Improved photocatalytic activity of MoS₂ nanosheets decorated with SnO₂ nanoparticles

Abstract

MoS₂ nanosheets decorated with SnO₂ mesoporous nanoparticles were successfully prepared by a facile two-step method. The MoS₂ nanosheets were pre-synthesized using a solvothermal method and then decorated with the SnO₂ mesoporous nanoparticles through a wet chemical method. The nanocomposite was characterized with powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersed spectrometry (EDX), high-resolution transmission electron microscopy (HRTEM), thermal gravimetric and differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). SnO₂ mesoporous nanoparticles can be selectively formed and attached to the peripheral surface of the layered MoS₂, which was confirmed by FESEM and HRTEM. The photocatalytic activity of the nanocomposite was examined with Rhodamine B (RhB) in aqueous solution under UV light irradiation. The SnO₂ nanoparticles remarkably suppressed the electron–hole recombination effect on the MoS₂ photocatalyst and improved the photocatalytic activity compared to a pristine MoS₂ catalyst. A higher rate of pollutant degradation was accomplished within 50 min that was three times higher than that of the pristine MoS₂ catalyst.

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Introduction

Molybdenum disulfide (MoS₂) can be made into a type of layered structure material similar to graphene. MoS₂ has hexagonally packed crystals comprising two S–Mo–S tri-layers that are weakly bonded by van der Waals interactions.^1,2 The S–Mo–S tri-layer consists of terrace sites on the basal plane and edge sites at the edges of the nanoparticles.³ Catalytically active edge sites of MoS₂ are difficult to understand and use for advanced catalyst design and developing improved catalytic materials,³ and effective synthesis of MoS₂ with well-defined edge sites has been challenging and rarely reported.

MoS₂ is used as a solid lubricant and a catalyst for petroleum purification.^4,5 It has been gaining increasing attention in the semiconductor industry due to its unique properties. In particular, different morphological forms of MoS₂ have attracted a recent surge of attention due to their morphology-dependent properties. Such forms include nanotubes,⁶ nanorods,^7,8 nanosheets,⁹ nanospheres,^4,10 single-layer structures, and multi-layer structures.¹¹ These forms can be suitable for application to solar cells,¹² nanoelectronics,¹³ optoelectronics,¹⁴ tribology,⁴ and catalysis.⁵ Monolayer MoS₂ could possibly be produced reliably by mechanical exfoliation¹⁵ or chemically from the bulk material.¹⁶ The development of heterostructure devices has gained scientific interest for emerging applications, such as controlled photoluminescence, photocatalysts, plasmonic devices, and sensors.^17–19 Nanointerfacing can provide an avenue for controlling the electronic, electrical, optical, and thermal properties.

The performance characteristics of MoS₂ sheets could be improved by incorporation of semiconductor nanoparticles with good distribution. MoS₂ sheets decorated with nanoparticles might result in particular properties due to synergetic effects.^19,20 MoS₂ nanosheets decorated with semiconductor nanoparticles have recently been reported.^19–21 Heterostructured catalysts exhibit better performance and have proved more effective compared to individual catalysts.^13,22 One of the most difficult challenges in the development of heterostructured photocatalysts is finding specific materials that have both high photocatalytic activity and suppressed recombination effects for pollutant degradation.

MoS₂ nanosheets with well-distributed semiconductor nanoparticles can offer more versatility for charge-carrier transportation and selective catalysis. This effects result from the greater specific surface area and porosity. Thus, SnO₂-decorated MoS₂ composites may provide new opportunities to develop new photocatalyst materials.

To the best of our knowledge, there have been no reports about the synthesis of SnO₂ incorporated on MoS₂ nanosheets as a photocatalyst. We report a two-step method for synthesizing such nanosheets and used them as a photocatalyst for the first time. The metal-interfacing mechanism and its influence on the structural and photocatalytic properties of SnO₂@MoS₂ are discussed. The greatest advantage of this method is that mesoporous SnO₂ nanoparticles are formed via wet oxidation–reduction reaction between metal precursors in the presence of CTAB. Photocatalysis experiments prove that this heterogeneously structured composite has higher photocatalytic activity than pristine MoS₂.

Experimental procedure

All chemicals were analytical grade and used as received without further purification.

