Insight into l -cysteine-assisted growth of Cu2S nanoparticles on exfoliated MoS2 nanosheets for effective photoreduction removal of Cr(VI)

Applied Surface Science 518 (2020) 146191

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Insight into L-cysteine-assisted growth of Cu2S nanoparticles on exfoliated MoS2 nanosheets for effective photoreduction removal of Cr(VI)

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Yiming Zhanga, Xiaoyan Yangb, Yonglin Wangc, Peng Zhangc, Dan Liuc,d, , Yongwei Lia, ⁎ Zhouzheng Jina, Bhekie B. Mambad, Alex T. Kuvaregad, Jianzhou Guia,c,d, a

State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Key Laboratory of Green Chemical Technology and Process Engineering, and School of Material Science and Engineering, Tiangong University, Tianjin 300387, China b School of Chemistry and Chemical Engineering, Shangqiu Normal University, Shangqiu 476000, China c School of Chemistry and Chemical Engineering, Tiangong University, Tianjin 300387, China d University of South Africa, College of Science, Engineering and Technology, Nanotechnology and Water Sustainability Research Unit, Florida Science Campus 1710, South Africa

A R T I C LE I N FO

A B S T R A C T

Keywords: Cu2S-MoS2 In-situ conjugation Interfacial effects Photoreduction Cr(VI)

For obtaining ultra-fine Cu2S nanoparticles with reduced recombination of their photogenerated electron/hole pairs, in-situ growth of Cu2S nanoparticles on exfoliated MoS2 nanosheets was achieved in this study. We have exhaustively studied L-cysteine in-situ conjugation with sulfur defects on the surface of MoS2 nanosheets, which makes particle-level superlattice structured Cu2S nanoparticles (average 6.2 nm) well disperse on MoS2 nanosheets. Further, XPS and Raman analysis confirm that the intense interfacial effects are formed between Cu2S and MoS2 for the 2-Cu2S-MoS2 composite, which significantly promote separation of photogenerated electronhole pairs and prolong their lifetime, as shown by PL and TRPL. As a result, 2-Cu2S-MoS2 exhibits the fastest photoreduction rate of Cr(VI) (0.0058 min−1) under visible light irradiations, which are almost 8.3 times and 2.9 times higher than that of MoS2 nanosheets (0.0007 min−1) and pure Cu2S (0.002 min−1), respectively. Meanwhile, 2-Cu2S-MoS2 also possesses an optimal composite proportion between Cu2S and MoS2, which presents outstanding photocatalytic activity among series of x-Cu2S-MoS2 composites. The photoreduction mechanism of Cr(VI) is investigated in detail. As a matter of fact, L-cysteinecoupled with sulfur defects on MoS2 nanosheets could produce the coordination inner heterojunction interface of 2-Cu2S-MoS2 samples, which greatly improves migration efficiency of photoinduced carriers and ultimately endows it outstanding photoreduction activities. Furthermore, the strategy was extended to fabricate CdS-MoS2 and ZnS-MoS2 heterojunctions, implying that this strategy is of promising potentials for designing more active metal sulfides heterojunction photocatalysts.

1. Introduction In past decades, heavy metal Cr(VI) pollution has caused serious threats to the environment and human health due to its strong oxidizing power and high mobility. Photocatalytic technology gradually becomes one of the most promising strategies to solve environment problems. For instance, semiconductor-based photocatalysis has been widely studied to reduce Cr(VI) into Cr(III) under light irradiations [1,2]. Among various photocatalysts, TiO2 semiconductor has attracted significant attention because of its low cost, high abundance, non-toxicity, good chemical stability, and suitable reduction potential for Cr(VI) reduction [3–5]. Unfortunately, TiO2 materials exhibit poor response to the solar spectrum and encounter a high rate of recombination of ⁎

charge carriers, which limits its large-scale applications [6,7]. Herein, it is imperative to seek for visible-light-driven photocatalysts with a high efficiency and low cost. Attributed to interesting optical and electronic properties, transition metal sulfides (TMCs) hold the great potential for applications in diverse fields including optoelectronic devices, transistors, lithium-ion batteries, lubrication and catalysis [8,9]. Particularly, as an important p-type semiconductor, cuprous sulfide (Cu2S) nanomaterials exhibit ideal light-absorbing performance and are thus widely employed in photocatalytic reactions [10]. The narrow bandgap (1.2 eV) is beneficial well with solar energy utilization and the more negative potential on Cu2S conduction band makes it feasible for photoreduction reaction [11,12]. Until now, Cu2S photocatalyst materials with various

Corresponding authors at: School of Chemistry and Chemical Engineering, Tiangong University, Tianjin 300387, China. E-mail addresses: [email protected] (D. Liu), [email protected] (J. Gui).

https://doi.org/10.1016/j.apsusc.2020.146191 Received 13 February 2020; Received in revised form 17 March 2020; Accepted 21 March 2020 Available online 28 March 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.

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Scheme 1. Schematic illustration of the synthetic process of Cu2S/MoS2 heterostructures.

synthetic targets for surface modification and functionalization [30–32]. These characteristics indicate that MoS2 nanosheets could replace graphene for in situ growth of Cu2S nanoparticles, towards enhanced the photocatalytic performance. Until now, only a few studies on decorating MoS2 nanosheets with CdS, and Ag2S, were reported, significantly inhibiting the recombination of photogenerated electron-hole pairs and improving the photocatalytic performance compared with respective monomer [33,34]. However, the issue of large size and severe aggregation of TMCs nanoparticles on MoS2 nanosheets are still not well solved. Xie et al. developed a general route for the fabrication of 2D Cu2S/MoS2 heterostructures by domain-matching epitaxial growth of Cu2S materials on 2D MoS2 nanosheets [35]. Even so, the limited sulfur source just from the dissolved smaller MoS2 nanosheets caused a low and un-adjustable deposited ration of Cu2S nanoparticles. Recently, thiol-chemistry directed techniques have been extensively employed to heal or functionalize sulfur vacancies, aiming to modify the properties of MoS2 nanosheets [32,36,37]. As common sulfur sources, organic thiols coupling with sulfur defects on MoS2 nanosheets might provide more growth sites for forming the small-sized TMCs nanoparticles on MoS2 nanosheets, further its conjunction could produce the coordination between formed TMCs nanoparticles and MoS2 nanosheets, which would greatly improve quantum efficiency of hybrid photocatalyst. However, in previous reports [33,34,38,39], organic thiols were seldom used for anchoring TMCs nanoparticles onto MoS2 nanosheets, and the roles were hardly investigated in detail. In this direction, L-cysteine-assisted growth of superlattice structured Cu2S nanoparticles on exfoliated MoS2 nanosheets is expected to achieve. In this paper, in-situ growth of particle-level superlattice structured Cu2S nanoparticles on MoS2 nanosheets was designed and achieved. Moreover, L-cysteinein-situ conjunction with sulfur defects on the surface of MoS2 nanosheets was systematically investigated. The possible formation mechanism of Cu2S-MoS2 heterostructures was proposed (Scheme 1) and the strategy was extended and applied to fabricate CdSMoS2 and ZnS-MoS2 heterostructure. Structural characterizations (XRD, XPS and Raman) confirm that the intense interfacial effects are formed between Cu2S and MoS2 for 2-Cu2S-MoS2 photocatalyst, which greatly promote the separation and transfer of photogenerated electron-hole pairs. Furthermore, photocatalytic activities of as-prepared Cu2S-MoS2

