Preparation and improved photocatalytic activity of mesoporous WS2 using combined hydrothermal-evaporation induced self-assembly method

Accepted Manuscript Title: Preparation and improved photocatalytic activity of mesoporous WS2 using combined hydrothermal-evaporation induced self-assembly method Author: S.V. Prabhakar Vattikuti Chan Byon Ch. Venkata Reddy PII: DOI: Reference:

S0025-5408(15)30254-3 http://dx.doi.org/doi:10.1016/j.materresbull.2015.11.059 MRB 8539

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16-7-2015 26-11-2015 30-11-2015

Please cite this article as: S.V.Prabhakar Vattikuti, Chan Byon, Ch.Venkata Reddy, Preparation and improved photocatalytic activity of mesoporous WS2 using combined hydrothermal-evaporation induced self-assembly method, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.11.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation and improved photocatalytic activity of mesoporous WS2 using combined hydrothermal-evaporation induced self-assembly method S.V. Prabhakar Vattikuti1,*, Chan Byon 1, * and Ch.Venkata Reddy1 1

School of Mechanical Engineering, Yeungnam University, Gyeongsan, South Korea, 712-749

Corresponding authors Address: Dr S.V. Prabhakar Vattikuti & Dr. Chan Byon, School of Mechanical Engineering Yeungnam University 214-1 Dae-dong Gyeongsan-si, Gyeongsangbuk-do (712-749, Republic of Korea) Mobile: +82-10-4017-8527 Fax: +82-53-810-4627 *Corresponding author E-mail: [email protected] (S. V. Prabhakar Vattikuti) & [email protected] (Chan Byon )

Graphical abstract

Highlights

1. One-step method for synthesis of mesoporous WS2 was proposed. 2. Role of CTAB surfactant on formation of mesoporous WS2 was elucidated. 3. Possible growth mechanism of the mesoporous structure is also reported 4. 0.1wt% mesoporous WS2 catalyst exhibited high photocatalytic activity under UV light. Abstract In this paper, we report mesoporous WS2 nanosheets with a crystalline network that were synthesized using CTAB as a structure-directing agent via self-assembly induced by hydrothermal and thermal evaporation. Powder X-ray diffraction, Raman spectra, and high-resolution X-ray photoelectron spectroscopy results confirmed the formation of WS2 structures. Scanning electron microscopy and transmission electron microscopy were used to observe the as-prepared mesoporous frameworks. The mesoporous WS2 nanosheets have a surface area of 197 m2g-1. A possible growth mechanism is reported for these mesoporous WS2 nanosheets. The mesoporous WS2 nanosheets demonstrate high photocatalytic activity. Among different concentrations, 0.1 wt% mesoporous WS2 shows superior catalytic activity compared to pristine WS2 nanosheets. Keywords: A. chalcogenides; A. nanostructures; B. solvothermal; D. catalytic properties; D. optical properties

1. Introduction Layered transition metal disulfides MX2 (M = Mo, W, and X= S, Se and Te) show high promise for utilization in catalysis because of both the large surface area and excellent photocatalytic activity, which could be enhanced easily [1-3]. Among these, tungsten disulphide (WS2) is a promising candidate for the conversion of solar energy due to (i) its narrow band gap, (ii) a less electronegative valence band and cationic characteristics of the conduction band with high reducing activity, and (iii) a strong W-S bond [4-6]. This high photocatalytic activity is related to the large ratio of the surface area to the volume, which is improved by increasing the crystallinity of the material, as well as the high density of active sites for combining with organic compounds [7]. To date, two-dimensional (2D) mesoporous materials have attracted sustained research interest due to their novel and superior properties compared with their bulk structures. Mesoporous nanostructured materials have been widely used as catalysts, gas sensors, probes for scanning probe microscopy [8], and supercapacitors [9-10]. In addition, mesoporous nanostructured materials are key components in the field of biology as chemically active templates [11]. Mesoporous nanoparticles based on zeolite, nano-Ag or Au, and oxides (SiO2, Al2O3, ZrO2, Fe3O4, and carbon nanotubes (CNTs)) are suitable as additives to polymer materials for membrane

