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Synergistic effect in the reduction of Cr(VI) with Ag-MoS2 as photocatalyst
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Synergistic effect in the reduction of Cr(VI) with Ag-MoS2 as photocatalyst Kaige Sun a , Feifei Jia b,∗ , Bingqiao Yang c , Changsheng Lin b , Xianghui Li b , Shaoxian Song a,b,∗ a
Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China c School of Xingfa Mining Engineering, Wuhan Institute of Technology, Xiongchu Avenue 693, Wuhan, Hubei, 430073, China b
a r t i c l e
i n f o
Article history: Received 9 June 2019 Received in revised form 16 August 2019 Accepted 8 September 2019 Keywords: Ag-MoS2 Cr(VI) Reduction Photocatalysis Oxidation
a b s t r a c t In this work, Cr(VI) reduced into Cr(III) in aqueous solution with Ag-MoS2 as photocatalyst has been studied in order to approach into the mechanism of the reduction. The results have shown that the mechanism was not only due to the photocatalytic reduction through excited photoelectrons by AgMoS2 , but also attributed to the chemical reduction (redox reaction) from the self-oxidation of MoS2 nanosheets. This synergetic effect is not reported elsewhere. In dark condition, chemical reduction played a predominant role, in which MoS2 acted as an electron donor through its self-oxidation, while Cr(VI) anions were electron acceptor. In visible light condition, photocatalytic reduction started working, in which photoexcited electrons had an effective impact on Cr(VI) reduction, while chemical reduction also played a role. This finding might be really significant for the application of MoS2 as an effective photocatalyst for the reduction of Cr(VI). © 2019 Published by Elsevier Ltd.
1. Introduction Heavy metal ions have been excessively released into environment and caused serious environmental pollution and threated human health due to their high toxicity and non-biodegradable property [1–3]. Among the toxic heavy metals, hexavalent chromium (Cr(VI)) is one of the most common pollutant from industrial effluents including pigment manufacturing, corrosion control, metal plating, textile manufacturing and so on [4–7]. Various purification methods have been applied for removing toxic Cr(VI) such as chemical precipitation [8], adsorption [9,10], membrane separation [11], and photocatalytic degradation [12]. Photocatalytic degradation as one of the most promising technology, has drawn wide attention due to its low cost, high efficiency and environment friendly [13–16]. Molybdenum disulfide (MoS2 ) is a typical layered transitionmetal dichalcogenite with a special sandwich structure of three stacked atomic layers (S-Mo-S) [17–19]. MoS2 displays many intriguing physical and chemical properties with a wide range of applications in adsorbents, electrodes, electronic devices and catalysis [20–22]. Previous work revealed that MoS2 was a superb adsorbent for removal heavy metals form aqueous solutions and
∗ Corresponding authors at: School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China. E-mail addresses: [email protected] (F. Jia), [email protected] (S. Song).
an excellent capacitive deionization electrode for desalination [21,23]. Meanwhile, MoS2 exhibits strong absorption in the visible light region owing to its narrow bandgap in the range about 1.2 eV – 1.8 eV, which is regarded as an ideal visible light responding photocatalyst [24]. Nowadays, some researches have been done for studying the performance of MoS2 on photoreduction of Cr(VI). For instance, Borthakur and Wang et al. revealed that the reduction of Cr(VI) under visible light irradiation was realized through photoelectrons produced by MoS2 [25,26]. However, there is a serious problem to use MoS2 as photocatalyst, which is that the recombination of the charge carriers in MoS2 has an adverse impact on photocatalytic efficiency. Previous work had revealed that noble metal nanoparticles had great advantages on refraining charge recombination as well as enhancing the absorption of light [27–29]. Liu used Ag doted 1T@2H-MoS2 as photocatalyst for degradation of Cr(VI) and MB and he emphasized that Ag nanoparticles could significantly enhanced charges separation [30]. However, the surface properties of MoS2 was totally ignored in their studies. Previous work revealed that MoS2 was easily oxidized in aqueous solution and the oxidative products were Mo(VI) and SO4 2− presented in the solution [31]. Therefore, oxidation would generate lots of electrons which might be as a source of reducing Cr(VI). Thus in this study, Ag0 decorated MoS2 (Ag-MoS2 ) composite was synthesized and the role of oxidation of MoS2 in the reduction of Cr(VI) was revealed. The structure, morphology, optical properties and electrochemical properties of MoS2 and Ag-MoS2 samples were characterized, while the catalytic performance was inves-
https://doi.org/10.1016/j.apmt.2019.100453 2352-9407/© 2019 Published by Elsevier Ltd.
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tigated through the dark reaction and visible light reaction. The cycling performance of Ag-MoS2 composite was also studied. The object was to obtain a clear understanding in the role and mechanism of Ag-MoS2 in the catalytic performance of Cr(VI) as well as to give a guidance for application of MoS2 as photocatalyst. 2. Experimental 2.1. Materials and reagents Hexaammonium heptamolybdate tetrahydrate ((NH4 )6 Mo7 O24 ·4H2 O), thiourea (CN2 H4 S), silver nitrate (AgNO3 ), potassium dichromate (K2 Cr2 O7 ), L-ascorbic acid (C6 H8 O6 ) and sodium hydroxide (NaOH) used in this study were purchased from the Sinopharm Chemical Reagent Co., Ltd. (China). Concentrated sulfuric acid (H2 SO4 ) was supplied by Xinyang chemical reagent (China). All the reagents were of analytical grade. Deionized water (Millipore Q USA) with a resistivity of 18.2 M cm was used in the experiments. 2.2. Methods 2.2.1. Preparation of flower-like molybdenum disulfide Flower-like molybdenum disulfide (MoS2 ) was prepared according to our previous work [21]. Firstly, 2.48 g of (NH4 )6 Mo7 O24 ·4H2 O and 4.56 g of CN2 H4 S were dissolved in 72 ml deionized water, followed by magnetic stirring the mixture for 30 min to form a homogeneous solution. Then, the solution was transferred into a 100 ml Teflon-lined stainless steel autoclave at 220 ◦ C for 6 h. After cooling down to room temperature, the synthetic black precipitate was collected by filtration, and washed with ultrapure water for several times to remove the residual chemical reagents followed by lyophilization. 2.2.2. Preparation of Ag decorated molybdenum disulfide A certain mass of as-synthesized MoS2 (0.1 g) was scattered in 40 mL deionized water followed by sonication for 10 min to disperse uniformly. Then, 0.1 M of AgNO3 solution (46.35 L) was added into MoS2 suspension under vigorous agitation for 30 min. Subsequently, 6.9 mL of L-ascorbic acid (0.1 M) solution was added followed by agitation for another 2 h to reduce Ag+ . The mass ratio of Ag measured by ICP was 0.15% (the specific element contents (Mo, S and Ag) are listed in Table S1). Hereafter, the composite was purified with deionized water to remove the residual chemical reagents. Finally, the black precipitate was freeze-dried for 24 h. The resulting sample was named as Ag-MoS2 .