morphological structures have been constructed [13,14]. For instance, Chen et al. fabricated both atomic- and particle-level superlattice structures of Cu2S nanoparticles and achieved greatly enhanced photocatalytic activity [15]. The superlattice Cu2S particles, typically at the nanometer scale, could expose more active sites and exhibit unique properties for the quantum effect, which is a great benefit for the photocatalytic reaction [15,16]. However, fabrication of superlattice Cu2S nanoparticles of small size (< 10 nm) is still a challenging problem due to their high surface energy and reactivity [17,18]. Furthermore, numerous studies have indicated that the most serious problem of Cu2S photocatalyst is the fast recombination of photogenerated electron/hole pairs during the photocatalytic reaction, which causes a low quantum efficiency and severely restricts in actual applications [19]. To obtain small-sized Cu2S nanoparticles and inhibit severe recombination of the photogenerated electrons and holes, graphene nanosheets depositing Cu2S nanoparticles have been widely studied [20,21]. It is reported that oxygen-containing functional groups (e.g., eCOOH and eOH) on graphene could make TMCs in situ grow, which plays a key role in controlling the size, morphology, and distribution of anchored TMCs nanoparticles [22,23]. Therefore, small-sized Cu2S nanoparticles can be also easily generated on graphene nanosheets, which could highly promote the charge spatial separation and enhance photocatalytic activities. However, with a little regret, pristine graphene displays a semi-metallic character and cannot initialize any photocatalytic treatment alone. As a graphene-like structured semiconductor, recently MoS2 nanosheets have aroused more and more attention for their outstanding photoelectronic, physicochemical, and mechanical properties [24,25]. MoS2 nanosheets, especially the monolayer MoS2, are direct bandgap semiconductors and could be directly excited for photocatalytic reaction under visible light as compared with graphene. Bandgap structure and electronic properties of MoS2 nanomaterials can also be feasibly modulated by staking layers, lateral dimension, and lattice vacancies of nanosheets, offering more potential applications for photocatalytic field [26–28]. Most importantly, exfoliated MoS2 nanosheets exhibit absolute electronegativity and all sulfur atoms exposed on their surfaces are soft Lewis base, which could produce strong affinity for metal ions (act as soft Lewis acids) [29]. Meanwhile, abundant sulfur atom defects on exfoliated MoS2 nanosheets surface have been identified as potential 2

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heterostructures were evaluated by the removal of Cr(VI) under visiblelight irradiation. Meanwhile, the photoreduction mechanism of Cr(VI) is discussed in detail.

3. Results and discussion

2. Experimental section

The classical synthetic route for the Cu2S/MoS2 heterostructures is outlined in Scheme1. The bulk MoS2 is exfoliated into nanosheets in IPA solvent, which further produce abundant S defects and facilitate its electronegativity. In the reaction solution, the electrostatic interaction will impel Cu2+ ions to anchor onto the exfoliated MoS2 nanosheets [40,41]. Meanwhile, the free S2− ions dissolved from smaller-sized MoS2 nanosheets will combine with part of those anchored Cu2+ ions on the surface of larger MoS2 nanosheets [35]. For further addition of Lcysteine, the thiol groups will preferentially couple with sulfur atom vacancies, while carboxyl groups will bind to Cu2+ ions in the reaction system. Under hydrothermal process, those surface Cu2+ ions will crystallize into well-defined superlattice structure of Cu2S nanoparticles, which are strongly supported on the MoS2 nanosheets. In addition, at the absence of L-cysteine, only a few Cu2S nanoparticles with a larger size can grow on MoS2 nanosheets by direct hydrothermal process. Therefore, as the main sulfur source, L-cysteineendows more sites for in situ growth of Cu2S nanoparticles on the MoS2 nanosheets. Most importantly, its conjugation with S atom defects could act as barriers, which facilitate the formation of small-sized Cu2S nanoparticles. The characteristic structure and properties of exfoliated MoS2 nanosheets were studied (Fig. 1). AFM topographical image of the MoS2 nanosheets is shown in Fig. 1a, indicating the successful preparation of MoS2 nanosheets through solvent exfoliation. The MoS2 nanosheets height analysis of sections marked were plotted at the top right corner of Fig. 1a, and the precise thickness of sections 1 and 2 is 8.94 nm and 9.64 nm, respectively. The TEM image of exfoliated MoS2 nanosheets further confirms the typical nanosheets structure (Fig. 1b). Zeta potential is shown in Fig. 1c, and the exfoliated MoS2 nanosheets possessed a high negative charge of −34 mV, while the bulk MoS2 carry with a negative charge of −22.6 mV. The strengthened negative charge makes the exfoliated MoS2 nanosheets highly stable in the solution and facilitates the affinity for heavy metal ions. Moreover, the exfoliated MoS2 nanosheets and bulk MoS2 exhibit an ESR signal at g = 2.006 (Fig. 1d), which could be attributed to the unpaired electrons on coordinatively unsaturated sulfur defective sites [42]. The signal intensity illustrates that the MoS2 nanosheets exfoliated from bulk MoS2 possess a higher S-vacancy concentration, which could be conveniently couple with organic thiol. To illuminate the conjunction between L-cysteineand MoS2 nanosheets, UV–Vis, FTIR and XPS measurements were carried out. When re-dispersing the MoS2 NS and Cys-MoS2 in IPA solvent, electronic extinction spectrum of Cys-MoS2 exhibits a high degree of nanosheets aggregation in comparison with MoS2 NS (Fig. 2a). The conjunction between L-cysteineand the exfoliated MoS2 nanosheets caused big changes to the relative intensities or energies of excitonic transitions. This result reveals that the conjunction structure yielded a slightly modified MoS2 nanosheets. Fig. 2b shows the FTIR spectra of MoS2 NS, Cys-MoS2, and L-cysteinesamples. Comparing the spectra of MoS2 NS and Cys-MoS2, a sharp feature at 470 nm which is typical of 2HMoS2[43], indicating that the conjunction had not affected the vibrational properties of MoS2 in the Cys-MoS2. From the spectra of L-cysteine, the peak located at 2563 cm−1 is corresponding with S-H bond, which was also reported in previous works [37]. However, as observed in Cys-MoS2, this characteristic peak does not appear in the thiol functionalized MoS2 spectra. This result suggests that thiol has successfully been coupled with the MoS2 nanosheets via the S-H bond. Furthermore, all the C-H and C-C bonds of L-cysteineappear in spectra of Cys-MoS2, confirming the introduction of L-cysteineon the surface of the MoS2 nanosheets. Meanwhile, these peaks are slightly changed and do not appear as they do in the unconjugated L-cysteinespectra. The distinguish could be ascribed to low concentrations of the purified

