Self-assembly of ultrathin Cu₂MoS₄ nanobelts for highly efficient visible light-driven degradation of methyl orange

Abstract

We demonstrate ultrathin self-assembled Cu₂MoS₄ nanobelts synthesized by using Cu₂O as the starting sacrificial template via a hydrothermal method. The nanobelts exhibit strong light absorption over a broad wavelength spectrum, suggesting their potential application as photocatalysts. The photocatalytic activity of nanobelts is evaluated by the degradation of Methyl Orange (MO) dye under visible light irradiation. Notably, the nanobelts can completely degrade 100 mL of 15 mg mL⁻¹ MO in 20 minutes with excellent recycling and structural stability, suggesting their excellent photocatalytic performance. In comparison with a sheet-like sample, the high efficiency of the self-assembled Cu₂MoS₄ nanobelts is attributed to a high surface area and a unique band gap, agreeing with the nitrogen adsorption analysis and photoluminescence spectra. This study offers a self-assembled synthetic route to create new multifunctional nanoarchitectures composed of atomic layers, and thus may open a window for greatly extending potential applications in water pollution treatment, photocatalytic water-splitting, solar cells and other related fields.

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1. Introduction

The research on the photocatalytic and photoelectrocatalytic features of various nanomaterials not only provides useful insights into fundamental questions of photochemistry and photoelectrochemistry, but also creates next-generation catalysts.^1–3 During the past few years, tremendous efforts have been made to resolve the increasingly serious energy and environmental problems by exploring various solar-driven photocatalysts.^4,5 Irrespective of traditional photocatalytic materials, such as TiO₂ ^6–9 and ZnO,^10,11 some new photocatalysts have been developed in order to extend the absorption range to the visible-light region. In particular, plasmonic-enhanced photocatalysts¹² have been recently studied as promising visible-light-absorbing materials, i.e. Ag@AgCl,¹³ and the Cu₂O–Au hybrid structure.¹⁴ However, the use of noble metals (Ag, Au) in these photocatalysts causes high cost, limiting their practical applications. The booming of graphene has brought about new opportunities for hybrid photocatalysis,^15,16 but the ideal formation of graphene compositions is usually inconvenient, and the controllable construction of a robust interface between graphene and active materials is still a big challenge, which seriously influences their photocatalytic activity and durability. So it is crucial to explore new photocatalysts which are not only facilely-prepared and well-stable, but also earth-abundant and environmentally-friendly.

Among various candidates, transition metal sulphides (TMSs) are one of the ideal catalysts because of their excellent absorption in the visible-light region and their being an extensive source, especially ternary transition metal sulphide (TTMS) nanomaterials.^17,18 Until now, much effort has been put into exploring different approaches to enhance the photocatalytic performance of TTMS materials, such as ZnIn₂S₄,¹⁹ and CuInS₂.²⁰ As one of the typical TTMS, ternary Cu₂MoS₄ is known as an earth-abundant, environmentally-friendly and low-cost semiconductor material.^21,22 It has been demonstrated by Guo et al. that the Cu₂MoS₄ material had intensive absorption in the visible light region, which is predictive to exhibit good performance in visible-light-driven photocatalysis.²³ Recently, Tran et al. successfully synthesized Cu₂MoS₄ samples as novel electro- and photoelectrocatalysts for the hydrogen evolution reaction (HER). The excellent catalytic performance of Cu₂MoS₄ is ascribed to accelerate the multi-electronic nature of chemical reactions in HER.²⁴ However, it's still a problem to prepare Cu₂MoS₄ samples with high surface area, high uniformity and no-impurity for further enhancing their catalytic efficiency.

Herein, we successfully prepared self-assembled Cu₂MoS₄ (space group I42m) nanobelts via a modified hydrothermal method. And their photocatalytic behavior for the degradation of Methyl Orange (MO) dye under visible-light irradiation was investigated. The photocatalytic tests revealed that the nanobelts exhibited excellent photocatalytic performance by degrading 100 mL of 15 mg mL⁻¹ MO in 20 min completely. Compared with our previous sheet-like sample,²⁵ the reasons for the high efficiency of Cu₂MoS₄ nanobelts are further discussed. Thanks to their unique bundle-assembled structure, the ultrathin Cu₂MoS₄ nanobelts show higher surface areas and visible light absorption in contrast to the sheet-like sample. Meanwhile, the weaker photoluminescence spectra of the nanobelts further confirmed the better efficiency of separation of the photogenerated electron–hole pairs. This work offers a common method to prepare Cu₂MX₄ (M = Mo, W; X = S, Se) nanomaterials with various morphologies and also provides useful insights for designing other excellent catalysts for many specific applications.