synthesis due to antibacterial and organic antifouling functionalities [12-17]. These additives can improve the filtration performance and lifespan due to high membrane permeability and their strong affinity to water molecules. However, the main drawbacks of metal oxide additives are their negative impact on the rejection of solutes, low water stability, and low thermal stability, which may limit the applications in membrane synthesis [18-21]. The synthesis techniques for the preparation of mesoporous materials with controllable pore size and shape have gained immense attention from researchers in recent years. In a reduction environment, the exfoliation of precursors yields plate-like WS2 sheets, and increased reaction time and temperatures lead to spherical fullerene-like nanoparticles [22]. Porous-structured tungsten oxide nanowire has been synthesized using facile template-based methods involving a porous anodic alumina template combined with colloidal particle repeat filling techniques [23]. These nanowires exhibit high surface-to-volume ratio with nanosized lacuna-like structure. Porous MoS2 has developed using ultrasonic spray pyrolysis with the help of colloidal silica particles as a sacrificial template [24]. However, developing an alternative method for synthesizing different porous inorganic materials without a template is still a challenge in materials research. The solvothermal method is considered as one promising method owing to its benefits of an easy, simple fabrication process, environmental friendliness, and high product purity. Hard template methods have become a general pathway for the synthesis of different mesoporous materials [23]. However, removal of the template is not easy, and networks of these materials are destroyed due to disordered arrangement of the pores. The synthesis of ideal mesoporous WS2 with large surface area is a challenge to achieve through template-free synthesis. Therefore, in most approaches, synergistically hard or soft templates have been utilized to fabricate

WS2 with tunable mesoporous structures. To the best of our knowledge, there are limited reports on nanostructured WS2 with different morphologies synthesized using a hydrothermal technique with cetyltrimethyl ammonium bromide (CTAB) as a surfactant. Cao et al. reported the synthesis of low-dimensional WS2 nanostructures with different morphologies by a hydrothermal process with CTAB as a surfactant [25]. In this study, WS2 materials with mesoporous structure have been produced using a hydrothermal method followed by a thermal decomposition-induced self-assembly method using 0.3 M of CTAB as a structure-directing agent. Possible growth mechanisms of the mesoporous WS2 nanostructure are also proposed. 2. Experimental procedure 2.1 Materials and synthesis method The mesoporous WS2 nanosheets were synthesized via a hydrothermal method followed by a thermal decomposition process in a vacuum dryer. Fig. 1 shows a schematic representation of the evolution process of the WS2 nanosheets to a mesoporous structure. Typically, 0.165 g of sodium tungstate dehydrate (Na2O4W•2H2O), 1.9 g of thiourea (CH4N2S), 1.74 g of hydroxylamine hydrochloride (HONH3Cl), and 10 mL of ethanol were added to 30 mL of deionized water. The white suspension was continuously stirred at 80 °C for 1 hr. After several trials, we optimized the concentration of CTAB to 0.3 mmol, which was added to the suspension and stirred at 120°C for 2 h. Then, the homogenous aqueous solution was transferred to a Teflon-lined stainless-steel autoclave, which was maintained at 180°C for 6 h. After heating, the reactor was left to cool down to room temperature. Finally, precipitates were washed three times with acetone and

filtered. The final precipitates were dried under vacuum at 120°C for 3 h. 2.2 Characterization Powder X-ray diffraction (XRD) patterns of the product were recorded using a Shimadzu Labx XRD 6100 with Cu-Kα radiation (k=0.14056 nm) and a scan range of 10–80°. The morphologies of the samples were studied using a field-emission scanning electron microscope (FESEM) (HITACHI S-4800) operated at an acceleration voltage of 15 kV. A high-resolution TEM (HRTEM) study was done with a Tecnai G2 F 20 S-Twin TEM at an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out on an SDT Q600 thermogravimetric analyzer under nitrogen flow at a rate of 40 cm3/min, and the furnace temperature was increased from 25 °C to 900˚C at a heating rate of 6 °C/min. X-ray photoelectron spectroscopy (XPS) was carried using a Thermo Scientific K-alpha surface analysis instrument. Raman measurements were performed with a Raman laser spectrometer (DXR) with an excitation line of 632.8 nm. The optical characterizations of the samples were performed using UV–visible spectroscopy (Cary 5000). Photocatalytic degradation of rhodamine B (RB) was performed under UV light irradiation with a 150-W UV lamp with a wavelength of 254 nm. 100 mg of the WS2 nanoparticle catalyst and 100 mL of the RB solution (4 mg/L) were added to a reactor. This solution was stirred at 25°C in the dark for 60 min to obtain an equilibrium mixture. Then, 5 mL of the reaction mixture was collected with a syringe and immediately centrifuged to detect the effect of RB adsorption on the catalyst. After turning on the UV lamp, 5 mL of the reaction mixture was collected at each 5-min interval. The surface area and pore size distribution were measured using a Micromeritics ASAP