Fig. 1. XRD patterns of MoS2 and Ag-MoS2 .
the concentration of total Cr and Mo ions in solution were determined by atomic absorption spectrometer (AAS). 2.4. Measurements X-ray diffraction (XRD) patterns were gained by an advance diffractometer (D8, Bruker, Germany). Inductively Coupled PlasmaOptical Emission Spectra (ICP-OES, Agilent 720ES) was used to obtain the element contents of the composite. UV–vis diffuse reflectance spectra were obtained on a UV-VIS-NIR spectrometer (Lambad 750 S, PerkinElmer). The photoluminescence (PL) spectra were obtained using a FLS 980 spectrophotometer (Edinburgh, Britain) with an excitation wavelength of 532 nm. A scanning electron microscope (SEM) (Zeiss, Germany) was applied to obtain the morphology and the energy dispersive spectra (EDS). The concentration of total Cr and Mo ions were detected using an atomic absorption spectrometer (AAS, AA-6880, SHIMADZU, Japan). Electrochemical impedance spectroscopy (EIS) was measured in a 1 M Na2 SO4 solution over the frequency range of 10−2 -105 Hz with an alternating current voltage of 5 mV by using a conventional three-electrode system on an electrochemical workstation (VersaSTAT-450, PAR, USA). X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB 250Xi (USA). Raman spectra were obtained from INVIA Raman microscope (Renishaw, UK) to obtain the Ag-MoS2 photocatalyst structure before and after cycles. 3. Results and discussion
2.3. Reduction of Cr(VI) 3.1. Structure and morphology of molybdenum disulfide Source of Cr(VI) was prepared by using K2 Cr2 O7 and a Xe lamp (300 W) with UV cut off filter (>420 nm) was used as irradiation source. The distance of visible light source to the surface of solution was about 10 cm. The typical catalytic experiments were conducted as follows. Initially, 20 mg of catalysts were dispersed in simulated Cr(VI) wastewater (50 mg/L, 100 mL) with adding 0.2 mL ethanol as hole scavenger. The pH of the solution was adjusted to 2.2 by H2 SO4 and NaOH. Subsequently, the suspension was kept in the dark condition or visible light irradiation condition to react accompanied with continuous stirring. After certain intervals, 3 mL of the treated solution was extracted and filtered by 0.22 m membrance. The real time concentration of Cr(VI) was determined by a Thermo Scientific Orion Aquamate 8000 UV–vis spectrophotometer at 540 nm according to the diphenylcarbazide colorimetric method, and in order to confirm the reduction of Cr(VI) and its mechanism exactly,
XRD patterns of molybdenum disulfide are illustrated in Fig. 1. The typical diffraction at 2 of 13.6◦ , 32.4◦ , 35.5◦ , 57.6◦ correspond to the primary diffractions of (002), (100), (103) and (110) planes of MoS2 , respectively, which indicated the successful synthesis of MoS2 [3,21]. Besides, it could be noticed that the XRD pattern of AgMoS2 was similar with that of MoS2 indicating that the decoration of Ag on MoS2 had no effect on their crystalline structure. In addition, there were no obvious peaks associated with Ag, the reason might be attributed to the low content of Ag in the composite. Fig. 2 shows the SEM images of flower-like MoS2 and AgMoS2 composite. The MoS2 were petal-shaped sheets with uniform dimension and all nanosheets were linked together like a flower (Fig. 2(a)), which confirmed the flower-like MoS2 was successfully synthesized. Compared with flower-like MoS2 , the Ag-MoS2
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Fig. 2. SEM images of MoS2 (a) and Ag-MoS2 (b), Element mapping images of Mo, S and Ag (c) and the EDXA spectrum of Ag-MoS2 (d).
