, a scavenger of h+) and isopropanol (IPA, a scavenger of •OH), respectively [34]. The trapping of reactive species experiments are the same as the MB photoelectric degradation process. Before starting the degradation, scavengers were added into the MB solution. 3. Results and discussion 3.1. Structural and composition characterizations Fig. 1 shows the XRD patterns of the as-prepared SnS2, AOSS-4, AOSS-11, AOSS-18 and AOSS-22, as well as commercial Al2O3; AOSSX refers to X wt%-Al2O3-SnS2. As seen from Fig. 1, the diffraction peaks at 2ɵ=14.95°, 28.29°, 32.15°, 41.83°, 50.04°, and 52.52° correspond to the (001), (110), (101), (102), (110) and (111) planes, which are in good agreement with a high purity and the crystallization of 2T-type hexagonal SnS2 (JCPDS, NO. 23-0677) [35,36]. All of the diffraction peaks of commercial Al2O3 can be well indexed to the rhombohedral ɑ-phase structure (JCPDS, NO. 081-2267) [28]. The diffraction pattern of the commercial Al2O3 shows peaks at 2ɵ=25.55°, 35.11°, 37.75°, 43.29°, 52.49°, 57.43°, and 61.23°, which correspond to the (012), (104), (110), (113), (024), (116) and (018) planes, respectively. The observed XRD results are well matched with the literature [28,35,36]. With regard to the Al2O3/SnS2 composite, both Al2O3 and SnS2 can be detected. It is also seen that the Al2O3 XRD peaks increase, and the SnS2 peaks at 14.95° decrease with increasing

Fig. 2. N2 adsorption–desorption isotherms of the pure SnS2 and 11 wt% Al2O3/SnS2 (AOSS-11) composite.

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and reveals that the Al2O3 was fully mixed with the SnS2 nanosheets. The hexagonal SnS2 and irregular Al2O3 are clearly found. The coexistence of SnS2 and the irregular Al2O3 is proved by the highresolution transmission electron microscopy (HRTEM) image in Fig. 4b. The HRTEM image reveals that the nanosheets display main distinct fringe intervals: the spaces of 0.238 nm and 0.348 nm coincide with the Al2O3 of (110) and (012) crystal planes; whereas the spaces of 0.279 nm and 0.316 nm correspond to the hexagonal phase SnS2 (101) and (100) crystal planes, respectively. The selected area electron diffraction (SAED) pattern demonstrates the polycrystalline structure of the samples [37]. Fig. 4c displays the SAED pattern of SnS2 nanosheets, revealing a 2T-type hexagonal structure, which is consistent with the XRD results. Clearly, three crystal planes of (100), (111), and (201) are marked in the image. Fig. 4d shows the high-resolution TEM (HRTEM) image of the top view of the SnS2 nanosheets loaded on the Al2O3 surface. It can be seen that an interplanar distance of 0.316 nm and 0.276 nm are in agreement with the d-spacing distances of the (100) and (101) crystal planes of hexagonal SnS2, respectively. The corresponding fast Fourier transform (FFT) in the lower right inset of Fig. 4d resembles the diffraction pattern of a hexagonal phase along the [001] zone axis. This assignment is supported by the schematic

Table 1 SBET values and pore volumes of the pure SnS2 and AOSS-11 composite. Samples

Surface area (m2 g−1)

Pore volume (cm3 g−1)

SnS2 AOSS-11

9.704 11.337

0.037 0.051

higher than 18%. As described in the experimental section, it is deduced that an appropriate amount of Al2O3 can make a better dispersion of SnS2 and a more specific morphology, which are confirmed by the SEM results. The energy-dispersive X-ray spectrometry (EDS) analysis of AOSS-11 is shown in Fig. 3f, and the presence of primary elements O, Al, S, and Sn are detected in the sample, further suggesting the existence of Al2O3 and SnS2. Meanwhile, The EDS analysis also indicates the percentages of the elemental content present in the composite. The atomic percentages of Sn and S are 26.44% and 44.16%, which are closed to the nominal composition of SnS2. Transmission electron microscopy (TEM) was used to verify the microscopic structure of the samples. Fig. 4a shows the TEM results

Fig. 3. SEM images of (a) pure-SnS2, (b) AOSS-4, (c) AOSS-11, (d) AOSS-18, (e) AOSS-22 and (f) EDS for the sample of AOSS-11; the insert of the chart is the atomic content data.