Preparation of MoS₂ nanosheets

To obtain the MoS₂ nanosheets, ammonium hepta molybdate tetrahydrate, citric acid, and thiophene (C₄H₄S) were used as the starting materials and sulfur source. 2.05 g of ammonium heptamolybdate tetra hydrate and 1.56 g of citric acid were dissolved in distilled water and magnetically stirred for 10 min at 90 °C on a hot plate to form a homogeneous solution. The white suspension was vigorously stirred and the pH was adjusted to 3.5 with the addition of ammonia water. Then, 3.15 g of thiophene in water was added dropwise to the solution and transferred to an autoclave, which was maintained at 200 °C for 10 h. After natural cooling of the reactor down to 25 °C, the resulting precipitates were collected through centrifugation, filtered, and washed three times with distilled water and acetone. The final precipitates were dried under vacuum at 130 °C for 2 h.

Preparation of MoS₂ sheets decorated with SnO₂ nanoparticles

The MoS₂ precipitates (20 mg) were exfoliated in distilled water (25 mL) and heated at 75 °C under magnetic stirring for 30 min to form a colloidal suspension. Then, SnCl₂·2H₂O (2.7 g) was dissolved in the colloidal solution. HCl (1.3 mL) and H₂O (30 mL) were added drop-wise to the solution with continuous heating and stirring for 2 h. After several trials, we optimized the concentration of CTAB to 0.05 mmol. 0.05 mmol of CTAB was added to the suspension and stirred at 110 °C for 6 h. The final products were collected, filtered, and washed three times with acetone. The obtained precipitates were dried in a vacuum oven at 120 °C for 12 h.

Characterization of samples

The structural properties of the obtained precipitates were characterized by powder X-ray diffraction (XRD) with a Shimadzu Labx XRD 6100 using Cu-Kα radiation (λ = 0.14 nm). The scan range was 10–80°, and the scan speed was 5 deg min⁻¹. The nanoparticles were analyzed with a transmission electron microscope (TEM, Hitachi H-7000) at 110 kV and a high-resolution TEM (HRTEM, Tecnai G² F 20 S-Twin TEM) at an accelerating voltage of 210 kV. The optical properties of the nanoparticles were studied using UV-visible spectroscopy (Cary 5000 UV-Vis spectrophotometry). Thermogravimetric (TG) and differential thermal analysis (DTA) were carried out on a SDT Q600 thermogravimetric analyzer under N₂ flow at a rate of 28 cm³ min⁻¹. The furnace temperature was increased from room temperature to 900 °C at a heating rate of 6 °C per minute. The specific BET surface area and pore size distribution were determined by N₂-adsorption using the BJH method with a Micromeritics ASAP 2000 instrument. The purity of the final product was examined by an Avatar 370 Fourier transform infrared spectroscope (FTIR) with a spectral range of 4000–400 cm⁻¹ and X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-alpha surface analysis instrument). Electrochemical impedance spectroscopy (EIS) measurements were carried out on a CHI 630B workstation. The measurement was conducted in 0.1 M KCl solutions containing 3 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆].

Photocatalytic experiments were performed at the natural pH of a solution of Rhodamine B (RhB), an organic pollutant. The experiments involved a photoreactor that has a 150 W mercury lamp with a main emission wavelength of 254 nm as an internal light source, which is surrounded by a quartz vessel. The light source is completely surrounded by a suspension of the SnO₂-decorated MoS₂ nanosheet catalyst and aqueous RhB (100 mL, 10 mg L⁻¹). Before irradiation, the suspension was stirred in the dark for 15 min to obtain a good dispersion and to ensure adsorption–desorption equilibrium between the organic pollutant molecules and the catalyst. During light irradiation, the samples of the reaction solution were collected at particular intervals and examined using an optical spectrophotometer.

Results and discussion

Fig. 1 shows the formation mechanism of the SnO₂-decorated MoS₂ nanocomposite and processing conditions. CTAB cationic surfactant was used to obtain the mesoporous SnO₂ nanoparticles. The optimized concentration of CTAB is 0.3 mmol, and the effect of CTAB in the formation of the mesoporous structured SnO₂ particles was elucidated. After addition of a small account of CTAB, aggregation of CTA⁺ ions occurs on the surface of MoS₂ nanosheets. A few CTA⁺ ions form a complex structure on the surface, and excess CTA⁺ ions go to the air–water interface to generate mesoporous structured SnO₂ particles.^23,24 This result suggests that the CTA⁺ ions interact with the MoS₂ units of the molecules and are involved in the assembly process to create the mesostructures. Also, CTAB plays an important role in the assembly process of SnO₂ and MoS₂ nanosheets.