2. Experimental section

The Cu₂MoS₄ nanobelts are synthesized using a modified hydrothermal method based on our previous work.²⁴ First, 40 mg Cu₂O powder was dispersed in 25 ml deionized water by sonication for 5 min, then 108 mg (NH₄)₂MoS₄ was dissolved in the solution. After stirring for 5 min, the precursor was transferred into a 45 mL Teflon-lined stainless steel autoclave and maintained at 160 °C for 9 h. The final products were washed with deionized water and ethanol several times to remove any possible ions, and then dried at 60 °C under vacuum for a couple of hours.

The composition and structure of the as-prepared products were characterized by X-ray powder diffraction (XRD, Philips X'Pert Pro Super X-ray diffractometer, Cu Kα (λ = 1.54178 Å) radiation). The morphologies were observed by using a scanning electron microscope (SEM, JSM-6700F, and 5 kV) and a transmission electron microscope (TEM, JEOL JEM-2100F microscope, 200 kV). Energy dispersive X-ray (EDX) element mapping analysis was carried out using a JEOL ARM-200 microscope at 200 kV. The samples were dispersed in ethanol and dropped onto a gold grid with a carbon film coated for TEM characterization. The AFM analysis was carried out using a Veeco MultiMode V. The UV-vis spectra were recorded using a Shimadzu DUV-3700 spectrophotometer. N₂ sorption isotherms were recorded with a surface area and pore size analyzer (Micromeritics TriStar 3020). All of the samples were degassed under vacuum for 12 h before measurements. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method using adsorption data. The photoluminescence spectra of the samples were recorded on a Jobin Yvon Horiba FluoroLog-3-Tau Spectrofluorometer.

The photocatalytic activities of the Cu₂MoS₄ samples were evaluated by the degradation of methylene orange (MO) dyes under a 300 W Xe lamp with a UV cut-off filter (providing visible light with a wavelength longer than 420 nm). The samples (20 mg, 50 mg) were put into a solution of MO (100 mL, 15 mg L⁻¹) in a 200 mL glass beaker. Before the lamp was switched on, the solution was stirred in the dark for 120 min to ensure adsorption/desorption equilibrium between Cu₂MoS₄ and dye. Under constant stirring, about 5 mL of the mixture solution was taken out at different intervals. After centrifugation, the UV/Vis spectrum of the supernatant was recorded to monitor the degradation behaviour.

3. Results and discussion

Fig. 1A shows the EDX spectrum of the obtained samples, indicating the presence of Cu, Mo and S elements, whereas Fig. 1B shows the corresponding powder X-ray diffraction (XRD) patterns. According to previous reports, the XRD peaks can be well assigned to Cu₂MoS₄ with I-phase.²⁵ Notably, no peak is observed for impurities (i.e. MoS₂ or Cu₂S), indicating that the as-prepared sample is highly crystalline and pure quality of Cu₂MoS₄.

Fig. 1 (A) EDX spectrum and (B) XRD patterns of as-prepared Cu₂MoS₄ nanobelts.

Fig. 2A and B show representative SEM and TEM images of the obtained Cu₂MoS₄ samples, respectively. It is noted that the samples exhibit uniform bundle structures with no particle-like impurities. The average length and width of the bundle are 1–1.5 μm and 50–100 nm, respectively. The typical TEM image in Fig. 2B reveals that the resulting products are composed mainly of belt-like nanostructures with well-defined facets. The high-resolution TEM (HRTEM) analysis in Fig. 2C further confirms the highly crystalline nature of nanobelts. The crystal lattices of 2.72 Å and 1.91 Å correspond to the d-spacing of both (200) and (220) facets. The selected-area electron diffraction (SAED) pattern is shown in Fig. 2D, which reveals that the nanobelts were single crystals. To accurately detect the thickness of the nanobelt, atomic force microscopy (AFM) analysis was done in tapping-mode (see Fig. S1 in the ESI†). The AFM profile calculation of the nanobundles shows the thickness range of 5–25 nm, suggesting that the nanobundles are formed by the stack of several ultrathin nanobelts. In addition, Fig. 2E shows the elemental mapping of the samples. It can be seen that Cu, Mo, and S are uniformly distributed within the samples. The calculated molar ratio of Cu : Mo : S is about 2 : 1.1 : 4, further suggesting the as-prepared Cu₂MoS₄.