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Fig. 4. (a) and (b) show TEM and HRTEM images of the nanocomposite AOSS-11, respectively. (c) The SAED pattern of the hexagonal structure of SnS2. (d) HRTEM images and the insert is an FFT pattern of AOSS-11.

UV–vis spectroscopy was used to reveal the optical property of all of the as-prepared samples. As shown in Fig. 6a, a sharp absorption peak is detected in the ultraviolet region at 232 nm. Such a peak has been reported for both SnS2 nanowires and nanosheets [42,43], and its exact position may reflect quantum size effects characteristic of the different nanostructures [44]. It is also found that the photoabsorption ability of the samples can be enhanced in UV–vis region by adding Al2O3. The reason for the absorption in the visible region can be attributed to two parts: One is the wide energy band gap of Al2O3, which displays a significant special response in the ultraviolet region. The SnS2 shows a strong absorbance for visible light because of its narrow energy band gap of 2.214 eV, as presented in Fig. 6b. The other is that Al2O3, as a supporting material with good reflectively (~98%), is beneficial to promote SnS2 secondary light absorption owning to its characteristic high reflection [45] to improve the utilization rate of light. Fig. 6a. reveals that all of the samples display a remarkable absorption of visible-light with an absorption edge at approximately 560 nm. The band gap energy of the semiconductors can be calculated by the following equation [46–48]:

illustration of the structure of hexagonal SnS2 viewed along the c-axis in the upper right inset of Fig. 4d, where the atomic arrangement resembles the lattice image [38]. Importantly, the FFT pattern reveals the simultaneous presence of the SnS2 (100), (101) and Al2O3 (012), (104) planes, further confirming the formation of the Al2O3/SnS2 composite. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition and bonding configuration of the pure SnS2 and AOSS-11 samples. Fig. 5a clearly shows the main characteristic peaks of O 1s, Al 2p, Sn 3d, S 2p, and C 1s. It should be noted that the peak of C 1s (284.6 eV), which is related to the C bonds of carbon source impurities, and carbon source pollution [39], was used as a reference to calibrate the spectra. The peaks at binding energies of 74.3 eV and 531.1 eV (Fig. 5b-c) belong to Al 2p and O 1s, respectively. The binding energy of O 1s at 531.1 eV is assigned to the lattice oxygen [40], so the above means Al 2p3/2 and O 1s correspond to Al2O3. Fig. 5d reveals the evolution of Sn 3d with two binding energies that are observed at 486.5 eV and 494.9 eV, which are attributed to Sn 3d5/2 and Sn 3d3/2, respectively [41]. A splitting energy of 8.4 eV between Sn 3d5/2 and Sn 3d3/2 is a typical value for Sn4+ in SnS2 [18]. Those peaks in the spectrum of AOSS-11 shift to 487.2 and 495.6 eV, being 0.7 eV higher than the pure SnS2. Fig. 5e shows the evolution of S 2p with two binding energies of 160.67 eV and 162.07 eV, which can be assigned to S 2p3/2 and S 2p1/2, respectively, and are 0.51 eV lower than the corresponding two peaks in the S 2p spectrum of AOSS-11. The observed binding energies of the Sn 3d and S 2p peaks are in good accordance with Sn4+ and S2- of SnS2. The binding energies in AOSS-11 are 0.7 eV and 0.51 eV higher than those in pure SnS2, which could be attributed to the interaction between Al2O3 and SnS2, or the formation of Al2O3/SnS2 heterojunctions.

αhυ = A (hυ − Eg )n/2

(1)

By applying the above equation, the band gaps of pure SnS2, AOSS4, AOSS-11, AOSS-18 and AOSS-22 are found to be 2.214 eV, 2.246 eV, 2.172 eV, 2.197 eV and 2.294 eV, respectively. Compared with other composites, the band gap of the composite AOSS-11 is the minimum value. The band gap of SnS2 after coupling with 11 wt% Al2O3 is reduced by 0.042 eV compared with pure SnS2 (2.214 eV), which can be shown in Fig. 6b. The band edge positions of the CB and VB for a semiconductor can be estimated according to the following equations [49]: 5

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Fig. 5. XPS spectra of the as-fabricated samples: (a) AOSS-11 and pure SnS2, (b) Al 2p core-level spectrum, (c) O 1s core-level spectrum, (d) Sn 3d core-level spectrum, and (e) S 2p core-level spectrum.