Fig. 1 Schematic illustration of the synthesis method of SnO₂-decorated MoS₂ nanosheets.

The powder XRD patterns of the pristine MoS₂ and SnO₂-decorated MoS₂ nanosheets are shown in Fig. 2. The diffraction peaks of pristine MoS₂ (2θ = 14.1°, 28.4°, 32.9°, 39.5°, 49.4°, 55.4°, 58.6°, and 69.4°) matched with the JCPDS card (no. 75-1539) and were confirmed by previous reports.^25,26 In the case of SnO₂-decorated MoS₂, the main diffraction peaks coincided well with those of pristine MoS₂, and SnO₂ peaks also appeared with high intensity. This is attributed to the high dispersion of SnO₂ on the surface of MoS₂. The average crystallite size of MoS₂ and SnO₂-decorated MoS₂ was determined based on the (102) peak using the Debye–Scherrer formula.²⁷ The crystallite sizes were 23.53 nm and 27.46 nm for pristine MoS₂ and SnO₂-decorated MoS₂, respectively. The crystallite size of MoS₂ slightly increased after surface modification with SnO₂.

Fig. 2 XRD patterns of: (a) pristine MoS₂ and (b) SnO₂-decorated MoS₂ nanocomposite.

FESEM images of pristine MoS₂ and SnO₂-decorated MoS₂ are shown in Fig. S1(a–d).† The shape of pristine MoS₂ is a sheet form and it is completely agglomerated. The SnO₂-decorated MoS₂ is composed of both spherical (SnO₂) and sheet (MoS₂) shapes. SnO₂ appeared as bright spheres with mesoporous structure, while the MoS₂ appeared as gray sheets. Tiny SnO₂ spheres were deposited on the MoS₂ sheet surface, which was confirmed by the XRD results. TEM and HR-TEM images of pristine MoS₂ and SnO₂-decorated MoS₂ are shown in Fig. 3. The TEM images confirm that the SnO₂ particles are aggregated on the MoS₂ sheets. For MoS₂ nanosheets of a length more than 300 nm, their thickness is around 20–100 nm. It is clear that the SnO₂ particles are spread on the MoS₂ sheets, which is in good agreement with the FESEM results. The lattice fringes of the SnO₂-decorated MoS₂ have lattice spacing of 0.61 and 0.33 nm, which can be ascribed to the (002) and (110) planes of 2H–MoS₂ and SnO₂, respectively. The elements present were detected using EDX analysis, as shown in Fig. S2.† Mo, Sn, S, and O elemental peaks can clearly be seen for the SnO₂-decorated MoS₂ sample. This result also confirms that SnO₂ spheres are randomly deposited on the MoS₂ surface. No other elements appeared, and the present elements were observed at their consistent keV values. The atomic percentage of all the elements was further confirmed by XPS analysis. HRTEM mapping analysis was done to further understand the distribution and elements present in the SnO₂-decorated MoS₂ nanocomposite, as shown in Fig. S2(c–g).† Mo, Sn, S, and O are predominately distributed within the selected area of the SnO₂-decorated MoS₂ sample, which is in good agreement with the FESEM-EDX results.

Fig. 3 TEM and HRTEM images of (a and b) pristine MoS₂, (c–f) SnO₂-decorated MoS₂ nanocomposite.

TG analysis and DTA of the SnO₂-decorated MoS₂ nanocomposite were performed in nitrogen atmosphere, as shown in Fig. S3.† The weight loss of 2.1% between room temperature and 150 °C is ascribed to residual solvent evaporation, which favors crystal growth. The weight loss of 9.1% between 150 °C and 390 °C is due to the dehydroxylation process of the sample. Dehydroxylation promotes the partial breaking of sulfur and hydrogen when in contact.²⁸ These phenomena give rise to the small exothermic peak centered at 355 °C in the DTA curve. Sulfur breaking during dehydroxylation favored the formation of strong Mo–O–Sn bonds. The small exothermal peaks between 390 °C and 780 °C in the DTA curve are attributed to the recrystallization and deformed nanocrystals. Negligible endothermic peaks appeared above 590 °C. Further sintering above 630 °C caused more transformations that generated more weight loss and one more exothermic peak. The total weight loss at 900 °C for the SnO₂-decorated MoS₂ sample is 21%, indicating that it was stable with temperature.