3.1. Formation mechanism of Cu2S/MoS2 heterostructures

2.1. Materials 2H-MoS2 powders (particle sizes ~6 μm), Cu(NO3)2·2H2O, Cd (NO3)2·4H2O, Zn(NO3)2·6H2O, isopropanol (IPA), L-cysteine, were purchased from Aladdin. All chemicals were of the analytical grade and used without further purification. Deionized water was used throughout this study. 2.2. Preparation of MoS2 nanosheets MoS2 nanosheets were produced by a solvent stripping method. On the initial exfoliation, bulk MoS2 powders (10 mg/mL in IPA) were sonicated for 12 h in 200 mL IPA using ultrasound equipment (Branson S800), in which the circulating pump keeps the water at room temperature. The resultant raw was centrifuged for 60 min at 3000 rpm to remove unexfoliated MoS2. The supernatant was recentrifuged for 60 min at 10000 rpm to obtain sediments (the exfoliated MoS2 nanosheets). As-obtained sediments were completely dried and the weight is estimated for approximately 32 mg (0.2 mmol), which was used to determine the theoretical atom ratio of Cu/Mo in the Cu2S-MoS2 composites. 2.3. Fabrication of Cys-MoS2 The as-obtained sediments during preparation of MoS2 nanosheets were re-dispersed in 40 mL (30 v/v % IPA/water) mixture, 0.4 mmol Lcysteinewas added into the solution and stirring for 60 min. In the end, the products were washed using water and alcohol for several times and dried in a vacuum oven. The product is called Cys-MoS2. 2.4. Fabrication of Cu2S-MoS2 composites During the preparation of MoS2 nanosheets, the as-obtained sediments (about 0.2 mmol MoS2) were re-dispersed in 40 mL (30 v/v% IPA/water) mixture. After ultrasonic treatment for 30 min, suitable amount Cu(NO3)2·2H2O was dissolved in mixture for sonication another 30 min to ensure mental ions to be adsorbed on the surface of MoS2 nanosheets. After that, L-cysteine(the same mole ratio with Cu2+) was added into the mixed solution and kept sonicating for another 60 min. Finally, the resulting solution was transferred to 50 mL Teflon-lined stainless-steel autoclave and heated at 180 °C for 12 h in an electronic oven. After cooling down to room temperature, the formed precipitates were collected through centrifugation, then washed with distilled water and alcohol several times, and dried completely. The product was denoted to be x-Cu2S-MoS2 (x for 0.2, 1, 2, 3 with atom ration of Cu/Mo). Furthermore, relying on a similar process of as-obtained 2-Cu2S-MoS2, while pure Cu2S was obtained without the addition of MoS2. 2-Cu2SMoS2-blank was also synthesized without the addition of L-cysteine, which follows the methods reported by Xie et al., as mentioned in the introduction. 2-Cu2S-bulk MoS2 was also prepared, in which exfoliated MoS2 nanosheets were replaced by bulk MoS2. 2.5. Fabrication of 2-CdS-MoS2 and 2-ZnS-MoS2 To extend this strategy to be a general approach, CdS and ZnS nanoparticles were grown on MoS2 nanosheets with the optimal ratios of 2 for Cd (Zn)/Mo. Based on the above-mentioned protocol, CdS and ZnS nanoparticles were deposited on the MoS2 nanosheets, after that 2-CdSMoS2 and 2-ZnS-MoS2 samples were then obtained. 3

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Fig. 1. (a) AFM image and corresponding height profile of MoS2 NS. (b) TEM image of MoS2 NS. (c) Zeta potential and (d) EPR analysis of Bulk MoS2 and MoS2 NS.

Fig. 2. (a) UV/Vis extinction spectra of MoS2 NS and Cys-MoS2 in IPA. (b) FTIR spectra of MoS2 NS, Cys-MoS2 and L-cysteine. Fitted XPS spectra of Mo 3d core-level spectra (c), and S 2p core-level spectra (d). 4

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conjugated nanosheets, leading to the decreased signal-to-noise-ratio and hence, peak reduction and broadening [36]. To examine the atomic-level interaction of MoS2 and L-cysteinein Cys-MoS2, high-resolution X-ray photoelectron spectroscopy (XPS) analysis was performed. As shown in Fig. 2c, for MoS2 NS samples, S 2 s located at 226.2 eV correspond to S2−, Mo 3d5/2, and Mo 3d3/2 located at 229.5 eV and 232.7 eV are ascribed Mo4+. In particular, the weak peak on MoS2 NS located at 235.9 eV is due to Mo6+ in Mo-O [44], which might be from the surface sulfur defects on exfoliated MoS2 nanosheets. As Cys-MoS2 sample, the new peak located at 227.4 eV can be attributed to S 2 s of the surface L-cysteineentities in comparison with MoS2 NS. Meanwhile, the Mo-O is disappearing and the peak position of Mo4+ 3d shift toward low energy in Cys-MoS2, which implies that the thiol group of L-cystine might repair the sulfur defects and form strong interaction between L-cysteineand MoS2 nanosheets. As shown in Fig. 2d, the XPS spectrum of MoS2 NS exhibits a single doublet of S 2p peaks with the S 2p3/2 binding energy at 162.09 eV. On the contrary, the Cys-MoS2 present two doublets of the S 2p peaks: the first doublet with the S 2p3/2 binding energy at 162.14 eV corresponds with S2− in MoS2 and the second doublet with the S 2p3/2 binding energy at 163.82 eV is mainly attributed to the surface L-cysteineentities [45]. Based on the above results and previous reports [37,46–48], we conclude that the L-cysteinecould couple with S vacancies on exfoliated MoS2 nanosheets.