Fig. 6. (a) UV–vis diffuse reflection spectra of SnS2, AOSS-4, AOSS-11, AOSS-18, and AOSS-22; (b) band gap energies of SnS2 and AOSS-11.

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reducing electron-hole pair recombination. The photocurrent densities of AOSS-18 and AOSS-22 are lower than pure SnS2. This result might be interpreted as the Al2O3 content increased, and the pure SnS2 content decreased in the composite material per unit area, which results in the amount of photoinduced electron-hole pairs being reduced; thus, the photocurrent density was reduced when compared with the pure SnS2. SnS2 has a good visible light response, while Al2O3 has a significant special response in the ultraviolet region. Under light illumination, the best photocurrent density of AOSS-11 (130 μA/cm2) is the result of the SnS2 and Al2O3 responses together. This is difficult to interpret, as the Al2O3 defect levels capture electrons. However, the photocurrent density of AOSS-11 is also higher than that of the pure SnS2 under visible light, merely the light of the SnS2 response function. Furthermore, this proves that the defect levels of Al2O3 capture the electrons. Al2O3 can reduce electron-hole pair recombination and promote the photocurrent density because of the defect levels. Electrochemical impedance spectroscopy (EIS) measurements were performed to characterize the difference between the intrinsic electronic and charge-transfer properties under simulated solar light irradiation [1]. Fig. 8 shows the Nyquist plots of the samples under irradiation with 0.1 V (vs. Ag/AgCl). In the Nyquist diagram, the radius of each arc is associated with the charge-transfer process at the corresponding electrode/electrolyte interface, and a smaller radius corresponds to a lower charge-transfer resistance [23]. Clearly, the sample of AOSS-11 exhibits minimal charge transfer resistance under simulated sunlight irradiation, indicating that effective shuttling of charges between the electrode and electrolyte occurs. A fast interfacial charge transfer occurred at the composite's interface thereby generating the increased photocurrent. Therefore, Al2O3 serves as electron capture centers, speeding up the charge transfer.

Table 2 Calculation of the CB and VB potentials of SnS2. Samples

χ

Eg (eV)

ECB (eV)

EVB (eV)

SnS2

5.467

2.214

−0.14

2.074

EVB = χ–Ee + 0.5Eg

(2)

ECB = EVB − Eg

(3)

where EVB and ECB are the valence and conduction band edge potentials; χ is the absolute electronegativity of the semiconductor, expressed as the geometric mean of the absolute electronegativity of the constituent atoms, and defined as the arithmetic mean of the atomic electron affinity and the first ionization energy [50]; and Ee is the energy of free electrons (approximately 4.5 eV on the hydrogen scale). Eg is the band gap energy of the semiconductor calculated from the UV–vis spectra. It is known that the ECB of SnS2 is −0.14 eV [23], thus, the CB and VB potentials of SnS2 are listed in Table 2. 3.2. The PEC characteristics of the photoelectrode To investigate the photocurrent response of the as-prepared photoelectrodes, the photoelectrochemical measurements were conducted in the three-electrode system containing a Na2SO4 (0.5 M) aqueous solution with an applied potential of 0.49 V vs. Ag/AgCl (i.e., 1.0 V vs. RHE). Fig. 7a demonstrates typical time-dependent current change curves of the different photoelectrodes under light illumination, while Fig. 7b shows the change curves under visible light illumination. Comparing Fig. 7a and Fig. 7b, the photocurrent densities of the pure SnS2 and AOSS-11 photoelectrodes are 100 and 130 μA/cm2 under light illumination at an applied potential of 0.49 V (vs. Ag/AgCl), respectively. Notably, the photocurrent density of the AOSS-11 photoelectrode rapidly increased by 30 μA/cm2, which means the photoinduced electron-hole pairs were separated more efficiently on AOSS11. Moreover, the photocurrent densities of the pure SnS2 and AOSS11 photoelectrodes are 30 and 37 μA/cm2 under visible light illumination at an applied potential of 0.49 V (vs. Ag/AgCl), respectively. From the UV–vis spectra in Fig. 6a, it appears that the composite can enhance the absorption of light, which in turn densifies the photoinduced electrons [51]. The photocurrent densities of AOSS-4 and AOSS-11 are higher than that of the pure SnS2, and the main reason for this can be divided into two parts: On the one hand, photoinduced electrons can be captured at the defect levels of Al2O3 and then timely transported to the Pt electrode through FTO. On the other hand, Al2O3 plays as a supporting material, enhancing the specific surface area of the Al2O3/SnS2 composite. A high specific surface area is conducive to the fast transfer of photoinduced electrons, similar to a “highway,”