(Fig. 2(b)) displayed more orderless and the nanosheets were partially connected together because of vigorous agitation and ultrasonic dispersion. Fig. 2(c) and (d) depicts the element mapping and EDS spectrum of Ag-MoS2 , respectively. Element mapping confirmed a uniform distribution of Mo, S and Ag throughout the composite and the EDS spectrum gave a rough content of the elements. The result indicated the successful immobilization of Ag on MoS2 sheets. In order to explore the chemical states and elements composition of Ag-MoS2 composite, XPS was performed and conducted as shown in Fig. 3. The full survey spectrum (Fig. 3(a)) exhibited that the composite contained the element of Mo, S and Ag, which was consistent with Fig. 2(c–d). The narrow peaks of Ag (Fig. 3(b)) was fitted into two peaks at 368.4 eV (Ag 3d5/2 ) and 374.4 eV (Ag 3d3/2 ), respectively, which could be assigned to the Ag in the zero valence state [32–35]. This result demonstrated that the Ag nanoparticles were deposited onto the MoS2 nanosheets and preserved their metallic nature without further oxidation. 3.2. Optical and electrochemical properties of molybdenum disulfide For further investigating the optical properties of the two samples, the UV–vis absorption spectra of MoS2 and Ag-MoS2 were tested. As shown in Fig. 4, the pure MoS2 exhibited an optical absorption response from ultraviolet (UV) to visible light due to its narrow band gap. Meanwhile, the Ag-MoS2 also showed complete absorption in both UV and visible light region. Remarkably, it was worth noting that Ag-MoS2 composite showed a stronger absorption intensity at visible light region, which was likely to be a superior visible light responsive photocatalyst for reduction of pollutants. Above results demonstrated that the Ag-MoS2 was a superior photocatalyst for visible light responding. The performance
of transfer of electron was further investigated by photoluminescence (PL) spectra and electrochemical impedance spectroscopy (EIS) curve. Fig. 5(a) shows the PL spectra of MoS2 and Ag-MoS2 . The PL spectra for both of the samples showed some principal emission peaks centered at about 580 nm, 630 nm and 660 nm. The lower PL emission intensity of Ag-MoS2 suggested that decoration of Ag nanoparticles hindered the recombination of the charge carriers [36]. The Nyquist plots for MoS2 and Ag-MoS2 are illustrated in Fig. 5(b) to analyze the electron transport properties. There are two main parts in the both of the plots: (1) an incomplete semicircle at high-frequency zone, reflecting the charge-transfer resistance (Rct ), and (2) a sloped line in low-frequency region, relating to the ion-diffusion resistance. A smaller radius in charge-transfer zone represents a smaller charge-transfer resistance, which means a higher mobility and separation of electrons and holes pairs [37–41]. Obviously, the semicircle radius decreased after loading of Ag nanoparticles. Furthermore, the Rct could be calculated by equivalent circuit. Fig. 5(b) revealed that the Rct for MoS2 and Ag-MoS2 were 5.56 and 3.14 , respectively, indicating that Ag-MoS2 possessed a better interfacial charge-transfer ability than pure MoS2 . The results were in good agreement with that of Fig. 5(a). All above results indicated that Ag-MoS2 had the optimum efficiency in separation and transfer of electron and hole pairs. 3.3. Reduction of Cr(VI) under dark and visible light condition Toxic Cr(VI) ions can be reduced by photocatalytic reduction, whereas a number of Cr(VI) ions may also be reduced directly due to the special properties of catalyst, such as oxidation of MoS2 surface which can produce lots of electrons. Therefore, the Cr(VI) reduction mechanism need to be clear. The photo and dark reactions of Cr(VI) and MoS2 based photocatalyst were carried out separately. As shown in Fig. 6(a), the removal of Cr(VI) was realized by
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Fig. 3. Full survey XPS spectrum of Ag-MoS2 (a) and high resolution spectrum of Ag 3d (b).
Fig. 4. UV–vis spectra of Ag-MoS2 and MoS2.
both of photocatalysts even under dark condition. Although both reduction and adsorption could lead to Cr(VI) removal, the reduction was the only reason here, which would be discussed carefully below. Compared with pure MoS2 photocatalyst, the reduction efficiency of Cr(VI) ions by Ag-MoS2 decreased slightly under dark condition (Fig. 6(a)), which illustrated the existence of Ag nanopar-
ticles nearly did not occupy the active site on MoS2 . In visible light irradiation, the reduction efficiency of pure MoS2 had a slight improvement, which indicated charges (photoelectrons and holes) recombined rapidly in MoS2 . However, in regard to Ag-MoS2 catalyst, the reduction efficiency of Cr(VI) ions increased dramatically under visible light irradiation and the reduction efficiency reached nearly 100% at 100 min. The higher photocatalytic reduction efficiency of Ag-MoS2 was ascribed to the stronger absorption in the visible region and better transmission of photoelectrons and holes pairs than pure MoS2 . The concentration change of Cr(VI) ions only with hole scavenger under visible light irradiation is shown in Fig. S1. It could be seen that Cr(VI) solution by itself was very stable over time. However, the concentration of Cr(VI) decreased dramatically after adding photocatalyst. As shown in Fig. 6(b), the suspension (containing Ag-MoS2 catalyst and Cr(VI) contaminant) was exposed to visible light irradiation after reacting in the dark for different time (0, 1 and 2 h). It could be seen that the concentration of Cr(VI) decreased under dark condition and the reduction rate was totally enhanced after visible light irradiation. These results indicated that photoexcited electrons could promote the reduction of Cr(VI) ions. The reasons for the decrease in Cr(VI) concentration were revealed and discussed. The concentration of chromium and molybdenum ions in the solution were determined. The total chromium ions (T-Cr) and molybdenum ions (Mo) were analyzed by AAS, the concentration of specific Cr(VI) was determined by UV–vis spectroscopy and the Cr(III) concentration could be calcu-
Fig. 5. PL spectra of MoS2 and Ag-MoS2 (a) and EIS Nyquist plots of MoS2 and Ag-MoS2 (b).
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Fig. 6. Reduction efficiency of Cr(VI) under dark and visible light condition (a) and effect of light irradiation on reduction efficiency using Ag-MoS2 as catalyst (b).
Fig. 7. Concentration changes of various ions: MoS2 system under visible light irradiation (a), MoS2 system under dark condition (b), Ag-MoS2 system under visible light irradiation (c) and Ag-MoS2 system under dark condition (d).
lated by the concentration difference between the T-Cr and Cr(VI) ions. The results are shown in Fig. 7. Apparently, the total Cr ions concentration (T-Cr) were nearly stable in all images of Fig. 7, indicating only few amount of Cr ions was adsorbed on MoS2 nanosheets. However, the concentration of Cr(VI) decreased significantly and Cr(III) concentration increased synchronously in all the experimental condition, demonstrating that the toxic Cr(VI) was reduced to harmless Cr(III). As a result, it was not because of adsorption behavior leading to Cr(VI) ions concentration decreasing in Fig. 6. Expectedly, the concentration of molybdenum ions increased during the continuous reduction process, indicating Mo(IV) atoms from MoS2 structure were oxidized into Mo(VI) and existed in the form of HMoO4 − or MoO4 2- in aqueous solutions [21,31,42]. There-
fore, the spontaneous reduction of Cr(VI) under dark condition occurred due to electrons of Mo(IV) oxidation in MoS2 structure. What’s more, Mo ions could also be detected under visible light condition (Fig. 7(a) and (c)), which showed the oxidation of MoS2 also made positive effect on Cr(VI) reduction. In conclusion, photoreduction and chemical reduction (redox between MoS2 and Cr(VI)) made a collaborative contribution on reducing Cr(VI) to Cr(III) ions under light condition. To further confirm the significant role of MoS2 oxidation during the reduction of Cr(VI), X-ray photoelectron spectroscopy technique was used to characterize the Ag-MoS2 composites before and after Cr(VI) reduction under light irradiation. The high resolution XPS spectra of Mo 3d and S 2p were illustrated in Fig. 8. In the high
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Fig. 8. XPS analysis of Ag-MoS2 before and after Cr reduction: Mo 3d before reaction (a), Mo 3d after reaction (b), S 2p before reaction (c) and S 2p after reaction (d).