Fig. 2 Electron microscopy characterization of the self-assembling Cu₂MoS₄ nanobelts. (A) Typical SEM and (B) TEM images. (C) High-resolution TEM image. (D) Selected area electron diffraction patterns. (E) Elemental mapping images of Cu, Mo, and S, respectively.

For better understanding of the growth mechanism of the Cu₂MoS₄ nanobelts, time-dependent experiments were carried out by preparing samples at different reaction times (0 h, 3 h, 6 h, 9 h, 12 h), then characterized by TEM and SEM. The results are shown in Fig. 3A (also see Fig. S3 in the ESI†). It can be observed that the starting Cu₂O materials, acting as sacrificial templates, exhibit sizes from 100 to 200 nm with a sphere-like morphology. Further, dispersion of Cu₂O nanospheres in deionized water, followed by hydrothermal reaction can change them to a hollow-sphere structure. Meanwhile, some branches can also be formed on the surface of hollow-sphere (like a meteor hammer). It is noted that with the increasing reaction time, the length of the branches can be continuously increased and self-assembled into nanobundles. Finally, the sacrificial Cu₂O hollow-sphere is fractured, and the fall-off nanobundles can attach to each other to form the stacking nanobelts. The above mentioned experimental observations may suggest a synthetic route for the formation of Cu₂MoS₄ nanobelts, as illustrated in Fig. 3B. During the synthesis of Cu₂MoS₄ nanobelts, the free MoS₄²⁻ ions in the solution tend to react with Cu₂O and form Cu₂MoS₄ at the surface of nanospheres. After a 3-hour hydrothermal process, the hollow structure is formed because of the Kirkendall effect.²⁶ By controlling the reaction parameters, we can selectively achieve the oriented growth of Cu₂MoS₄ nanobelts along the (100) crystal direction.

Fig. 3 (A) TEM images of various samples at different reaction times (0 h, 3 h, 6 h, 9 h, 12 h). (B) The schematic illustration proposes the growth formation of Cu₂MoS₄ nanobelts.

The UV-vis diffuse reflectance spectra of nanobelts are shown in Fig. 4A. The nanobelts exhibit an enhanced strong light absorption in the wavelength range of 300–800 nm. The photographs of samples dispersed in ethanol are shown in the inset of Fig. 4A. It can be seen that the nanobelt solution displays a bright-red colour. The unique behaviors of strong light absorption and well dispersion motivate us to investigate the obtained Cu₂MoS₄ nanobelts as potential photocatalysts.

Fig. 4 (A) Diffuse reflectance absorption spectra of the Cu₂MoS₄ nanobelts. The inset shows the photograph of the sample dispersed in ethanol. (B) Photocatalytic activities of Cu₂MoS₄ nanobelts for MO degradation under visible-light irradiation (λ > 420 nm). (C) Visible light-driven degradation of MO repeated cycles in 100 min by using Cu₂MoS₄ nanobelts. (D) Raman spectra of the nanobelts before and after tens of MO degradation cycles, the excited laser wavelength is 532 nm.

The photocatalytic activity of the as-prepared samples was investigated by the photodegradation of Methyl Orange (MO) under visible light irradiation. As shown in Fig. 4B, the blank experiment shows that the MO degradation without catalysts contributes a little under visible light, while the nanobelts exhibited excellent photocatalytic activity. By adding 20 mg Cu₂MoS₄ nanobelts in the 100 mL of 15 mg mL⁻¹ MO, 95% MO dye can be degraded in 60 min. What's more, by adding 50 mg Cu₂MoS₄ nanobelts, 90% MO dye can be degraded within 15 min, and completely degraded in 20 min. To the best of our knowledge, this means that the ultrathin Cu₂MoS₄ nanobelt is one of the most efficient photocatalysts for MO degradation under visible-light irradiation, sometimes even more efficient than some high-cost photocatalysts, like Ag₃PO₄ (i.e. 200 mg Ag₃PO₄ nanosphere catalyst were put into a solution of 100 mL of 8 mg mL⁻¹ MO, and the dye was completely degraded in 28 min).²⁷ Thus, we suggest that the Cu₂MoS₄ nanobelt can be one of the ideal photocatalysts for MO degradation in combination with low-cost, low-dosage and high efficiency.