3.3. Photoelectrocatalysis activity and reusability To evaluate the photoelectrode photoelectrocatalytic properties owning to the better photocurrent density, we performed the photoelectric degradation of MB experiment. The electrochemical (EC), photocatalytic (PC) and photoelectrochemical (PEC) processes of MB (the initial concentration was 10 mg/l) were evaluated over the pure SnS2 electrode under the simulated solar light irradiation at an applied potential of 0.49 V (vs. Ag/AgCl). As shown in Fig. 9a, it is found that the PEC process is clearly superior to EC and PC, and 72.5% of MB was removed by the PEC process after 3 h compared with EC and PC, where the degradation amounts of MB are 10.5% and 41.1%. The PEC process is the fastest among these processes, indicating that the applied potential boosted the separation of the photoinduced electron–hole pairs effectively and prolonged the lifetime of the photoinduced charge carriers [52]. The PEC degradation curves of MB as a function of time

Fig. 7. Photocurrent responses of the different sample electrodes to on/off cycles: (a) under light illumination and (b) under visible light illumination (λ > 420 nm) at the applied potential of 0.49 V (vs. Ag/AgCl).

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ments indicated the composite material possesses an excellent photoelectrocatalysis efficiency, good reusability, and super stability, which are advantageous for potential practical applications in wastewater treatment. The superoxide anion radical (•O2-), photoinduced holes (h+) and the hydroxyl radical (•OH) have been reported to be the major reactive species for the catalytic oxidation [53]. To uncover the mechanism of the PEC degradation of MB by the AOSS-11 electrode, various scavengers were applied individually to eliminate the corresponding active species so that the efficiency of others active species were determined in the degradation process. Herein, three chemicals including p-benzoquinone (BQ), triethanolamine (TEOA) and isopropanol (IPA) were utilized as the scavengers of •O2-, h+ and •OH, respectively. As shown in Fig. 10a, the PEC degradation efficiency of MB is 85.9% without any scavengers. When the IPA was added into the reaction system, the PEC degradation efficiency of MB drops to 42.3%. Adding BQ and TEOA into the reaction solution, the degradation efficiency decreases to 60.7% and 68.9%, respectively. Comparing the trapping experiments data, the oxidation activity follows •OH > •O2- > h+. It is found that •OH is the major active species for the photooxidation of MB. The synergistic effect of active species of such as •OH, •O2and h+ can further facilitate the PEC degradation of MB. A possible PEC mechanism behind using the Al2O3/SnS2 electrode under the simulated solar light irradiation at an applied potential of 0.49 V (vs. Ag/AgCl) is proposed. The separation of photoinduced electrons-hole pairs and the oxidation-reduction process are described directly in Fig. 10b. Through the interaction with photons, the SnS2 valance band (VB) electrons (e-) are promoted to the conduction band (CB), leaving holes in the valance band. The electron-hole pairs participate in a series of oxidation-reduction reactions on the electrode's surface. A portion of the photoinduced electrons recombine with holes in the VB, while others are captured by the defect levels of Al2O3 or are directly transferred to the Pt wire through the outer circuit.

Fig. 8. Nyquist plots of the as-prepared samples under standard solar simulator illumination (0.1 V vs. Ag/AgCl). The inset shows the equivalent circuit. Rsol, Rct, and Cq represent the resistance of the electrolyte, the resistance of charge transfer at the interface between FTO and electrolyte, and the capacitance of the interface, respectively.