2000 instrument. The specific surface areas and pore size distribution were investigated using the Brunauer-Emmett-Teller (BET) and the Barrett-Joyner-Halenda (BJH) methods along with the nitrogen adsorption-desorption isotherms. 3. Results and discussion 3.1 Powder X-ray diffraction The as-synthesized WS2 nanosheets exhibited a WS2 phase with 100% purity, as evidenced by the XRD pattern shown in Fig. 2. The intense diffraction peaks at 14.3 °, 28.9°, 33.5°, 35.9°, 39.5 °, 44 °, 49.7°, 55.9°, and 58.4° correspond to the refection of (002), (004), (101), (102), (103), (006), (105), (106), and (110) planes of the WS2 crystal, respectively. All obtained diffraction peaks could be matched to the standard WS2 hexagonal structure corresponding to JCPDS card No: 841398. No impurity peaks were observed, indicating a complete transformation of the starting precursors into WS2. In the XRD pattern, the (002) peak corresponding to multi-layered sheets is vertically stacked along the c-axis. The (100) and (110) peaks indicating the ab plane imply a twodimensional nanosheet structure of crystalline WS2 and are in good agreement with previous reports [26-27]. 3.2 FESEM and HRTEM analysis The FESEM images of WS2 nanosheets are shown in Fig. 3a-b. The images show that the aggregate morphology consists of several small sheet-like aggregates. During the calcination treatment, microstructure changes occurred within the sheets at the nanometer scale with cracks and mesopores forming due to decomposition of the surfactant. This favors the creation of new interfaces and voids inside the small sheet-like aggregates, as shown in Fig. 3c–d. Macropores

were made without an obvious change of the particle morphology at the nanoscale. This feature confirms that the sizes of the crystalline domains were in good agreement with the high intensity peak of (002) from the XRD pattern. Cross-sectional FESEM images of the mesoporous WS2 structure are displayed in Fig. 3e–f. Interestingly, high active edge portions of the mesoporous WS2 material clearly appear, which enable enhancement of the photocatalytic activity compared to a simple nanosheet form. HRTEM images of the WS2 nanosheets are shown in Fig. 4 and provide more details on the microstructures and mesoporous structure. Agglomerated tiny sheets of a few nanometers were randomly distributed with wormhole-like irregularly shaped pores and spaces between them (Fig. 4a–b). Mesoporous framework structures were observed, which is in good agreement with the FESEM results. The lattice fringe of 0.35 nm can be attributed to the (002) plane of mesoporous WS2 (Fig. 4c). The selective area electron diffraction (SAED) pattern reveals the single-crystalline nature of the mesoporous WS2 (Fig. 4d). Fig. 5a–g show the local composition of the mesoporous WS2 studied by FESEM-EDX and HRTEM-EDX area and line mapping. The figures clearly show the existence of W and S. The FESEM image (Fig. 5a) and elemental peaks of W and S are similar to those of standard WS2. EDX spectra confirmed that the final composition of WS2 nanosheets had an S:W ratio equal to the standard value (2.06). An elemental weight percentage graph is included in Fig. 5b. The presence of elements in the mesoporous WS2 was determined using HRTEM mapping analysis, and the results are shown in Fig. 5c–g. It is evident that W and S were indeed present in the mesoporous WS2, which confirms the purity of the final product.