The FT-IR spectra of the SnO₂-decorated MoS₂ nanocomposite are shown in Fig. S4.† The bands at 3404, 3143, 1647, 1410, and 603 cm⁻¹ are associated with pristine MoS₂ nanomaterials.^21,29 Among these, an absorption band at 3154 cm⁻¹ was generated by the stretching vibration of hydroxyls.³⁰ However, when the sample is heated at 400 °C, this peak disappeared, which is in good agreement with the TG-DTA results. The band at 1410 cm⁻¹ corresponds to in-plane bending vibration of O–H.³¹ The absorption bands at about 1647 and 603 cm⁻¹ are ascribed to the in-plane OH⁻ group bending vibration and γ_as Mo–S vibration, respectively.³² New peaks were obtained from the SnO₂-decorated MoS₂ nanocomposite. The band at 938 cm⁻¹ is assigned to γ_as Mo–O vibration, the one at 720 cm⁻¹ is attributed to out-of-plane bending vibration of OH⁻,²⁹ and the one at 603 cm⁻¹ is attributed to stretching vibration of Sn–O.³³

The specific area and pore volume of the SnO₂-decorated MoS₂ nanosheets were measured using nitrogen adsorption. The N₂ adsorption–desorption isotherm in Fig. 4 shows that the surface area and pore volume (inset) of SnO₂-decorated MoS₂ nanosheets were 103.7 m² g⁻¹ and 0.16 cm³ g⁻¹ respectively. This indicates moderate N₂ adsorption capability due to the SnO₂ nanoparticles being mesoporous. A hysteresis loop appears when the relative pressure is higher than 0.87 and is ascribed to capillary condensation in the mesopores. This is also in good agreement with the FESEM results.

Fig. 4 N₂ adsorption–desorption curve of the SnO₂-decorated MoS₂ nanocomposite; inset: pore diameter distribution curve.

The X-ray photoelectron survey spectra of pristine MoS₂ and the SnO₂-decorated MoS₂ nanocomposite are shown in Fig. S5.† Mo 3d, Mo 3p, and S 2p peaks were obtained from pristine MoS₂, whereas additional peaks of Sn 3d, Sn 3p, and O 1s appeared for the SnO₂-decorated MoS₂ nanocomposite. The XPS analysis confirms that the as-obtained SnO₂-decorated MoS₂ nanocomposite is composed of pristine MoS₂ and SnO₂. Fig. S6† shows the high-resolution spectra of individual elements of the as-obtained SnO₂-decorated MoS₂ nanocomposite. The main comparison between pristine MoS₂ (Mo 3d_5/2 = 228.45 eV, Mo 3d_3/2 = 231.17 eV, S 2p_3/2 = 161.31 eV)³⁴ and SnO₂-decorated MoS₂ (Mo 3d_5/2 = 228.86 eV, Mo 3d_3/2 = 231.83 eV, S 2p_3/2 = 161.69 eV, Sn 3d_5/2 = 487.38 eV)³⁵ reveals that both the Mo 3d_5/2 and S 2p_3/2 peaks shifted significantly to higher binding energies (BE) by about 0.41 eV and 0.38 eV, respectively, which is ascribed to the formation of the hybrid structure.

The difference in BE (ΔBEsn-s) obtained for the SnO₂-decorated MoS₂ nanocomposite was 325.69 eV, which varied according to the tin content and preparation conditions.³⁵ The main concentrations of S for both pristine MoS₂ and the SnO₂-decorated MoS₂ nanocomposite are about 38.1 with negligible reduction. In addition, the O 1s peak (531.1 eV) of the SnO₂-decorated MoS₂ sample shifted to higher binding energy by about 0.24 eV (O 1s = 531.34 eV) due to formation of the heterogeneous compound. The higher energy peak is associated with O₃⁻ and the formed Sn–Mo–S phase of the sample. The existence of apical S²⁻ and terminal S²⁻ suggests that an S-rich MoS₂ structure in the SnO₂-decorated MoS₂ nanocomposite. It was reported that MoS₂ with more active S edge sites is highly active for catalytic applications.^6,36 The mesoporous SnO₂ has increased active sites for enhancing the photocatalytic activity.