are shown in Fig. 9b. The decrease of MB concentration in the presence of AOSS-11 is the most evident, approximately 85.9% of MB decomposed after 3 h, while approximately 72.5% for the pure SnS2. Fig. 9c shows the kinetic constants (Кapp) of MB over different electrodes, which were measured to the follow a pseudo-first-order kinetics model: -ln(C/C0)=Кappt, where C is the concentration at irradiation time t, and C0 is the initial concentration of MB before light irradiation. The Кapp value of the AOSS-11 electrode is 0.01903 min−1, corresponding to the degradation curve in Fig. 9b. In addition to the excellent photoelectrocatalysis efficiency, the stability and reusability of the as-prepared electrode was also measured by the cycling experiments. Selecting the AOSS-11 electrode, the measurement was carried out four times, and the corresponding results are shown in Fig. 9d, with amounts of approximately 85.9%, 82.7%, 81.1%, and 78.7%. The cycling experi-

Fig. 9. (a) Degradation of MB by an electrochemical process, photocatalysis and photoelectrocatalysis over the pure SnS2; (b) Photoelectrocatalysis degradation curves of MB over the as-prepared samples under the visible light irradiation (λ > 420 nm) at the applied potential of 0.49 V (vs. Ag/AgCl); (c) The apparent rate constants of samples for the photoelectric degradation of MB; and (d) The cycling experiments of AOSS-11.

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4. Conclusion In summary, an efficient and stable Al2O3/SnS2 composite was successfully fabricated via a one-step hydrothermal synthesis method. The structural adjustment role for SnS2 by Al2O3 in the hydrothermal process can be observed by SEM, which favored the formation of a high specific surface area, increased the active sites, and enhanced the absorption of light for the composites. AOSS-11 demonstrated the highest photocurrent density and fast photoelectric degradation MB under visible light and an applied potential 0.49 V (vs. Ag/AgCl). The improved PEC performance of AOSS-11 could be mainly attributed to the Al2O3 defect levels and a high SBET achieving photoinduced electron-hole pair separation and transfer. The photoelectrocatalytic degradation of MB over the AOSS-11 electrode was mainly attributed to the hydroxyl radical (•OH), and the cycling experiments indicated the high reusability of the AOSS-11 electrode. The Al2O3/SnS2 composite may serve as a promising candidate for use as an efficient catalytic material removing other organic dyes, waste water or noxious substances, etc. Acknowledgement The National Natural Science Foundation of China (Grant Nos. 21476183, 21676213, 51372201), and the Research Fund for the Doctoral Program of Higher Education (Grant Nos. 20136101110009). References [1] H. Miao, G.W. Zhang, X.Y. Hu, J.L. Mu, T.X. Han, J. Fan, C.J. Zhu, L.X. Song, J.T. Bai, X. Hou, A novel strategy to prepare 2D g-C3N4 nanosheets and their photoelectrochemical properties, J. Alloy. Compd. 690 (2017) 669–676. [2] H. Miao, X.Y. Hu, Q. Sun, Y.Y. Hao, H. Wu, D.K. Zhang, J.B. Bai, E.Z. Liu, J. Fan, X. Hou, Hydrothermal synthesis of MoS2 nanosheets films: microstructure and formation mechanism research, Mater. Lett. 166 (2016) 121–124. [3] S.C. Yan, Z.S. Li, Z.G. Zou, Photodegradation performance of g-C3N4 fabricated by directly heating melamine, Langmuir 25 (2009) 10397–10401. [4] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76–80. [5] H.M. Xiong, Y. Xu, Q.G. Ren, Y.Y. Xia, Stable aqueous ZnO@ polymer core−shell nanoparticles with tunable photoluminescence and their application in cell imaging, J. Am. Chem. Soc. 130 (2008) 7522–7523. [6] J.Q. Wen, X. Li, H.Q. Li, S. Ma, K.L. He, Y.H. Xu, Y.P. Fang, W. Liu, Q.Z. Gao, Enhanced visible-light H2 evolution of g-C3N4 photocatalysts via the synergetic effect of amorphous NiS and cheap metal-free carbon black nanoparticles as cocatalysts, Appl. Surf. Sci. 358 (2015) 204–212. [7] G.C. Bi, J.Q. Wen, X. Li, W. Liu, J. Xie, Y.P. Fang, W.W. Zhang, Efficient visiblelight photocatalytic H2 evolution over metal-free g-C3N4 co-modified with robust acetylene black and Ni (OH)2 as dual co-catalysts, RSC Adv. 6 (2016) 31497–31506. [8] J.L. Yuan, J.Q. Wen, Y.M. Zhong, X. Li, Y.P. Fang, S.S. Zhang, W. Liu, Enhanced photocatalytic H2 evolution over noble-metal-free NiS cocatalyst modified CdS nanorods/g-C3N4 heterojunctions, J. Mater. Chem. A 3 (2015) 18244–18255. [9] D.D. Song, M.C. Li, Y.J. Jiang, Z. Chen, F. Bai, Y.F. Li, B. Jiang, Facile fabrication of MoS2/PEDOT–PSS composites as low-cost and efficient counter electrodes for dyesensitized solar cells, J. Photochem. Photobiol. A: Chem. 279 (2014) 47–51. [10] Y.G. Li, H.L. Wang, L.M. Xie, Y.Y. Liang, G.S. Hong, H.J. Dai, MoS2 nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction, J. Am. Chem. Soc. 133 (2011) 7296–7299. [11] J.J. Lee, H. Kim, S.H. Moon, Preparation of highly loaded, dispersed MoS2/Al2O3 catalysts for the deep hydrodesulfurization of dibenzothiophenes, Appl. Catal. B: Environ. 41 (2003) 171–180. [12] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Single-layer MoS2 transistors, Nat. Nanotechnol. 6 (2011) 147–150. [13] D.J. Late, B. Liu, H.S.S.R. Matte, V.P. Dravid, C.N.R. Rao, Hysteresis in single-layer MoS2 field effect transistors, ACS Nano 6 (2012) 5635–5641. [14] V. Salgueiriño-Maceira, C.E. Hoppe, M.A. Correa-Duarte, Formation of fractal-like structures driven by carbon nanotubes-based collapsed hollow capsules, J. Phys. Chem. B 111 (2007) 331–334. [15] O. Khaselev, J.A. Turner, A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting, Science 280 (1998) 425–427. [16] Z.X. Liu, H.Q. Deng, P.P. Mukherjee, Evaluating pristine and modified SnS2 as a lithium-ion battery anode: a first-principles study, ACS Appl. Mater. Interfaces 7 (2015) 4000–4009. [17] D.L. Greenaway, R. Nitsche, Preparation and optical properties of group IV–VI2 chalcogenides having the CdI2 structure, J. Phys. Chem. Solids 26 (1965) 1445–1458.