3.3 Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) The TGA and DTA curves of WS2 nanosheets under N2 are shown in Fig. 6. Three steps mainly appear in the TGA curve at 180°C, 464 °C, and 592 °C corresponding to 12%, 18%, and 24% weight loss, respectively. The weight loss of 12% between 25°C and 180°C corresponds to evaporation of residual solvents, which promotes crystal growth. Between 464 °C and 592 °C, 6% weight loss occurred due to dehydroxylation and allows the breaking of sulfur bonds, which was confirmed by the overlapping exothermic effects in the DTA curve. Above 600°C, negligible endothermic peaks can be observed, probably due to incipient crystallization or stable phase transformation. Four exothermic peaks were observed in the DTA curves of WS2 nanosheets. The first exothermic peak at 84 °C indicates the hydrogenation of precursor molecules, while the second exothermic peak at 185 °C is ascribed to the chemisorption process, and the remaining two exothermic peaks at 470 °C and 600°C are ascribed to transformation of WS2 into WO3 due to an oxidation effect. When increasing the temperature, WO3 changed to a crystalline structure. Therefore, approximately 550°C is considered as the maximum calcination temperature for WS2 nanosheets, which facilitated the attainment of a highly crystalline product. Further heating up to 900°C causes a small mass loss (2%) mainly due to disulfide decomposition, the formation of oxides with subsequent oxidation, and sulfur oxide loss. Therefore, these synthesized nanosheets are thermally stable up to 900°C under N2. 3.4 N2 adsorption-desorption studies The textural properties of WS2 nanosheets were revealed using N2 adsorption–desorption

measurements, as shown in Fig. 7. The N2 adsorption–desorption isotherms indicate mesoporous characteristics of the WS2 nanosheets. The hysteresis loops appear when the relative pressure is higher than 0.8, which is ascribed to capillary condensation in the mesopores. This also represents the aggregates of well-shaped sheet-like morphology, which is in agreement with the morphology observed in FESEM and HRTEM results. The BET surface area and pore volume (inset figure) of WS2 nanosheets were found to be 197 m2g-1 and 0.42 cm3g-1, respectively. After the thermal decomposition process, WS2 nanosheets generally have larger surface area and pore size of more than 30 nm with a small fraction of larger pores. The texture characteristics of WS2 become more stable with decomposition temperature. As a result, the formation of mesopores in WS2 can be attributed to the cracking induced by thermal decomposition. 3.5 High-resolution X-ray photoelectron spectroscopy studies The XPS scan of the mesoporous WS2 nanosheets is shown in Fig. 8. Only peaks for W, S, and C (from the reference for shift correction) were obtained in the XPS survey. The W peaks corresponding to W5p3/2, W4f5/2, and W4f7/2 were observed at 38.7, 35.2, and 33.1 eV, and peaks corresponding to the S2p 1/2 and S2p3/2 orbital of divalent sulfide ions were obtained at 163.27 and 162.07 eV, respectively. These results are in good agreement with previous reports [28, 29]. As a result, the obtained W peaks are attributed to a W valence of +4, which is good evidence for the existence of a pure WS2 phase [29]. 3.6 Photocatalytic activity UV–vis absorption spectroscopy was performed at room temperature for the mesoporous WS2 nanosheets, as shown in Fig. S1. A classical Tauc approach was employed to estimate the

band gap energy [28] and the tangent intercepts to the X-axis provide the band gap of the mesoporous WS2 nanosheets. A plot of (αhʋ)1/2 with respect to the photon energy (hʋ) is shown in inset Fig.S1. The band gap energy of the mesoporous WS2 nanosheets is estimated to be 1.44 eV. The photocatalytic activities of the different concentrations of WS2 nanosheets (0.05, 0.1, and 0.15 wt%) are shown in Fig. 9. The figure shows the UV-Vis absorption spectra of RB with mesoporous WS2 catalyst under UV light irradiation. Fig. 10 shows the photocatalytic degradations of RB solution in the presence of 0.1 wt% WS2 nanosheets and different concentrations of mesoporous WS2 catalyst under UV light irradiation. The mesoporous WS2 catalyst exhibits better photocatalytic activity compared to WS2 nanosheets. This is ascribed to the high surface area of the mesoporous structure, which enhances the photoactivity. To understand the reaction kinetics of the RB degradation, the pseudo-first-order rate constant k was determined using the equation ln (C/C0) = kt, where C is the initial concentration of RB and C0 is the concentration at time t. Fig. 11 shows a plot of the reaction kinetics vs. ln (C/C0) of RB. Interestingly, the highest photocatalytic activity was achieved using 0.1 wt% of the catalyst, with which more than 97% of the RB was degraded within 30 min. The mesoporous materials also have better photocatalytic performance than in previous reports [30, 31]. The RB removal rate of 0.1 wt% of the catalyst is higher than those of 0.05 wt% and 0.15 wt% of the catalysts, which have a kinetic constant k of 0.01965 min -1. The catalyst concentration of 0.1 wt% is thus the optimal mass ratio and effective as a photocatalyst for the degradation of RB pollutant. The stability or recycling performance of a photocatalyst is very important for industrial applications. Therefore, the recycling performance was estimated for 0.1 wt% mesoporous WS2 nanosheets for the degradation of RB under stimulated UV light irradiation. The nanosheets were