The UV-vis absorption edge of the semiconductor catalyst depends on the electronic structure feature. The optical absorption properties of pristine MoS₂ and SnO₂-decorated MoS₂ nanocomposite were analyzed, and the diffuse absorption spectrum is shown in Fig. S7.† The pristine MoS₂ had a weak UV response with an absorption band edge at 220 nm. In the case of the SnO₂-decorated MoS₂ nanocomposite, the light absorption is enhanced with a slight red-shift in the absorbance region, which is ascribed to the high absorption ability of SnO₂ in UV light. This red-shift is caused by the presence of SnO₂ mesoporous nanoparticles in the composite, resulting in a possible charge-transfer transition between SnO₂ and MoS₂.³⁷ The intensity of optical absorption was enhanced, and the optical absorption edges of the SnO₂-decorated MoS₂ nanocomposite were extended. This result favors the photocatalytic activity of the SnO₂-decorated MoS₂ nanocomposite.

The band gap energy of pristine MoS₂ and SnO₂-decorated MoS₂ nanocomposite can be estimated from a plot of (αhν)^1/2 versus the photon energy. The intercept of a tangent to the x-axis was recorded, which gives the band gap energies of the sample, as shown in Fig. 5. The estimated band gap energies of pristine MoS₂ and the SnO₂-decorated MoS₂ nanocomposite were approximately 2.31 eV and 2.21 eV, respectively.

Fig. 5 Tauc plots of (a) pristine MoS₂ and (b) SnO₂-decorated MoS₂ nanocomposite.

The variation in the absorption spectrum of RhB (10 ppm) with respect to irradiation time using 50 mg L⁻¹ of SnO₂-decorated MoS₂ is shown in Fig. 6. The absorption maximum obtained at 556 nm decreases gradually when the irradiation time increases. The adsorption peak nearly disappeared and the red color of the RhB pollutant faded after 50 min of irradiation, which indicates the complete degradation of RhB. The results obtained with pure SnO₂, MoS₂, and SnO₂-decorated MoS₂ in the photocatalytic degradation of RhB under UV light irradiation are shown in Fig. 7. The blank experiment was carried out without any photocatalyst and result showed that hardly degraded and its degradation percentage is below 4.3% after 50 min of exposure (Fig. 7). In contrast, when the photocatalyst is added and noticeable RhB degradation occurred. No significant concentration decay occurred when keeping the SnO₂-decorated MoS₂ suspension in dark conditions for 50 min. It can be seen from Fig. 7, the pure SnO₂ showed lower photocatalytic activity compared to pristine MoS₂. The order of degradation rates of RhB pollutant is SnO₂-decorated MoS₂ > pristine MoS₂ > pure SnO₂. It is clear that the coupled SnO₂–MoS₂ photocatalyst demonstrated higher photocatalytic activity than pure SnO₂ and MoS₂. Hence, the SnO₂ content should be a significant factor affecting the photocatalytic reaction of the SnO₂-decorated MoS₂ nanosheets. The degradation of RhB pollutant occurs as a photocatalytic process, and the SnO₂-decorated MoS₂ is more efficient than the pure forms of SnO₂ and MoS₂, as shown by the degradation curve in Fig. 7.

Fig. 6 Time-dependent UV-vis absorbance spectra of the SnO₂-decorated MoS₂ nanocomposite.

Fig. 7 Photodegradation rate of the RhB pollutant under UV light and light irradiation time (50 min) of the SnO₂-decorated MoS₂ nanocomposite.

The kinetic fitting semilog plot of −ln(C/C₀) with respect to irradiation time is shown in Fig. 8. If the concentration of pollutant is proportional to the reaction rate or the pollutant concentration is in the millimolar range, the effective reaction rate constant (k) of the photocatalytic reaction is calculated by assuming a pseudo-first-order kinetic law: C/C₀ = exp(−kt).^6,38 The value of k was determined to be 0.0072 and 0.047 min⁻¹ for the MoS₂ and SnO₂-decorated MoS₂ catalyst, respectively. The SnO₂-decorated MoS₂ nanocomposite showed significantly more activity in photocatalyzing the degradation of RhB than the pure SnO₂ and MoS₂ nanoparticles, which is ascribed to the specific surface area of the SnO₂-decorated MoS₂ nanocomposite. In addition, the mesoporous structure of SnO₂ nanoparticles may also contribute to suppressing the recombination effect.