further confirms the formed Cu2S nanoparticles with highly crystalline and particle-level superlattice structure (Fig. 3d). Meanwhile, the HRTEM image exhibits two sets of distinct lattices spacing with the distance of 0.278 and 0.198 nm, corresponding to the (1 0 0) lattice plane of the hexagonal 2H-MoS2 and (2 2 0) lattice plane of hexagonal Cu2S, respectively. The HRTEM results reveal heterojunction between Cu2S and MoS2 is successfully constructed by in situ growth of superlattice structure of Cu2S nanoparticles on MoS2 nanosheets. Besides, particle size statistics (Fig. 3e) display that the size of Cu2S nanoparticles in 2-Cu2S-MoS2-blank sample (average 20.2 nm) was not changed obviously in comparison with pure Cu2S nanoparticles (average 19.4 nm). However, with the addition of L-cysteinein 2-Cu2SMoS2 sample, the size of Cu2S nanoparticles deposited on the MoS2 nanosheets can be greatly reduced to average 6.2 nm. Moreover, the TEM images of different rations of Cu/Mo are studied and presented in Fig. S1(a, b, c), it is observed that all these composites possess smallsized Cu2S nanoparticles, while nanoparticles become dense on MoS2 nanosheets with increasing contents of Cu2S in nanocomposites. The added L-cysteinewill facilitate the formation of well-dispersed and small-sized Cu2S nanoparticle on the MoS2 nanosheets. Meanwhile, the actual ratios of Cu/Mo measured by ICP-OES are 0.19, 0.92, 1.84, 2.72, and 0.42 for 0.2-Cu2S-MoS2, 1-Cu2S-MoS2, 2Cu2S-MoS2, 3-Cu2S-MoS2, and 2-Cu2S-MoS2-blank, respectively. The actual Cu/Mo ratios of x-Cu2S-MoS2 were close to the theoretical values, while 2-Cu2S-MoS2-blank sample displays a lower content of Cu2S as expected. In comparison with 2-Cu2S-MoS2, the lower content of Cu2S on 2-Cu2S-MoS2-blank sample was mainly attributed to limited sulfur source for dissolution of the smaller-sized MoS2 nanosheets. Therefore, it is found L-cysteinealso effects the Cu2S loading in the Cu2S-MoS2 composites under the equal amount of Cu source. The phase structures of pure Cu2S, exfoliated MoS2 nanosheets, and x-Cu2S-MoS2 composite were studied by XRD analysis, as shown in Fig. 4. MoS2 NS only exhibits the main diffraction peak at 14.4°, which could be attributed to the ultrathin structure and indexed to hexagonal 2H-MoS2 (JCPDS No. 37-1492). Meanwhile, all characteristic peaks of pure Cu2S match well with cubic Cu2S (JCPDS No.03-1071). There are no diffraction peaks of impurities or any other phases, indicating the pure-phase structure of MoS2 NS and Cu2S samples. As for x-Cu2S-MoS2

3.2. Structural and chemical characterization Morphology and structure of the Cu2S, 2-Cu2S-MoS2-blank, and 2Cu2S-MoS2 samples are investigated by TEM and HRTEM images, as shown in Fig. 3. It can be observed that pure Cu2S photocatalyst is of serious self-aggregation because of the high surface free energy for nanoparticles with a small average size (Fig. 3a). 2-Cu2S-MoS2-blank nanocomposite was fabricated without the addiction of L-cysteine, where large Cu2S nanoparticles are sparsely distributed on MoS2 nanosheets (Fig. 3b). After introducing L-cysteine, the representative TEM images of 2-Cu2S-MoS2 (Fig. 3c) show that Cu2S nanoparticles with relatively uniform shapes and sizes were homogeneously deposited on the surfaces of MoS2 nanosheets. The HRTEM image of 2-Cu2S-MoS2

Fig. 3. TEM images of Cu2S (a), 2-Cu2S-MoS2-blank (b), and 2-Cu2S-MoS2 (c); HRTEM image of 2-Cu2S-MoS2 (d). Particle size statistics of Cu2S (e). 5

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Fig. 4. XRD patterns of MoS2 NS, x-Cu2S-MoS2, and Cu2S samples.

Furthermore, the presence of interfacial effects in 2-Cu2S-MoS2 heterostructure is also confirmed by Raman analysis (Fig. 6a). The Raman spectra reveal changes in the vibration modes for the 2-Cu2SMoS2 heterostructure, according to the above-mentioned hypothesis. In detail, via in-situ growth of Cu2S on MoS2, the in-plane vibration mode E2g1 and the out-of-plane vibration mode A1g have been blue-shifted compared to MoS2 NS (from 378.5 to 380 cm−1 for E2g1 and from 403.7 to 406 cm−1 for A1g). This could be attributed to the in-situ growth of Cu2S, which interacts with the S layer of MoS2 and further changes the primitive vibration mode of the Mo-S bonds. The optical properties of the as-obtained samples are characterized by UV–vis absorption spectroscopy. As displayed in Fig. 6b, the UV–vis diffuse reflection spectra exhibits absorption edge near-infrared region for all photocatalysts, while the adsorption intensity of x-Cu2S-MoS2 samples was enhanced in 650–800 nm with the increase of Cu2S contents. As shown in Fig. S3, the bandgap energies of MoS2 NS and Cu2S are estimated by intercept of straight-line portion of (Ahν)2 versus the energy of absorbed light (hν). The bandgap values of MoS2 NF and Cu2S are 1.65 and 1.24 eV, respectively.