resolution Mo 3d spectrum (Fig. 8(a)), two characteristic peaks centered at 229.05 eV and 232.25 eV correspond to the Mo 3d5/2 and Mo 3d3/2 in MoS2 , respectively [43,44]. Another two small peaks at 230.4 eV and 233.4 eV and an additional peak located at 234.47 eV were attributed to the Mo2 S5 and MoS3 , respectively, which was ascribed to incomplete reaction during hydrothermal synthesis [43,45–47]. Notably, after photocatalytic reaction, an obvious peak occurred at 235.75 eV (Fig. 8(b)), indicating that MoS2 was partially oxidized to MoO3 [31,48], while the Mo 3d5/2 , Mo 3d3/2 , Mo2 S5 and MoS3 were consistent with that of Ag-MoS2 aforementioned In addition, S 2p spectrum (Fig. 8(c)) could be split into three peaks, from which the peaks at 161.95 eV and 163.15 eV were attributed to S 2p3/2 and S 2p1/2 in MoS2 , respectively, and peak located at 164.25 eV was MoS3 [43,47]. Similarly, after reduction process (Fig. 8(d)), a new peak emerging at 168.9 eV was ascribed to the S6+ , indicating that S(II) was also partially oxidized [31,49]. The above results indicated that oxidation of MoS2 was a crucial contributor for the reduction of Cr(VI). 3.4. Mechanism of Cr(VI) reduction on Ag-MoS2 On the basis of above experimental results and discussion, a probable mechanism for Ag-MoS2 composite is proposed and schematically illustrated in Fig. 9. MoS2 was partially oxidized and further released quantities of electrons. After that, some electrons were gathered by Ag nanoparticles anchored on MoS2 structure, and Ag nanoparticles passed them to Cr(VI), thus the Cr(VI) ions were reduced spontaneously to Cr(III) under dark condition. In visible light irradiation, MoS2 was excited, and the photogenerated
electrons and holes were in its conduction and valence bands. Then, the Ag nanoparticles served as an electrons pool, gathering photoelectrons from conductive band of MoS2 , which assisted efficient electron – hole separation. At the same time, the oxidation of MoS2 was also underway. As a consequence, the reduction of Cr(VI) under visible light condition was synergistic effect through photocatalytic and chemical reduction. Besides, the reduction could be expressed by the following equations: Chemical reduction process: Anodic : MoS2 +12H2 O → MoO4 2− +2SO4 2− +24H+ +18eCathodic : Cr2 O7 2− +6e- + 14H+ → 2Cr3+ +7H2 O MoS2 +3Cr2 O7 2− +18H+ → MoO4 2− +6Cr3+ +2SO4 2− +9H2 O Photocatalytic reduction: MoS2 +hv→ h+ +e− e− → Ag(e-) Ag(e− ) + Cr(VI) → Cr(III) + Ag h+ +C2 H5 OH → CO2 +H2 O
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Fig. 9. Reduction mechanism of Ag-MoS2 composite.
Fig. 10. Cyclic performance of Ag-MoS2 for reduction Cr(VI) under visible light irradiation.
3.5. Cyclic performance of Ag-MoS2 The cyclic performance of Ag-MoS2 for reduction Cr(VI) under visible light irradiation was investigated. The oxidation of MoS2 produced residual electrons and promoted the reduction of Cr(VI) in the beginning. However, the structural change of MoS2 decreased the photocatalytic activity during the cyclic test. As shown in Fig. 10, the photocatalytic efficiency of Cr(VI) decreased to 71% in the fourth cycle. In order to investigate the decreased performance for recycles, the Raman spectra of Ag-MoS2 before and after cycles were investigated and the results are shown in Fig. 11. As shown in Fig. 11, two characteristic peaks of MoS2 at 377.05 and 404.01 cm−1 were corresponded to the in-plane displacement in Mo and S atoms (E2 g 1 ) and out-plane symmetric displacement of S atoms along the c-axis (A1g ). After four cycles, a decreasing intensity and a red-
Fig. 11. Raman spectra of Ag-MoS2 before and after cycles. Highly efficient visible-light-driven plasmonic photocatalysts based on graphene oxide-hybridized one-dimensional Ag/AgCl heteroarchitectures
shift occurred, which could be ascribed to a reduction of thickness and oxidation of MoS2 [21,50,51]. In addition, the XPS of Ag-MoS2 after four cycles are illustrated in Fig. S2. Two strong oxidation peaks were observed from Fig. S2(a) and (b), which indicated that MoS2 was oxidized during the photocatalytic process [31,48,49]. Therefore, the decreased cyclic performance could be ascribed to structural change after oxidation of MoS2. 4. Conclusions Ag-MoS2 composite has been facilely fabricated through two-step processes: hydrothermal synthesis of MoS2 and Ag nanoparticles were anchored on MoS2 by chemical reduction method. The introduction of Ag0 hindered the recombination of the charge carriers efficiently and increased the photocatalytic efficiency. The oxidation of MoS2 played a key role in reduction of
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Cr(VI) under dark condition, while the reduction of Cr(VI) under visible light was attributed to the synergistic effect through photoreduction and oxidation of MoS2 . This study provided insights into the surface properties of MoS2 , and emphasized the significance of surface properties in photocatalytic fields.
[15]
[16]
Declaration of Competing Interest There are no conflicts of interest to declare. Acknowledgements The financial supports for this work from the National Natural Science Foundation of China (51704220, 51674183 and 51704212), Natural Science Foundation of Hubei Province (2017CFB280) are gratefully acknowledged. Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apmt.2019. 100453.