Fig. 8 The kinetic plot of photocatalytic degradation of RhB with SnO₂-decorated MoS₂ nanocomposite under UV light irradiation.

The XRD peaks of the MoS₂ nanoparticles show somewhat larger widths than that of the SnO₂-decorated MoS₂ nanocomposite. This indicates that MoS₂ has a higher degree of crystallite agglomeration along with a larger number of grain boundaries, which may accelerate the electron–hole recombination. Therefore, the photodegradation of pollutant is much faster with the SnO₂-decorated MoS₂ nanocomposite rather than the MoS₂ catalyst. In addition, the SnO₂-decorated MoS₂ heterogeneous structure could facilitate the rectification of photogenerated charge carriers, as was demonstrated in previous studies. Examples are MoS₂ and WS₂ supported by SiO₂ and TiO₂,³⁹ MoS₂ developed on TiO₂,⁴⁰ and MoS₂ on CdS.⁴¹

The stability and reusability of a sulfide photocatalyst are significant matters of concern. Sulfide photocatalysts exhibit activity loss due to corrosion during the photooxidation reactions. Hence, it is important to verify the reusability of the photocatalyst. To check this, a fixed portion of SnO₂-decorated MoS₂ nanocomposite was recycled four times in the photocatalytic RhB degradation reaction under UV light irradiation while restoring the RhB concentration after 50 min by the addition of pristine compound to the solution. The RhB concentration was measured before and after completion of each cycle. The results are shown in Fig. 9, which clearly shows that even after the fourth cycle, the photocatalyst continues to be highly active.

Fig. 9 Recycling photocatalytic degradation of RhB in the presence of SnO₂-decorated MoS₂ nanocomposite under UV light irradiation.

The RhB concentration remaining after the first run (0.3%) is compared with the value for the fourth run (1.6%) in Fig. 9. After the last of these cycles, the nanocomposite was tested again for photocatalytic activity, and the apparent constant was k = 0.036 min⁻¹, which is lower by 11% than that measured for the fresh sample. When the same experiment was performed using pristine MoS₂ nanoparticles, the loss of activity was very high, and more importantly, the concentration of RhB remaining in the solution increased from 61% in the first run to 78% in the fourth one.

In order to observe the relevant changes in composition and redox state of SnO₂-decorated MoS₂ nanocomposite after the four cyclic photodegradation, an XPS analysis was carried out and results are presented in Fig. 10. The used nanocomposite shows a small broad peak compared to fresh sample at E_B = 234.25 eV. E_B values above 234.25 eV are rarely reported in Mo compound, which is attributed to Mo bound to a ligand with lower electron donating capability.^35,36,42 The S 2p region a small compound peak appears at E_B = 163.4 eV in the 2p_3/2 peak, which is ascribed to oxidation of sulphide species. This indicate that possible reaction and formation of a bond with RhB and to enhance the degradation of pollutant. However, the consequence of these changes on the photoreaction is moderate. As a result, SnO₂-decorated MoS₂ nanocomposite has higher stability even after cyclic photoreaction due to their cohesive energies with strong Madelung lattice energies^36,42 and structural features, thanks to the higher charge and possible numbers of M–S bonds on the Sn cations. The XPS results of SnO₂-decorated MoS₂ nanocomposite before and after photoreaction do not show the presence of pollutant in it, which reflect the complete mineralization of the pollutant.

Fig. 10 X-ray photoelectron spectra of SnO₂-decorated MoS₂ nanocomposite sample obtained after 4 cyclic photoreaction (a) Mo, (b) S, (c) Sn and (d) O elements.