reused four times for the decomposition of RB (10 ppm) to test the chemical stability, and the results are shown in Fig. 12. After four cycles of RB photodegradation, the catalyst did not show an obvious loss of activity, which indicated high stability of the mesoporous WS2 in the photocatalytic reaction process. There was a slight reduction in activity (less than 8%) after four cycles, which is attributed to the loss of catalyst during washing and drying. The Raman shift spectra of mesoporous WS2 before and after four photoreactions are shown in Fig. 13. The major peak positions are not altered after the fourth photoreaction, which indicates that the mesoporous WS2 is a stable photocatalyst. 3.7 Formation mechanism In the thermal decomposition process, a cationic surfactant with anionic inorganic interactions is favorable for the precipitation reaction, which leads to the mesoporous structure of the WS2 nanosheets. The rapid precipitation reaction tends to promote the interface networks of the starting materials. However, during the hydrothermal process, interactive bonds occurred between the aggregates and surfactant micelles due to the electrostatic attraction. In addition, continuous mixed networks are formed around the aggregates and surfactant micelles. Depending on the nature of the bond, the aggregates wrap around the micelles and enclose them. The removal of surfactants followed by washing and calcination could produce ordered porous structures [32]. On the other hand, some added surfactant would probably accumulate at the interface of layers, making voids or defects more flexible with tendencies toward bending and deformation. 3.8 Degradation mechanism The proposed mechanism of the photocatalytic reaction for WS2 nanosheets with

mesoporous structure is illustrated in Fig. 14. The reaction mechanism for the WS2 photocatalyst is the same as that for MoS2, which was elucidated in previous work [33]. Under UV light irradiation of the WS2 photocatalyst, the valence band (VB) electrons of WS2 jump to the conduction band (CB), leaving behind a flaw, void, or absolutely charged hole [33, 34]. It is favorable for the photocatalyst to act more efficiently when these electrons and holes take more time to recombine. In this stage, the electrons react with O2 and H+ from water to form H2O2 then break into hydroxyl radicals (˙OH) and OH¯ ions as follows: WS2 + hʋ → e¯ + h+

(1)

h+ + H2O → H+ + OH·

(2)

e¯ + O2 → O2¯

(3)

2e¯ + O2 + 2H+ → H2O2

(4)

e¯ + H2O2 → OH· + OH¯

(5)

Similarly, holes (h+) react with water to give OH· radicals, as given in Eq. (2). Formation of these reactive species helps to suppress the recombination of electrons and holes and provides more time for interaction with the RB dye. The formation of OH¯ improves the photocatalytic reaction [33, 34]. Various features of this synthesis technique are as follows: (i) commercially available starting materials are used, and the process is cheap and simple; (ii) uniformly sized pores are obtained; (iii) the results serve as an ideal model for studying other types of inorganics; and (iv) there are numerous applications, such as sorption and bio-optics. The simple and facile method

can also be extended to synthesize other materials. 4. Conclusions A new methodology has been proposed for fabricating mesoporous WS2 nanosheets. This method is a promising and simple way of preparing mesoporous WS2 materials without a template. Mesoporous WS2 nanosheets can be produced with a self-assembly process using CTAB as a structure-stretching agent. CTAB can adjust the textural structure to influence the photocatalytic properties of mesoporous WS2. These meso-structures have uniform and highly ordered nanosheets morphology with large pore volume (0.42 cm3g-1) and specific surface area (197 m2g1