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Synergistic effect in the reduction of Cr(VI) with Ag-MoS2 as photocatalyst Kaige Sun a , Feifei Jia b,∗ , Bingqiao Yang c , Changsheng Lin b , Xianghui Li b , Shaoxian Song a,b,∗ a
Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China c School of Xingfa Mining Engineering, Wuhan Institute of Technology, Xiongchu Avenue 693, Wuhan, Hubei, 430073, China b
a r t i c l e
i n f o
Article history: Received 9 June 2019 Received in revised form 16 August 2019 Accepted 8 September 2019 Keywords: Ag-MoS2 Cr(VI) Reduction Photocatalysis Oxidation
a b s t r a c t In this work, Cr(VI) reduced into Cr(III) in aqueous solution with Ag-MoS2 as photocatalyst has been studied in order to approach into the mechanism of the reduction. The results have shown that the mechanism was not only due to the photocatalytic reduction through excited photoelectrons by AgMoS2 , but also attributed to the chemical reduction (redox reaction) from the self-oxidation of MoS2 nanosheets. This synergetic effect is not reported elsewhere. In dark condition, chemical reduction played a predominant role, in which MoS2 acted as an electron donor through its self-oxidation, while Cr(VI) anions were electron acceptor. In visible light condition, photocatalytic reduction started working, in which photoexcited electrons had an effective impact on Cr(VI) reduction, while chemical reduction also played a role. This finding might be really significant for the application of MoS2 as an effective photocatalyst for the reduction of Cr(VI). © 2019 Published by Elsevier Ltd.
1. Introduction Heavy metal ions have been excessively released into environment and caused serious environmental pollution and threated human health due to their high toxicity and non-biodegradable property [1–3]. Among the toxic heavy metals, hexavalent chromium (Cr(VI)) is one of the most common pollutant from industrial effluents including pigment manufacturing, corrosion control, metal plating, textile manufacturing and so on [4–7]. Various purification methods have been applied for removing toxic Cr(VI) such as chemical precipitation [8], adsorption [9,10], membrane separation [11], and photocatalytic degradation [12]. Photocatalytic degradation as one of the most promising technology, has drawn wide attention due to its low cost, high efficiency and environment friendly [13–16]. Molybdenum disulfide (MoS2 ) is a typical layered transitionmetal dichalcogenite with a special sandwich structure of three stacked atomic layers (S-Mo-S) [17–19]. MoS2 displays many intriguing physical and chemical properties with a wide range of applications in adsorbents, electrodes, electronic devices and catalysis [20–22]. Previous work revealed that MoS2 was a superb adsorbent for removal heavy metals form aqueous solutions and
∗ Corresponding authors at: School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China. E-mail addresses: [email protected] (F. Jia), [email protected] (S. Song).
an excellent capacitive deionization electrode for desalination [21,23]. Meanwhile, MoS2 exhibits strong absorption in the visible light region owing to its narrow bandgap in the range about 1.2 eV – 1.8 eV, which is regarded as an ideal visible light responding photocatalyst [24]. Nowadays, some researches have been done for studying the performance of MoS2 on photoreduction of Cr(VI). For instance, Borthakur and Wang et al. revealed that the reduction of Cr(VI) under visible light irradiation was realized through photoelectrons produced by MoS2 [25,26]. However, there is a serious problem to use MoS2 as photocatalyst, which is that the recombination of the charge carriers in MoS2 has an adverse impact on photocatalytic efficiency. Previous work had revealed that noble metal nanoparticles had great advantages on refraining charge recombination as well as enhancing the absorption of light [27–29]. Liu used Ag doted 1T@2H-MoS2 as photocatalyst for degradation of Cr(VI) and MB and he emphasized that Ag nanoparticles could significantly enhanced charges separation [30]. However, the surface properties of MoS2 was totally ignored in their studies. Previous work revealed that MoS2 was easily oxidized in aqueous solution and the oxidative products were Mo(VI) and SO4 2− presented in the solution [31]. Therefore, oxidation would generate lots of electrons which might be as a source of reducing Cr(VI). Thus in this study, Ag0 decorated MoS2 (Ag-MoS2 ) composite was synthesized and the role of oxidation of MoS2 in the reduction of Cr(VI) was revealed. The structure, morphology, optical properties and electrochemical properties of MoS2 and Ag-MoS2 samples were characterized, while the catalytic performance was inves-
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tigated through the dark reaction and visible light reaction. The cycling performance of Ag-MoS2 composite was also studied. The object was to obtain a clear understanding in the role and mechanism of Ag-MoS2 in the catalytic performance of Cr(VI) as well as to give a guidance for application of MoS2 as photocatalyst. 2. Experimental 2.1. Materials and reagents Hexaammonium heptamolybdate tetrahydrate ((NH4 )6 Mo7 O24 ·4H2 O), thiourea (CN2 H4 S), silver nitrate (AgNO3 ), potassium dichromate (K2 Cr2 O7 ), L-ascorbic acid (C6 H8 O6 ) and sodium hydroxide (NaOH) used in this study were purchased from the Sinopharm Chemical Reagent Co., Ltd. (China). Concentrated sulfuric acid (H2 SO4 ) was supplied by Xinyang chemical reagent (China). All the reagents were of analytical grade. Deionized water (Millipore Q USA) with a resistivity of 18.2 M cm was used in the experiments. 2.2. Methods 2.2.1. Preparation of flower-like molybdenum disulfide Flower-like molybdenum disulfide (MoS2 ) was prepared according to our previous work [21]. Firstly, 2.48 g of (NH4 )6 Mo7 O24 ·4H2 O and 4.56 g of CN2 H4 S were dissolved in 72 ml deionized water, followed by magnetic stirring the mixture for 30 min to form a homogeneous solution. Then, the solution was transferred into a 100 ml Teflon-lined stainless steel autoclave at 220 ◦ C for 6 h. After cooling down to room temperature, the synthetic black precipitate was collected by filtration, and washed with ultrapure water for several times to remove the residual chemical reagents followed by lyophilization. 2.2.2. Preparation of Ag decorated molybdenum disulfide A certain mass of as-synthesized MoS2 (0.1 g) was scattered in 40 mL deionized water followed by sonication for 10 min to disperse uniformly. Then, 0.1 M of AgNO3 solution (46.35 L) was added into MoS2 suspension under vigorous agitation for 30 min. Subsequently, 6.9 mL of L-ascorbic acid (0.1 M) solution was added followed by agitation for another 2 h to reduce Ag+ . The mass ratio of Ag measured by ICP was 0.15% (the specific element contents (Mo, S and Ag) are listed in Table S1). Hereafter, the composite was purified with deionized water to remove the residual chemical reagents. Finally, the black precipitate was freeze-dried for 24 h. The resulting sample was named as Ag-MoS2 .