In order to provide further evidence for proposed mechanism, EIS studies were conducted. It was observed from Fig. 11, the arc radius in the impedance spectrum of the SnO₂-decorated MoS₂ nanocomposite is smaller than the arc radius of the pristine MoS₂ sample in the spectrum, which indicates that the SnO₂-decorated MoS₂ nanocomposite has a lower resistance than pristine MoS₂, similar results have been reported.^43,44 This can attributed to the SnO₂@MoS₂ heterostructured interface can facilitate the interfacial electron–hole pairs transfer process.^43–46

Fig. 11 Electrochemical impedance spectra of pristine MoS₂ and SnO₂-decorated MoS₂ nanocomposite samples.

Photocatalytic mechanism

The schematic mechanism of photogenerated electron–hole transfer process in SnO₂-decorated MoS₂ nanosheets under UV-light irradiation was proposed and it shown in Fig. 12. For MoS₂ with the band gap energy of 2.31 eV, but it shows little photocatalytic activity due to the fast recombination effect of the electron/hole pairs in MoS₂ under UV irradiation in the current experimental conditions. The conduction band (CB) of MoS₂ is lower than that of SnO₂; hence the former can turn as a sink for the photogenerated electrons.^43–47 Then, the photogenerated holes might be trapped within the SnO₂ particles due to the holes move in the opposite direction from the electrons. As a result, the recombination of the electron/hole pairs is suppressed and, consequently the SnO₂-decorated MoS₂ nanocomposite exhibits higher photocatalytic activity than that of pure SnO₂ and MoS₂.

Fig. 12 The proposed schematic mechanism of photogenerated electron–hole transfer process in SnO₂-decorated MoS₂ nanosheets under UV-light irradiation.

The enhancement in photocatalytic activity of the SnO₂-decorated MoS₂ nanocomposite can be explained based on two mechanisms. The first is the adsorption ability of the MoS₂ nanosheets, since the adsorption is a key process in the catalytic destruction of pollutant. Second is the electron transfer from SnO₂ to the MoS₂ nanosheets. It is well know that a metal oxide semiconductor with greater photon energy could promote an electron from the valence band (VB) to the conduction band (CB).^46–49 The promoted electron creates a vacancy in the VB and a positive hole and resulting, the separation of the photogenerated electrons and holes was accomplished at the composite interface. This positive hole can react with hydroxyl groups and form a hydroxyl radical (˙OH), a powerful oxidant. The promoted electrons decrease the dissolved oxygen and yield superoxide anion radicals (O₂˙⁻).⁴⁷

The SnO₂-decorated MoS₂ heterogeneous structure facilitates the effective electron transfer from the CB into the MoS₂ nanosheets and suppresses the recombination effect. This proposed mechanism is supported by the higher photocatalytic activity of the SnO₂-decorated MoS₂ nanocomposite. The major proposed reaction steps during the photocatalytic process are as follows:

SnO₂ + hν → SnO₂ (e⁻ + h⁺)

SnO₂ (e⁻) + MoS₂ → SnO₂ + MoS₂ (e⁻)

MoS₂ (e⁻) + O₂ → + MoS₂ + ˙O₂⁻

SnO₂ (h⁺) + OH⁻ → SnO₂ + ˙OH

O₂ + e⁻ → ˙O₂⁻ → ˙OH

RhB + ˙O₂⁻/˙OH → products

RhB + h⁺ → CO₂ + H₂O + by products

Another reason for the lower photocatalytic activity of the individual MoS₂ nanoparticles is that they react with dissolved organic matter and with the surface sulfide ions. This gives rise to sulfate ions and dissolution of the solid. This phenomenon occurs due to photocorrosion and results in lower photocatalytic performance.

Conclusions

We have reported a solvothermal method for effective decoration of MoS₂ nanosheets with SnO₂ mesoporous nanoparticles. The characterization results reveal that the SnO₂ mesoporous nanoparticles are distributed on the MoS₂ nanosheet surface and form SnO₂@MoS₂ hetero nanostructures. The SnO₂-decorated MoS₂ nanocomposites are formed by van der Waals interaction. The morphology and density of the semiconductor nanoparticles can be tuned by changing the defective sites in MoS₂ nanosheets. The result of the photocatalyst test shows that the nanocomposite has outstanding photocatalytic activity with excellent stability compared to pristine MoS₂ and SnO₂ catalyst. The architecture of the nanocomposite makes a prominent contribution to the excellent photocatalytic performance.

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) and funded by the Ministry of Science ICT and Future Planning (2014R1A2A2A01007081).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15159g

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