). The results indicate that only W and S were detected in the XPS survey. XPS analysis

confirmed the pure form of WS2 with partial occupation of 5p, 4f, 3d, and 2p orbitals. The photocatalyst experiment indicated that the mesoporous WS2 nanosheets have good photocatalytic activity under UV light irradiation, and we found that 0.1 wt% of mesoporous WS2 is the optimal concentration for superior catalytic activity. In addition, the RB solution in the presence of 0.1 wt% mesoporous WS2 catalyst is very fast, and 97% of RB is degraded within 30 min under UV light irradiation. The better charge separation in the mesoporous semiconductor is the key issue for the interfacial charge transfer in the CB and VB. Based on this investigation, the mesoporous WS2 nanosheets are expected to find a wide range of applications in biological, catalytic, and optoelectronic fields. Hence, we are presently exploring the uses of mesoporous WS2 nanosheets in glucose detection sensors and other applications. Acknowledgement

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Fig.1. Schematic illustration of synthesis process from WS2 nanosheets to mesoporous structure

Fig.2. XRD pattern of mesoporous WS2 nanosheets

Fig.3. FESEM image of (a, b) as-synthesized WS2 nanosheets, (c, d) mesoporous WS2 and (e, f) cross sectional view of mesoporous WS2

Fig.4. HRTEM image of (a, b) mesoporous WS2 nanosheets, (c) lattice fringes and (d) SAED pattern

Fig.5. (a) FESEM image, (b) FESEM-EDX spectra (inset: %weight of W and S), (c, d) HRTEM image, HRTEM-elemental area mapping of (e) W element, (f) S element, and (g) line mapping spectra of mesoporous WS2 nanosheets

Fig.6. TG-DTA curves of 0.1 wt% mesoporous WS2 nanosheets

Fig.7. N2 adsorption/desorption isotherm of mesoporous WS2 nanosheets; inset: Barrett-joynerHalenda (BJH) pore size distribution data of the mesoporous WS2 nanosheets

Fig.8. X-ray photoelectron spectra of mesoporous WS2 nanosheets: (a) survey, (b) W4f, (c) W4d, and (d) S 2p

Dark 5min 10min 15min 20min 25min 30min

2.0

Intensity (a.u)

1.5

1.0

0.5

0.0 300

400

500

600

700

Wavelength (nm)

Fig.9 Time-dependent UV–vis absorbance spectra of the RB solution samples taken at different times of 0.1wt% mesoporous WS2 nanosheets

1.0

0.8

C/C0

Dark

UV irradiation

0.6

0.4

without catalyst 0.1 wt% WS2nanosheets 0.05 wt% WS2mesoporous

0.2

0.1 wt% WS2mesoporous 0.15 wt% WS2mesoporous -10

0

10

20

30

Time (min)

Fig.10. Photocatalytic activity curve of the photodegradation rate of the RB under UV light and light irradiation time of different concentration of mesoporous WS2 nanosheets

2.0 1.8

0.1wt% WS2 nanosheets 0.05 wt% WS2 mesoporous

1.6

0.1 wt% WS2 mesoporous

-ln C/C0

1.4

0.15 wt% WS2 mesoporous

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

5

10

15

20

25

30

Time (min)

Fig.11 The kinetic plot of photocatalytic degradation of RB under UV light irradiation of different concentration of mesoporous WS2 nanosheets

Photodegration (%) of RB

100

80

60

40

20

0 1

2

3

Number of experiment cycles

4

Fig.12. Recycling performance of 0.1wt% mesoporous WS2 nanosheets for the degradation of RB under stimulated UV light irradiation

2000 Mesoporous WS2 (Before photoreaction)

1800

th

Mesoporous WS2 (After 4 cyclic photoreaction)

419.53 cm-1

Counts

1600 348.47 cm-1

1400

1200 1000

800

600 200

250

300

350

400

450

500

550

600

-1

Raman shift [cm ]

Fig.13. Raman spectrum of 0.1wt% mesoporous WS2 nanosheets before and after 4 th cyclic photoreactions

Fig. 14. Schematic diagram illustrating the proposed mechanism for the photocatalysis of the mesoporous WS2 nanosheets