Fig. 1. XRD patterns of MoS2 and Ag-MoS2 .
the concentration of total Cr and Mo ions in solution were determined by atomic absorption spectrometer (AAS). 2.4. Measurements X-ray diffraction (XRD) patterns were gained by an advance diffractometer (D8, Bruker, Germany). Inductively Coupled PlasmaOptical Emission Spectra (ICP-OES, Agilent 720ES) was used to obtain the element contents of the composite. UV–vis diffuse reflectance spectra were obtained on a UV-VIS-NIR spectrometer (Lambad 750 S, PerkinElmer). The photoluminescence (PL) spectra were obtained using a FLS 980 spectrophotometer (Edinburgh, Britain) with an excitation wavelength of 532 nm. A scanning electron microscope (SEM) (Zeiss, Germany) was applied to obtain the morphology and the energy dispersive spectra (EDS). The concentration of total Cr and Mo ions were detected using an atomic absorption spectrometer (AAS, AA-6880, SHIMADZU, Japan). Electrochemical impedance spectroscopy (EIS) was measured in a 1 M Na2 SO4 solution over the frequency range of 10−2 -105 Hz with an alternating current voltage of 5 mV by using a conventional three-electrode system on an electrochemical workstation (VersaSTAT-450, PAR, USA). X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB 250Xi (USA). Raman spectra were obtained from INVIA Raman microscope (Renishaw, UK) to obtain the Ag-MoS2 photocatalyst structure before and after cycles. 3. Results and discussion
2.3. Reduction of Cr(VI) 3.1. Structure and morphology of molybdenum disulfide Source of Cr(VI) was prepared by using K2 Cr2 O7 and a Xe lamp (300 W) with UV cut off filter (>420 nm) was used as irradiation source. The distance of visible light source to the surface of solution was about 10 cm. The typical catalytic experiments were conducted as follows. Initially, 20 mg of catalysts were dispersed in simulated Cr(VI) wastewater (50 mg/L, 100 mL) with adding 0.2 mL ethanol as hole scavenger. The pH of the solution was adjusted to 2.2 by H2 SO4 and NaOH. Subsequently, the suspension was kept in the dark condition or visible light irradiation condition to react accompanied with continuous stirring. After certain intervals, 3 mL of the treated solution was extracted and filtered by 0.22 m membrance. The real time concentration of Cr(VI) was determined by a Thermo Scientific Orion Aquamate 8000 UV–vis spectrophotometer at 540 nm according to the diphenylcarbazide colorimetric method, and in order to confirm the reduction of Cr(VI) and its mechanism exactly,
XRD patterns of molybdenum disulfide are illustrated in Fig. 1. The typical diffraction at 2 of 13.6◦ , 32.4◦ , 35.5◦ , 57.6◦ correspond to the primary diffractions of (002), (100), (103) and (110) planes of MoS2 , respectively, which indicated the successful synthesis of MoS2 [3,21]. Besides, it could be noticed that the XRD pattern of AgMoS2 was similar with that of MoS2 indicating that the decoration of Ag on MoS2 had no effect on their crystalline structure. In addition, there were no obvious peaks associated with Ag, the reason might be attributed to the low content of Ag in the composite. Fig. 2 shows the SEM images of flower-like MoS2 and AgMoS2 composite. The MoS2 were petal-shaped sheets with uniform dimension and all nanosheets were linked together like a flower (Fig. 2(a)), which confirmed the flower-like MoS2 was successfully synthesized. Compared with flower-like MoS2 , the Ag-MoS2
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Fig. 2. SEM images of MoS2 (a) and Ag-MoS2 (b), Element mapping images of Mo, S and Ag (c) and the EDXA spectrum of Ag-MoS2 (d).