composites, the characteristic diffraction peaks of hexagonal 2H-MoS2 and cubic Cu2S phases can be observed in all nanocomposites, implying that x-Cu2S-MoS2 composites have been successfully fabricated. Notably, compared with MoS2 NS, diffraction peaks of 2H-MoS2 at 32.7°, 39.6°, and 49.8° are presented in x-Cu2S-MoS2 composites, which could be caused by the agglomeration of the nanosheets during the hydrothermal process. Furthermore, it is noted that the intensity of the main peak at 46.6° for cubic Cu2S gradually strengthens with increasing contents of Cu2S in nanocomposites. Especially, the main peak of cubic Cu2S in x-Cu2S-MoS2 composites shifts toward lower degree compared with pure Cu2S samples, which could be attributed to the coordination between Cu2S and MoS2 in x-Cu2S-MoS2 composites. In addition, the XRD pattern of 2-Cu2S-MoS2-blank is also studied, as presented in Fig. S2. The 2-Cu2S-MoS2-blank composite exhibits both hexagonal 2HMoS2 (JCPDS No. 37-1492) and hexagonal Cu2S (JCPDS No.84-0206), especially, different crystalline phase of Cu2S in 2-Cu2S-MoS2 and 2Cu2S-MoS2-blank composites could ascribe to the additive of L-cysteine. The XPS is employed to investigate the surface chemical composition and chemical state in 2-Cu2S-MoS2 heterostructure, along with that of pure Cu2S and MoS2 NS (Fig. 5). The XPS survey spectrum of 2-Cu2SMoS2 suggests that the composite contains Cu, S, Mo, which is consistent with the chemical composition of the photocatalyst (Fig. 5a). As shown in Fig. 5b, XPS spectrum of S 2 s, Mo 3d5/2 and Mo 3d3/2 located at 226.2, 229.5, and 232.7 eV, respectively, corresponding to S2− and Mo4+ in MoS2 NS. Interestingly, in the 2-Cu2S-MoS2 heterostructure, peaks of Mo4+ 3d3/2 and Mo4+ 3d5/2 shifts 0.4 eV towards a higher binding energy. It might be because the electronic interactions between MoS2 nanosheets and Cu2S nanoparticles lead to the charge redistribution on their interfaces. Moreover, in comparison with MoS2 NS, the disappearing Mo6+ peak on 2-Cu2S-MoS2 also indicate that L-cysteinehas coupled with S defects on MoS2 nanosheets, which eventually formed Cu2S-MoS2 heterostructure. Fig. 5c shows a high resolution spectrum of Cu 2p, the peaks at 932.2 and 952.1 eV of two samples were indexed to the binding energy of Cu+ 2p3/2 and Cu+ 2p1/2 for Cu2S. Notably, the peaks at binding energies 934.1 and 954.2 eV for Cu2S sample are attributed to Cu2+ 2p3/2 and Cu2+ 2p1/2, and it could be generated by surface oxidation of Cu2S [49]. However, in the 2Cu2S-MoS2 sample, the peak positions of Cu2+ 2p3/2 and Cu2+ 2p1/2 make slight movement compared to Cu2S sample, which could be ascribed to the intense interfacial effects between Cu2S and MoS2. The other two peaks of Cu 2p are satellite peaks, consistent with previous reports [50]. For S 2p spectrum of all samples (Fig. 5d), adjacent binding energy at 162.09 and 163.21 eV are attributed to the S 2p3/2 and S 2p1/2 orbitals of divalent sulfide ions (S2−) in Cu2S and MoS2.

3.3. Photoelectronic properties analysis Photoelectronic properties of photocatalysts are investigated to study the relationship between intrinsic structure and photocatalytic performance. The steady-state photoluminescence (PL) emission spectra of the MoS2 NS, Cu2S, and x-Cu2S-MoS2 are shown in Fig. 7a and all products are excited at 450 nm at room temperature. The higher PL intensity means the faster recombination rate of the photogenerated charges. It is found that the exfoliated MoS2 nanosheets and pure Cu2S nanoparticles show higher PL peak intensity, which implies the recombination of photoinduced electrons and holes. But for the x-Cu2SMoS2 photocatalysts, the relative peak intensity decreases dramatically, indicating that the recombination rate of photogenerated charges was greatly inhibited via the in-situ growth of superlattice structured Cu2S nanoparticles on the MoS2 nanosheets. It should be noted that 2-Cu2SMoS2 exhibits the lowest PL emission peaks, which can be attributed to the appropriate amount of Cu2S nanoparticles with uniform dispersion on MoS2 nanosheets. For more insights into the photogenerated charges, time-resolved photoluminescence (TRPL) measurement was performed. PL decay profiles of the MoS2 NS, Cu2S and x-Cu2S-MoS2 are shown in Fig. 7b, and the emission decay curves are fitted with a double-exponential decay kinetics: I(t) = A1exp(−t/τ1) + A2exp(−t/ τ2), where A and τ denote the amplitudes and emission lifetimes of each component [51,52]. The detailed kinetic parameters for the above 6

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Fig. 5. (a) XPS survey spectrum of 2-Cu2S-MoS2. (b) XPS of MoS2 NS and 2-Cu2S-MoS2 in the Mo 3d region. (c) XPS of Cu2S and 2-Cu2S-MoS2 in the Cu 2p region. (d) XPS of MoS2 NS, 2-Cu2S-MoS2 and Cu2S in the S 2p region.

and the optimal ratio of Cu/Mo, which could endow it the superior photocatalytic activities. Also, 3-Cu2S-MoS2 possesses the maximum contents of Cu2S and reduced fluorescent lifetime (7.86 ns), indicating overcrowding Cu2S nanoparticles on MoS2 nanosheets could provide more recombination centers of photogenerated electron-hole pairs. The electrochemical impedance spectroscopy (EIS) displays that Nyquist arc radius of 2-Cu2S-MoS2 heterostructure is smaller than those of MoS2 NS and Cu2S, implying that 2-Cu2S-MoS2 heterostructure has a lower resistance for interfacial charge transfer than MoS2 NS and Cu2S (Fig. 7c). The lower interfacial resistance of 2-Cu2S-MoS2 facilitates the separation and transfer of its photoinduced charge carriers, which also could contribute to the superior photocatalytic activity. The generation and transfer of photogenerated electrons and holes in MoS2 NS, 2-Cu2SMoS2, and Cu2S can also be illuminated by the transient photocurrent responses (Fig. 7d). The 2-Cu2S-MoS2 exhibits the highest photocurrent

photocatalysts are listed in Table 1. To further compare the PL decay behavior of above photocatalysts, the average lifetime, τ is calculated by the following equation.

τ = (A1τ12 + A2τ22)/(A1τ1 + A2τ2)

(1)

As the Table 1 shown, average PL lifetimes are calculated to be 5.23 ns for MoS2 NS, 6.65 ns for 0.2-Cu2S-MoS2, 7.43 ns for 1-Cu2SMoS2, 12.75 ns for 2-Cu2S-MoS2, 7.86 ns for 3-Cu2S-MoS2 and 5.8 ns for Cu2S. Depositing the Cu2S nanoparticles on MoS2 nanosheets could greatly prolong the lifetimes of photoinduced carriers, as compared to pure Cu2S nanoparticles or single MoS2 nanosheets. Among x-Cu2SMoS2 photocatalysts, with increasing the depositing contents of Cu2S nanoparticles on MoS2 nanosheets, the fluorescent lifetime initially accelerates from 6.65 ns to 12.75 ns and then reduces to 7.86 ns. The longest fluorescent lifetime (12.75 ns) of 2-Cu2S-MoS2 photocatalysts implies the higher transfer efficiency of photogenerated charge carriers

Fig. 6. (a) Raman spectrum of MoS2 NS and 2-Cu2S-MoS2. (b) UV–vis diffuse reflectance spectra of MoS2 NS, series of x-Cu2S-MoS2 and Cu2S samples. 7

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Fig. 7. (a) Photoluminescence (PL) and (b) Time-resolved photoluminescence (TRPL) of MoS2 NS, x-Cu2S-MoS2, and Cu2S samples. (c) The corresponding EIS Nyquist plots and (d) Transient photocurrent responses of MoS2 NS, 2-Cu2S-MoS2, and Cu2S samples.