Apart from high photocatalytic activity, other important factors of new photocatalysts for many practical applications are the recycling performance and stability. The recycling performance of the nanobelts is shown in Fig. 4C. After five recycles, they still show similar catalytic efficiency to the first recycle. In order to evaluate the structural stability, Raman spectroscopy was performed on the samples before and after degradation. Fig. 4D shows a comparative spectrum of the nanobelts before and after tens of photodegradation treatments. The main peaks can be assigned to four Raman active modes, similar to our previous report.²⁸ It is noted that no obvious change is found in comparison with the Raman peaks recorded before and after degradation, which strongly suggests the good structural stability of the nanobelts even after many catalytic reactions.

In order to explain the underlying mechanism for the excellent photocatalytic activity of the self-assembled Cu₂MoS₄ nanobelts, we carried out a comparison study by selecting sheet-like Cu₂MoS₄ as the control sample. The results for the Cu₂MoS₄ nanosheets are shown in the ESI† (see Fig. S3 and 4). In contrast to the Cu₂MoS₄ nanobelts, the sheet-like sample exhibits a comparably worse visible light absorption ability (the colour of the nanosheets dispersed in ethanol is dark-red). More notably, the calculated photodegradation efficiency of the Cu₂MoS₄ nanobelts is about 7.5 times (20 mg catalyst) and 6 times (50 mg catalyst) compared with the sheet-like sample. It is known that the large specific surface area of the photocatalyst benefits the photocatalytic activity.²⁹ The Brunauer–Emmett–Teller (BET) nitrogen adsorption analysis was performed on the two samples. Fig. 5A shows that the specific surface area of Cu₂MoS₄ nanobelts is around 35.4 m² g⁻¹, which is 2.6 times larger than that of the sheet-like sample. Another important factor for photocatalytic performance is the recombination of excited electrons and holes. Photoluminescence (PL) emission is used to obtain the separation efficiency of the photogenerated electrons and holes. As shown in Fig. 5B, the PL spectra of both samples exhibit emission peaks in the 300–800 nm range under the excitation at 320 nm. In comparison with the sheet-like sample, the self-assembled Cu₂MoS₄ nanobelts show weaker PL intensity, indicating their better efficiency for separating the photogenerated electrons and holes. In addition, the comparable small thickness may also contribute to good photocatalytic activity in the Cu₂MoS₄ nanobelts. Guo et al. have changed the ratio of (001)/(101) facets by controlling the thickness of Cu₂WS₄ decahedra via the PVP-assisted hydrothermal method.³⁰ And the change in (001)/(101) should play a more important role in its photoreactivity. Under these conditions, the ultrathin Cu₂MoS₄ nanobelts may show a higher ratio of high energy surface than the nanosheets.

Fig. 5 (A) The nitrogen adsorption–desorption isotherms of Cu₂MoS₄ nanobelts (upside) and nanosheets (bottom) show that the BET surface areas are about 34.5 m² g⁻¹ and 13.6 m² g⁻¹, respectively. (B) Photoluminescence spectra of the two samples, the excited laser wavelength is 325 nm.

4. Conclusion

Ultrathin self-assembled Cu₂MoS₄ nanobelts with a length of several microns and a thickness of 5–25 nm were successfully synthesized by a hydrothermal method. Our characterization revealed that the nanobelts were composed of numerous stacked Cu₂MoS₄ layers with a belt-like morphology. The time-dependent experiments indicate that the free MoS₄²⁻ ions in the solution tend to react with Cu₂O to form small Cu₂MoS₄ branches at the surface of Cu₂O hollow-sphere structures because of the Kirkendall effect. It is worth mentioning that as time passes during the hydrothermal reaction, the small branches were grown into the nanobundles. These nanobundles fall off and self-assembled to form the final stacking nanobelts. The visible light-driven photocatalytic performance of the as-obtained nanobelts was investigated systematically. Owing to the strong visible light absorption, large specific surface areas and low recombination of excited charges, the Cu₂MoS₄ nanobelts exhibited excellent photocatalytic activity, even better than some high-cost noble metal-photocatalysts. Meanwhile, the nanobelts also showed good recycle performance and durability. The presented results can provide a new self-assembly route to selectively prepare new multifunctional layered nanostructures with high surface areas and rich active edges, thus greatly extending their potential applications in catalytic and new energy related fields.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (2014CB848900), the National Natural Science Foundation of China (U1232131, U1532112, 11375198, 11574280) and the Fundamental Research Funds for the Central Universities (WK2310000035). We thank the Shanghai synchrotron Radiation Facility (14W1, SSRF), the Beijing Synchrotron Radiation Facility (1W1B, BSRF) and the NSRL soft-X-ray endstation for help in characterization. L.S. thanks the recruitment program of global experts and the CAS Hundred Talent Program of China.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr04741b

‡ These authors contributed equally to this work.

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