(Fig. 2(b)) displayed more orderless and the nanosheets were partially connected together because of vigorous agitation and ultrasonic dispersion. Fig. 2(c) and (d) depicts the element mapping and EDS spectrum of Ag-MoS2 , respectively. Element mapping confirmed a uniform distribution of Mo, S and Ag throughout the composite and the EDS spectrum gave a rough content of the elements. The result indicated the successful immobilization of Ag on MoS2 sheets. In order to explore the chemical states and elements composition of Ag-MoS2 composite, XPS was performed and conducted as shown in Fig. 3. The full survey spectrum (Fig. 3(a)) exhibited that the composite contained the element of Mo, S and Ag, which was consistent with Fig. 2(c–d). The narrow peaks of Ag (Fig. 3(b)) was fitted into two peaks at 368.4 eV (Ag 3d5/2 ) and 374.4 eV (Ag 3d3/2 ), respectively, which could be assigned to the Ag in the zero valence state [32–35]. This result demonstrated that the Ag nanoparticles were deposited onto the MoS2 nanosheets and preserved their metallic nature without further oxidation. 3.2. Optical and electrochemical properties of molybdenum disulfide For further investigating the optical properties of the two samples, the UV–vis absorption spectra of MoS2 and Ag-MoS2 were tested. As shown in Fig. 4, the pure MoS2 exhibited an optical absorption response from ultraviolet (UV) to visible light due to its narrow band gap. Meanwhile, the Ag-MoS2 also showed complete absorption in both UV and visible light region. Remarkably, it was worth noting that Ag-MoS2 composite showed a stronger absorption intensity at visible light region, which was likely to be a superior visible light responsive photocatalyst for reduction of pollutants. Above results demonstrated that the Ag-MoS2 was a superior photocatalyst for visible light responding. The performance
of transfer of electron was further investigated by photoluminescence (PL) spectra and electrochemical impedance spectroscopy (EIS) curve. Fig. 5(a) shows the PL spectra of MoS2 and Ag-MoS2 . The PL spectra for both of the samples showed some principal emission peaks centered at about 580 nm, 630 nm and 660 nm. The lower PL emission intensity of Ag-MoS2 suggested that decoration of Ag nanoparticles hindered the recombination of the charge carriers [36]. The Nyquist plots for MoS2 and Ag-MoS2 are illustrated in Fig. 5(b) to analyze the electron transport properties. There are two main parts in the both of the plots: (1) an incomplete semicircle at high-frequency zone, reflecting the charge-transfer resistance (Rct ), and (2) a sloped line in low-frequency region, relating to the ion-diffusion resistance. A smaller radius in charge-transfer zone represents a smaller charge-transfer resistance, which means a higher mobility and separation of electrons and holes pairs [37–41]. Obviously, the semicircle radius decreased after loading of Ag nanoparticles. Furthermore, the Rct could be calculated by equivalent circuit. Fig. 5(b) revealed that the Rct for MoS2 and Ag-MoS2 were 5.56 and 3.14 , respectively, indicating that Ag-MoS2 possessed a better interfacial charge-transfer ability than pure MoS2 . The results were in good agreement with that of Fig. 5(a). All above results indicated that Ag-MoS2 had the optimum efficiency in separation and transfer of electron and hole pairs. 3.3. Reduction of Cr(VI) under dark and visible light condition Toxic Cr(VI) ions can be reduced by photocatalytic reduction, whereas a number of Cr(VI) ions may also be reduced directly due to the special properties of catalyst, such as oxidation of MoS2 surface which can produce lots of electrons. Therefore, the Cr(VI) reduction mechanism need to be clear. The photo and dark reactions of Cr(VI) and MoS2 based photocatalyst were carried out separately. As shown in Fig. 6(a), the removal of Cr(VI) was realized by
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Fig. 3. Full survey XPS spectrum of Ag-MoS2 (a) and high resolution spectrum of Ag 3d (b).
Fig. 4. UV–vis spectra of Ag-MoS2 and MoS2.
both of photocatalysts even under dark condition. Although both reduction and adsorption could lead to Cr(VI) removal, the reduction was the only reason here, which would be discussed carefully below. Compared with pure MoS2 photocatalyst, the reduction efficiency of Cr(VI) ions by Ag-MoS2 decreased slightly under dark condition (Fig. 6(a)), which illustrated the existence of Ag nanopar-
ticles nearly did not occupy the active site on MoS2 . In visible light irradiation, the reduction efficiency of pure MoS2 had a slight improvement, which indicated charges (photoelectrons and holes) recombined rapidly in MoS2 . However, in regard to Ag-MoS2 catalyst, the reduction efficiency of Cr(VI) ions increased dramatically under visible light irradiation and the reduction efficiency reached nearly 100% at 100 min. The higher photocatalytic reduction efficiency of Ag-MoS2 was ascribed to the stronger absorption in the visible region and better transmission of photoelectrons and holes pairs than pure MoS2 . The concentration change of Cr(VI) ions only with hole scavenger under visible light irradiation is shown in Fig. S1. It could be seen that Cr(VI) solution by itself was very stable over time. However, the concentration of Cr(VI) decreased dramatically after adding photocatalyst. As shown in Fig. 6(b), the suspension (containing Ag-MoS2 catalyst and Cr(VI) contaminant) was exposed to visible light irradiation after reacting in the dark for different time (0, 1 and 2 h). It could be seen that the concentration of Cr(VI) decreased under dark condition and the reduction rate was totally enhanced after visible light irradiation. These results indicated that photoexcited electrons could promote the reduction of Cr(VI) ions. The reasons for the decrease in Cr(VI) concentration were revealed and discussed. The concentration of chromium and molybdenum ions in the solution were determined. The total chromium ions (T-Cr) and molybdenum ions (Mo) were analyzed by AAS, the concentration of specific Cr(VI) was determined by UV–vis spectroscopy and the Cr(III) concentration could be calcu-
Fig. 5. PL spectra of MoS2 and Ag-MoS2 (a) and EIS Nyquist plots of MoS2 and Ag-MoS2 (b).
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Fig. 6. Reduction efficiency of Cr(VI) under dark and visible light condition (a) and effect of light irradiation on reduction efficiency using Ag-MoS2 as catalyst (b).
Fig. 7. Concentration changes of various ions: MoS2 system under visible light irradiation (a), MoS2 system under dark condition (b), Ag-MoS2 system under visible light irradiation (c) and Ag-MoS2 system under dark condition (d).
lated by the concentration difference between the T-Cr and Cr(VI) ions. The results are shown in Fig. 7. Apparently, the total Cr ions concentration (T-Cr) were nearly stable in all images of Fig. 7, indicating only few amount of Cr ions was adsorbed on MoS2 nanosheets. However, the concentration of Cr(VI) decreased significantly and Cr(III) concentration increased synchronously in all the experimental condition, demonstrating that the toxic Cr(VI) was reduced to harmless Cr(III). As a result, it was not because of adsorption behavior leading to Cr(VI) ions concentration decreasing in Fig. 6. Expectedly, the concentration of molybdenum ions increased during the continuous reduction process, indicating Mo(IV) atoms from MoS2 structure were oxidized into Mo(VI) and existed in the form of HMoO4 − or MoO4 2- in aqueous solutions [21,31,42]. There-
fore, the spontaneous reduction of Cr(VI) under dark condition occurred due to electrons of Mo(IV) oxidation in MoS2 structure. What’s more, Mo ions could also be detected under visible light condition (Fig. 7(a) and (c)), which showed the oxidation of MoS2 also made positive effect on Cr(VI) reduction. In conclusion, photoreduction and chemical reduction (redox between MoS2 and Cr(VI)) made a collaborative contribution on reducing Cr(VI) to Cr(III) ions under light condition. To further confirm the significant role of MoS2 oxidation during the reduction of Cr(VI), X-ray photoelectron spectroscopy technique was used to characterize the Ag-MoS2 composites before and after Cr(VI) reduction under light irradiation. The high resolution XPS spectra of Mo 3d and S 2p were illustrated in Fig. 8. In the high
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Fig. 8. XPS analysis of Ag-MoS2 before and after Cr reduction: Mo 3d before reaction (a), Mo 3d after reaction (b), S 2p before reaction (c) and S 2p after reaction (d).