Fig. 10. (a) Effects of various scavengers on the photoelectrochemical degradation efficiency of AOSS-11. (b) A schematic illustration of the charge carrier transfer during the photoelectrocatalysis with the Al2O3/SnS2 electrode under simulated solar light irradiation (λ > 420 nm) at an applied potential of 0.49 V (vs. Ag/AgCl).

Then, the captured electrons are also transferred to the Pt wire through the outer circuit. Pt has a more negative potential than the standard redox potential of E°(O2/•O2-)=−0.33 eV (vs. NHE), thus, the surrounding oxygen molecules can consume electrons at the Pt wire, transforming themselves to the superoxide anion radical (•O2-). This species reacts with a proton (H+) from H2O forming hydrogen peroxide (H2O2) and then produces the highly active hydroxyl radical (•OH). By contrast, the generated holes (h+) in the VB of SnS2, which have a more positive potential than the standard redox potential of E°(•OH/OH-) =1.99 eV (vs. NHE), can directly generate hydroxyl radicals (•OH) by oxidizing OH-. The hydroxyl radical (•OH) has oxidation characteristics and can further react with the dye (MB) molecules, leading to degradation. In addition, the applied potential of 0.49 V (vs. Ag/ AgCl) accelerates the electron-hole pair separation and promotes the speed of all of the catalytic processes. Based on the discussion above, a majority of the PEC degradation mechanism can be provided, as illustrated by Eqs. (4)–(8): bias

Al2O3/SnS2 + hν ⎯⎯⎯→ Al2O3/SnS2 (eCB− +hVB+) bias

Al2O3/SnS2 (eCB−)+O2 ⎯⎯⎯→ Al2O3/Sn S2 +• O2− bias

bias

•O2− + H (H2O) ⎯⎯⎯→ H2O2 ⎯⎯⎯→ •OH bias

Al2O3/SnS2 (hVB+)+H2O /OH− ⎯⎯⎯→ Al2O3/SnS2 + •OH bias

•OH + MB ⎯⎯⎯→ Oxidized products

(4) (5) (6) (7) (8)

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