Cu2S-MoS2 exhibits the optimal adsorption capacity and achieves approximately 83% removal rate of Cr(VI) within 240 min. The distinguished structure of 2-Cu2S-MoS2 could make the maximum contacts between the photocatalyst surface and aqueous Cr(VI), which bring into the optimal adsorption capacity in comparison with other samples. To accurately compare the photocatalytic activities of different samples, the reaction kinetics are fitted by the first-order linear relationship between −ln(C/Co) and t, and the slope (k) refers to the rate constant (min−1). As shown in Fig. 8c, a good linear relationship between ln(C/ C0) and t is observed for the five photocatalysts, indicating the firstorder reaction process. The rate value (k) of different samples are intuitively shown in Fig. 8d. Rate constants are 0.0004, 0.0007, 0.0058, 0.0032 and 0.002 min−1 for bulk MoS2, MoS2 NS, 2-Cu2S-MoS2, 2Cu2S-Bulk MoS2, and Cu2S photocatalysts, respectively. It is concluded that 2-Cu2S-MoS2 with intense interfacial effects between Cu2S and MoS2 could greatly accelerate the photoreduction rate of Cr(VI), in comparison with bulk MoS2, MoS2 nanosheets, pure Cu2S nanoparticles and depositing Cu2S nanoparticles on bulk MoS2. The enhanced photoreduction activities of 2-Cu2S-MoS2 mainly attribute to the optimal adsorption capacity toward Cr(VI) and high separation efficiency of photogenerated electron/hole pairs, in comparison with other samples. Furthermore, the photoreduction activities of 2-Cu2S-MoS2-blank and x-Cu2S-MoS2 samples series were evaluated. The Cr(VI)-removed processes of 2-Cu2S-MoS2-blank and x-Cu2S-MoS2 products are shown in Fig. 9a. It is found that the adsorption capability of Cr(VI) seemed to improve with the increasing content of Cu2S for the x-Cu2S-MoS2 series. Though with different adsorption capacities, all the photocatalysts display good photocatalytic activities under visible light irradiations. Especially, 2-Cu2S-MoS2 exhibits optimal removal efficiency. Further, their reaction kinetics were fitted and the results are in Fig. 9b. The exact rate constants are displayed in Fig. 9c. It is observed that 2-Cu2SMoS2-blank possesses the slower photoreduction rate (0.0019 min−1), in comparison with x-Cu2S-MoS2 samples. Moreover, as for x-Cu2SMoS2 samples, the photoreduction reaction rates are initially increasing

Table 1 Average lifetime period (τ, ns) of the charge carriers with MoS2 NS, x-Cu2SMoS2, and Cu2S samples. Samples

A1/(A1 + A2) (%)

τ1 (ns)

A2/(A1 + A2) (%)

τ2 (ns)

τ (ns)

MoS2-NS 0.2-Cu2SMoS2 1-Cu2S-MoS2 2-Cu2S-MoS2 3-Cu2S-MoS2 Cu2S

89.34 77.78

0.412 0.406

10.66 22.22

7.44 6.33

5.23 5.65

84.89 57.23 83.83 62.63

0.427 0.52 0.435 0.417

15.11 42.77 16.17 37.37

9.14 12.7 9.6 6.36

7.43 12.75 7.86 5.8

density, in comparison with MoS2 NS and Cu2S. Generally speaking, the higher photocurrent density means the faster separation efficiency of photoinduced electron-hole pairs. Therefore, above all characterizations suggests that the in-situ growth of Cu2S nanoparticles on MoS2 nanosheets could greatly promote photogenerated charge carriers transfer rate and separation efficiencies, especially 2-Cu2S-MoS2 heterostructure possesses the optimal photoelectronic properties, which would endow it the outstanding photoreduction activities.

3.4. Evaluation of the photoreduction activity As a common heavy metal pollutant, Cr(VI) is adopted to evaluate the photocatalytic performance of as-obtained photocatalysts. Before visible light irradiations, the suspensions are magnetically stirred for 30 min in the dark to establish adsorption-desorption equilibrium. From the time-dependent absorption spectral pattern of DPC (diphenylcarbazide) - Cr(VI) solution over the 2-Cu2S-MoS2 (Fig. 8a), an apparent decreasing absorbance with the irradiation time implies the good photocatalytic activity. The adsorption and photoreduction efficiencies of Cr(VI) with bulk MoS2, MoS2 NS, 2-Cu2S-MoS2, 2-Cu2S-Bulk MoS2, and Cu2S samples are presented in Fig. 8b. It is found that 28

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Fig. 8. (a) Time-dependent absorption spectral pattern of DPC-Cr(VI) over 2-Cu2S-MoS2. (b) Photoreduction activities of Cr(VI) with Bulk MoS2, MoS2 NS, 2-Cu2SMoS2, 2-Cu2S-Bulk MoS2, and Cu2S samples under visible light irradiation. (c) Corresponding kinetic plots and (d) Reaction rate constants of different samples.

Fig. 9. (a) Photoreduction activities of Cr(VI) with x-Cu2S-MoS2 and 2-Cu2S-MoS2-blank samples under visible light irradiations. (b) Corresponding kinetic plots. (c) Reaction rate constants of different samples and (d) Presentation of error bar after repeating photocatalytic test for sample 2-Cu2S-MoS2 (Each data point and error bar represents the mean and the standard errors, respectively). 9

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100 80

pH=8 pH=10

60

O

C/C (%)

repeated reactions, which implies high stability and durability. The high catalytic activity and stability also endow the 2-Cu2S-MoS2 photocatalyst good potential practicability in the remediation of effluents. XRD patterns of the sample 2-Cu2S-MoS2 before and after reaction were examined to further investigate the stability of the photocatalysts. As can be seen from Fig. 11b, the crystalline structure of the catalysts was not changed significantly after five recycling tests. TEM images of the sample 2-Cu2S-MoS2 after five cycles have also been provided to check the photostability of the catalyst, as shown in Fig. S4. There is no obvious changes in the catalyst morphology, and no aggregation of the Cu2S nanoparticles occurs, further confirming the high photostability of 2-Cu2S-MoS2 composite. These results indicate that 2-Cu2S-MoS2 photocatalyst has good reusability and structural stability for Cr(VI) photocatalytic reduction.

pH=2 pH=4 pH=6

40 20 0 0

50

100

150

200

T/ min Fig. 10. Effect of pH on Cr(VI) photoreduction for 2-Cu2S-MoS2 sample.