resolution Mo 3d spectrum (Fig. 8(a)), two characteristic peaks centered at 229.05 eV and 232.25 eV correspond to the Mo 3d5/2 and Mo 3d3/2 in MoS2 , respectively [43,44]. Another two small peaks at 230.4 eV and 233.4 eV and an additional peak located at 234.47 eV were attributed to the Mo2 S5 and MoS3 , respectively, which was ascribed to incomplete reaction during hydrothermal synthesis [43,45–47]. Notably, after photocatalytic reaction, an obvious peak occurred at 235.75 eV (Fig. 8(b)), indicating that MoS2 was partially oxidized to MoO3 [31,48], while the Mo 3d5/2 , Mo 3d3/2 , Mo2 S5 and MoS3 were consistent with that of Ag-MoS2 aforementioned In addition, S 2p spectrum (Fig. 8(c)) could be split into three peaks, from which the peaks at 161.95 eV and 163.15 eV were attributed to S 2p3/2 and S 2p1/2 in MoS2 , respectively, and peak located at 164.25 eV was MoS3 [43,47]. Similarly, after reduction process (Fig. 8(d)), a new peak emerging at 168.9 eV was ascribed to the S6+ , indicating that S(II) was also partially oxidized [31,49]. The above results indicated that oxidation of MoS2 was a crucial contributor for the reduction of Cr(VI). 3.4. Mechanism of Cr(VI) reduction on Ag-MoS2 On the basis of above experimental results and discussion, a probable mechanism for Ag-MoS2 composite is proposed and schematically illustrated in Fig. 9. MoS2 was partially oxidized and further released quantities of electrons. After that, some electrons were gathered by Ag nanoparticles anchored on MoS2 structure, and Ag nanoparticles passed them to Cr(VI), thus the Cr(VI) ions were reduced spontaneously to Cr(III) under dark condition. In visible light irradiation, MoS2 was excited, and the photogenerated
electrons and holes were in its conduction and valence bands. Then, the Ag nanoparticles served as an electrons pool, gathering photoelectrons from conductive band of MoS2 , which assisted efficient electron – hole separation. At the same time, the oxidation of MoS2 was also underway. As a consequence, the reduction of Cr(VI) under visible light condition was synergistic effect through photocatalytic and chemical reduction. Besides, the reduction could be expressed by the following equations: Chemical reduction process: Anodic : MoS2 +12H2 O → MoO4 2− +2SO4 2− +24H+ +18eCathodic : Cr2 O7 2− +6e- + 14H+ → 2Cr3+ +7H2 O MoS2 +3Cr2 O7 2− +18H+ → MoO4 2− +6Cr3+ +2SO4 2− +9H2 O Photocatalytic reduction: MoS2 +hv→ h+ +e− e− → Ag(e-) Ag(e− ) + Cr(VI) → Cr(III) + Ag h+ +C2 H5 OH → CO2 +H2 O
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Fig. 9. Reduction mechanism of Ag-MoS2 composite.
Fig. 10. Cyclic performance of Ag-MoS2 for reduction Cr(VI) under visible light irradiation.
3.5. Cyclic performance of Ag-MoS2 The cyclic performance of Ag-MoS2 for reduction Cr(VI) under visible light irradiation was investigated. The oxidation of MoS2 produced residual electrons and promoted the reduction of Cr(VI) in the beginning. However, the structural change of MoS2 decreased the photocatalytic activity during the cyclic test. As shown in Fig. 10, the photocatalytic efficiency of Cr(VI) decreased to 71% in the fourth cycle. In order to investigate the decreased performance for recycles, the Raman spectra of Ag-MoS2 before and after cycles were investigated and the results are shown in Fig. 11. As shown in Fig. 11, two characteristic peaks of MoS2 at 377.05 and 404.01 cm−1 were corresponded to the in-plane displacement in Mo and S atoms (E2 g 1 ) and out-plane symmetric displacement of S atoms along the c-axis (A1g ). After four cycles, a decreasing intensity and a red-
Fig. 11. Raman spectra of Ag-MoS2 before and after cycles. Highly efficient visible-light-driven plasmonic photocatalysts based on graphene oxide-hybridized one-dimensional Ag/AgCl heteroarchitectures
shift occurred, which could be ascribed to a reduction of thickness and oxidation of MoS2 [21,50,51]. In addition, the XPS of Ag-MoS2 after four cycles are illustrated in Fig. S2. Two strong oxidation peaks were observed from Fig. S2(a) and (b), which indicated that MoS2 was oxidized during the photocatalytic process [31,48,49]. Therefore, the decreased cyclic performance could be ascribed to structural change after oxidation of MoS2. 4. Conclusions Ag-MoS2 composite has been facilely fabricated through two-step processes: hydrothermal synthesis of MoS2 and Ag nanoparticles were anchored on MoS2 by chemical reduction method. The introduction of Ag0 hindered the recombination of the charge carriers efficiently and increased the photocatalytic efficiency. The oxidation of MoS2 played a key role in reduction of
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Cr(VI) under dark condition, while the reduction of Cr(VI) under visible light was attributed to the synergistic effect through photoreduction and oxidation of MoS2 . This study provided insights into the surface properties of MoS2 , and emphasized the significance of surface properties in photocatalytic fields.
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Declaration of Competing Interest There are no conflicts of interest to declare. Acknowledgements The financial supports for this work from the National Natural Science Foundation of China (51704220, 51674183 and 51704212), Natural Science Foundation of Hubei Province (2017CFB280) are gratefully acknowledged. Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apmt.2019. 100453.
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Please cite this article in press as: K. Sun, F. Jia, B. Yang, et al., Synergistic effect in the reduction of Cr(VI) with Ag-MoS2 as photocatalyst, Appl. Mater. Today (xxxx), https://doi.org/10.1016/j.apmt.2019.100453