3.5. Photocatalytic mechanism study with the Cu/Mo ratio, which is 0.0017, 0.003, and 0.0058 min−1 for 0.2-Cu2S-MoS2, 1-Cu2S-MoS2, and 2-Cu2S-MoS2, respectively. And then, the photoreduction reaction rate of 3-Cu2S-MoS2 samples with highest Cu/Mo ratio is decreased to 0.0028 min−1. The result reveals that 2Cu2S-MoS2 possesses an optimal composite proportion between Cu2S and MoS2, which greatly promote the separation of photogenerated electron/hole pairs and ultimately endow it outstanding photocatalytic activity. Finally, the photoreduction activities tests of 2-Cu2S-MoS2 sample were repeated for independent triplicates, as shown in Fig. 9d, and each point and error bar respectively represent the mean and the standard errors. After adding the error bar, 2-Cu2S-MoS2 sample still kept a high photoreduction removal rate (82.92%) and all data points have small standard error values (≤4.10), which implying good photocatalytic repeatability of 2-Cu2S-MoS2 sample.

To better understand the role of photoinduced electrons, controlled experiments are carried out with the addition of the electron (Na2S2O8) and hole scavengers (formic acid, FA). As shown in Fig. 12a, the addition of Na2S2O8 largely weakens the photoreduction activity, while the addition of FA improves the photocatalytic performance. As electrons capturer, Na2S2O8 can capture photoinduced electrons, which greatly decrease the number of photogenerated electrons to participate in photoreduction reaction and further cause a low photoreduction efficiency. Meanwhile, as hole capturer, FA can combine with the photogenerated holes, restrain the recombination of photogenerated carriers, and then enhance photocatalytic activities. The results confirm that photoinduced electrons govern the whole process of photoreduction of Cr(VI), which is consistent with the previous reports [53,54]. To confirm the reduction of Cr(VI) over the photocatalyst, the used 2-Cu2SMoS2 photocatalyst after photocatalytic reduction of aqueous Cr(VI) was characterized by XPS (Fig. 12b). From high-resolution XPS spectra of Cr 2p, two peaks at about 577.6 and 587.5 eV can be assigned to Cr 2p3/2 and Cr 2p1/2 of Cr(III), respectively [55]. It indicates that the highly toxic Cr(VI) was converted into less toxic Cr(III) and covering on the surface of 2-Cu2S-MoS2 after the photoreduction. It is believed that the composite with suited band structure can promote the separation of photoinduced charge carriers and enhance the photocatalytic performances. The band edge positions of the conduction band (CB) and valence band (VB) of a semiconductor at the point of zero charges are calculated using the following empirical formulas [56–60]:

3.4.1. Effect of solution pH As pH strongly affects the photocatalytic reduction of Cr(VI), the pH was varied from 2 to 10 and photoreduction removal efficiencies for 2Cu2S-MoS2 photocatalyst are displayed in Fig. 10. It is found that the increasing pH will significantly reduce the adsorption capacity and photoreduction activities for 2-Cu2S-MoS2 composite. At low pH, Cr(VI) species exists as HCrO4−, and the HCrO4− would transfer to Cr2O72− with the pH increasing. Therefore, under low pH, the surface of photocatalysts becomes highly protonated and more positive, which preferably attracts for the existing predominant anionic of HCrO4−. At higher pH, the surface of photocatalysts becomes more negative, which repels the Cr2O72− and weakens the photoreduction activities of Cr(VI). 3.4.2. Reusability and stability of 2-Cu2S-MoS2 composite The long-term stability of a photocatalyst during reaction is very important for the practical application. As shown in Fig. 11a, 2-Cu2SMoS2 consistently maintains high Cr(VI) removal efficiency after five

ECB = X − Ee − 1/2 Eg

(2)

EVB = Eg + ECB

(3)

X=

1 [χ(A)a χ(B)b χ(C)c χ(D)d ] a + b + c + d

(4)

Fig. 11. As for 2-Cu2S-MoS2 photocatalyst, (a) Cycling runs of photoreduction of Cr(VI) under visible light irradiation. (b) XRD pattern before and after cycling runs of photoreduction of Cr(VI). 10

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Fig. 12. (a) Photoreduction of Cr(VI) by 2-Cu2S-MoS2 with the addition of hole and electron scavengers. (b) High-resolution XPS spectra of Cr 2p after photoreduction of aqueous Cr(VI) on 2-Cu2S-MoS2 photocatalyst.

Scheme 2. Schematic illustration of the photocatalytic mechanism of 2-Cu2S-MoS2 heterostructure under visible light irradiation.

where χ is the absolute electronegativity of semiconductor (a, b, c and d are the atomic number of semiconductor). Ee is the energy of free electrons on hydrogen scale (ca. 4.5 eV). Eg is the bandgap energy of photocatalyst and band gap value of Cu2S and MoS2 are 1.24 and 1.65 eV, respectively. Based on the band gaps of Cu2S and MoS2, the valance and conduction band potentials of Cu2S and MoS2 were calculated using Eqs. (2)–(4), and the results are illustrated in Scheme 2. The CB potential of Cu2S (−0.12 eV) is more negative than that of MoS2 (0.01 eV). Conversely, the VB potential of MoS2 (1.66 eV) is more positive than that of Cu2S (1.12 eV). That is a П-type heterojunction, in which photogenerated electrons will transfer from Cu2S to MoS2, while photogenerated holes will migrate from Cu2S to MoS2 under visible light irradiations. This phenomenon greatly facilitates the spatial separation of photogenerated electron-hole pairs, further suppresses recombination and prolong the lifetime of photogenerated charge carriers. Because CB potential (0.01 eV) of MoS2 is more negative than the Cr(VI)/Cr(III) potential (0.51 V, vs. NHE) [61,62], the electrons on CB of MoS2 would directly reduce the adsorbed Cr(VI) into Cr(III). Meanwhile, the H2O molecules in the Cr(VI) solution were oxidized into O2 on the VB of Cu2S.

XRD analysis indicates that heterostructures between CdS (ZnS) and MoS2 have been successfully fabricated. As expected, 2-CdS-MoS2 exhibits enhanced photocatalytic reduction activities of Cr(VI) compared with MoS2 NS and CdS, likewise 2-ZnS-MoS2 shows superior photoreduction activities, in comparison with MoS2 NS and ZnS (Fig. S6). These results imply that this strategy is of promising potential for constructing metal sulfides heterojunction photocatalysts. Because of various metal sulfides nanoparticles, it is justifiable to expect more enhanced photocatalytic activities over TMDs/MoS2 heterostructure. 4. Conclusions In summary, in-situ growth of particle-level superlattice structure of Cu2S nanoparticles on MoS2 nanosheets were successfully achieved. The L-cysteinecoupling with abundant sulfur atom defects on the surface of exfoliated MoS2 nanosheets would significantly affect the construction of Cu2S-MoS2 heterostructure, and a possible formation mechanism of Cu2S-MoS2 heterojunction was also proposed. XPS and Raman analysis confirm that the intense interfacial interaction exists between Cu2S and MoS2, which greatly promotes photogenerated electron-hole pairs separation efficiencies and prolong the lifetime of photogenerated charge carriers. Most importantly, the as-prepared 2Cu2S-MoS2 heterostructures also exhibit remarkably enhanced photocatalytic activity and high photostability in photoreduction removal of Cr(VI) under visible light irradiations. The П-type photocatalytic mechanism is adopted to illustrate efficient separation of photogenerated electron/hole pairs and enhanced photoreduction activities. At last, this strategy was extended as a general approach, and the photocatalytic

3.6. Extending the strategy To extend this strategy as a general approach to deposit different metal sulfide nanoparticles on MoS2, CdS and ZnS nanoparticles were grown on MoS2 nanosheets, which worked for photocatalytic removal of Cr(VI). As shown in Fig. S5, TEM images confirm CdS and ZnS nanoparticles are uniformly dispersed on the MoS2 nanosheets, and the 11

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efficiencies achieve great improvement when CdS and ZnS nanoparticles are in situ fabricated on the MoS2 nanosheets. It is proposed that the strategy is of promising potential for constructing metal sulfides heterojunction photocatalysts with more enhanced photocatalytic activities.

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CRediT authorship contribution statement Yiming Zhang: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Xiaoyan Yang: Validation, Formal analysis, Visualization, Software. Yonglin Wang: Formal analysis, Visualization, Software. Peng Zhang: Validation, Formal analysis, Visualization. Dan Liu: Validation, Formal analysis, Supervision, Data curation. Yongwei Li: Writing - review & editing, Data curation. Zhouzheng Jin: Writing - review & editing, Data curation. Bhekie B. Mamba: Writing - review & editing. Alex T. Kuvarega: Writing - review & editing. Jianzhou Gui: Resources, Writing - review & editing, Supervision, Data curation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors thank the financial support from the National Natural Science Foundation of China (No. 21576211 and No. 21706190), Tianjin 131 Research Team of Innovative Talents, Tianjin Innovative Research Team in Universities (Grant No. TD13-5031). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2020.146191. References [1] L. Wang, X. Li, W. Teng, Q. Zhao, Y. Shi, R. Yue, Y. Chen, Efficient photocatalytic reduction of aqueous Cr(VI) over flower-like SnIn4S8 microspheres under visible light illumination, J. Hazard. Mater. 244–245 (2013) 681–688. [2] B. Nanda, A.C. Pradhan, K.M. Parida, Fabrication of mesoporous CuO/ZrO2-MCM41 nanocomposites for photocatalytic reduction of Cr(VI), Chem. Eng. J. 316 (2017) 1122–1135. [3] L. Wang, C. Zhang, F. Gao, G. Mailhot, G. Pan, Algae decorated TiO2/Ag hybrid nanofiber membrane with enhanced photocatalytic activity for Cr(VI) removal under visible light, Chem. Eng. J. 314 (2017) 622–630. [4] X.F. Lei, C. Chen, X. Li, X.X. Xue, H. Yang, Characterization and photocatalytic performance of La and C co-doped anatase TiO2 for photocatalytic reduction of Cr (VI), Sep. Purif. Technol. 161 (2016) 8–15. [5] Y. Li, W. Cui, L. Liu, R. Zong, W. Yao, Y. Liang, Y. Zhu, Removal of Cr(VI) by 3D TiO2-graphene hydrogel via adsorption enriched with photocatalytic reduction, Appl. Catal. B: Environ. 199 (2016) 412–423. [6] J. Cai, X. Wu, F. Zheng, S. Li, Y. Wu, Y. Lin, L. Lin, B. Liu, Q. Chen, L. Lin, Influence of TiO2 hollow sphere size on its photo-reduction activity for toxic Cr(VI) removal, J. Colloid Interface Sci. 490 (2017) 37–45. [7] B.A. Marinho, R.O. Cristóvão, R. Djellabi, J.M. Loureiro, R.A.R. Boaventura, V.J.P. Vilar, Photocatalytic reduction of Cr(VI) over TiO2-coated cellulose acetate monolithic structures using solar light, Appl. Catal. B: Environ. 203 (2017) 18–30. [8] T. Wang, S. Chen, H. Pang, H. Xue, Y. Yu, MoS2-based nanocomposites for electrochemical energy storage, Adv Sci. 4 (2017) 1600289. [9] P. Hu, X. Liu, B. Liu, L. Li, W. Qin, H. Yu, S. Zhong, Y. Li, Z. Ren, M. Wang, Hierarchical layered Ni3S2-graphene hybrid composites for efficient photocatalytic reduction of Cr(VI), J. Colloid Interface Sci. 496 (2017) 254–260. [10] Y.-X. Yu, L. Pan, M.-K. Son, M.T. Mayer, W.-D. Zhang, A. Hagfeldt, J. Luo, M. Grätzel, Solution-processed Cu2S photocathodes for photoelectrochemical water splitting, ACS Energy Lett. 3 (2018) 760–766. [11] X. Zhang, Y. Guo, J. Tian, B. Sun, Z. Liang, X. Xu, H. Cui, Controllable growth of MoS2 nanosheets on novel Cu2S snowflakes with high photocatalytic activity, Appl. Catal. B: Environ. 232 (2018) 355–364. [12] J. Zhang, W. Li, Y. Li, L. Zhong, C. Xu, Self-optimizing bifunctional CdS/Cu2S with coexistence of light-reduced Cu0 for highly efficient photocatalytic H2 generation

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[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

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